1.
Human embryogenesis
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Human embryogenesis is the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, human development entails growth from a zygote to an adult human being. Fertilisation occurs when the cell successfully enters and fuses with an egg cell. The genetic material of the sperm and egg then combine to form a single cell called a zygote, embryogenesis covers the first eight weeks of development, at the beginning of the ninth week the embryo is termed a fetus. Human embryology is the study of development during the first eight weeks after fertilisation. The normal period of gestation is nine months or 38 weeks, the germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days, during this stage, the zygote begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus, embryogenesis continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow. In comparison to the embryo, the fetus has more recognizable external features, the entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in species, especially among chordates. Fertilization takes place when the spermatozoon has entered the ovum. This usually takes place in the ampulla of one of the fallopian tubes, the 46 chromosomes undergo changes prior to the mitotic division which leads to the formation of the embryo having two cells. Successful fertilization is enabled by three processes, which act as controls to ensure species-specificity. The first is that of chemotaxis which directs the movement of the sperm towards the ovum, secondly there is an adhesive compatibility between the sperm and the egg. The entry of the sperm causes calcium to be released which blocks entry to other sperm cells, a parallel reaction takes place in the ovum called the zona reaction. This sees the release of granules that release enzymes which digest sperm receptor proteins. The granules also fuse with the membrane and modify the zona pellucida in such a way as to prevent further sperm entry. The beginning of the process is marked when the zygote divides through mitosis into two cells
2.
Prenatal development
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Prenatal development is the process in which an embryo and later fetus develops during gestation. Prenatal development starts with fertilization the first stage in embryogenesis which continues in development until birth. In human pregnancy, prenatal development, also known as development, is the development of the embryo following fertilization. By the end of the week of gestational age the embryo has acquired its basic form and is referred to as a fetus. The next period is that of development where many organs become fully developed. This fetal period is described both topically and chronologically with major occurrences being listed by gestational age, in other animals the very early stages of embryogenesis are the same as those in humans. In later stages, development across all taxa of animals and the length of gestation vary, the perinatal period is around the time of birth. In developed countries and at facilities where expert neonatal care is available, in many developing countries, the starting point of this period is considered 28 completed weeks of gestation. In ICD-10, a classification list by the WHO, there is a particular chapter relating to certain conditions originating in the perinatal period. The antepartum period is equivalent to prenatal. Practically, however, antepartum usually refers to the period between the 24th/26th week of age until birth, for example in antepartum hemorrhage. When semen is released into the vagina, the travel through the cervix and body of the uterus. Fertilization of the egg cell, usually takes place in one of the Fallopian tubes, many sperm are released with the possibility of just one sperm cell managing to adhere to and enter the thick protective shell-like layer surrounding the ovum. The first sperm that penetrates fully into the egg donates its genetic material, the egg then polarizes, repelling any additional sperm. The resulting combination is called a zygote, a new and genetically unique organism, the term conception refers variably to either fertilization or to formation of the conceptus after its implantation in the uterus, and this terminology is controversial. Prior to fertilization, each ovum, as a gamete, contains half of the material that will fuse with the male gamete. The ovum only carries the X female sex chromosome whilst the sperm carries a single sex chromosome of either an X or a male Y chromosome. The resulting human zygote is similar to the majority of cells because it contains two copies of the genome in a diploid set of chromosomes
3.
Anatomical terminology
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Anatomical terminology is a form of scientific terminology used by anatomists, zoologists, and health professionals such as doctors. Anatomical terminology uses many terms, suffixes, and prefixes deriving from Ancient Greek. These terms can be confusing to those unfamiliar with them, but can be more precise reducing ambiguity, also, since these anatomical terms are not used in everyday conversation, their meanings are less likely to change, and less likely to be misinterpreted. By using precise anatomical terminology such ambiguity is eliminated, an international standard for anatomical terminology, Terminologia Anatomica has been created. Anatomical terminology has quite regular morphology, the prefixes and suffixes are used to add meanings to different roots. The root of a term refers to an organ, tissue. For example, in the disorder hypertension, the prefix hyper- means high or over, the roots, prefixes and suffixes are often derived from Greek or Latin, and often quite dissimilar from their English-language variants. Latin names of such as musculus biceps brachii can be split up and refer to, musculus for muscle, biceps for two-headed. The first word tells us what we are speaking about, the second describes it, when describing the position of anatomical structures, structures may be described according to the anatomical landmark they are near. These landmarks may include structures, such as the umbilicus or sternum, or anatomical lines, the cephalon or cephalic region refers to the head. This area is differentiated into the cranium, facies, frons, oculus, auris, bucca, nausus, oris. The neck area is called the cervicis or cervical region, examples of structures named according to this include the frontalis muscle, submental lymph nodes, buccal membrane and orbicularis oculi muscle. Sometimes, unique terminology is used to reduce confusion in different parts of the body, for example, different terms are used when it comes to the skull in compliance with its embryonic origin and its tilted position compared to in other animals. Here, Rostral refers to proximity to the front of the nose, similarly, in the arms, different terminology is often used in the arms, in part to reduce ambiguity as what is the front, back, inner and outer surfaces. For this reason, the terms below are used, Radial referring to the radius bone, ulnar referring to the ulna bone, medially positioned when in the standard anatomical position. Other terms are used to describe the movement and actions of the hands and feet. International morphological terminology is used by the colleges of medicine and dentistry and it facilitates communication and exchanges between scientists from different countries of the world and it is used daily in the fields of research, teaching and medical care. The international morphological terminology refers to morphological sciences as a biological sciences branch, in this field, the form and structure are examined as well as the changes or developments in the organism
4.
Embryogenesis
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Embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of development, whereas the terms fetus. Embryogenesis starts with the fertilization of the egg cell by a sperm cell, once fertilized, the ovum is referred to as a zygote, a single diploid cell. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, the egg cell is generally asymmetric, having an animal pole and a vegetal pole. It is covered with protective envelopes, with different layers, the first envelope - the one in contact with the membrane of the egg - is made of glycoproteins and is known as the vitelline membrane. Different taxa show different cellular and acellular envelopes englobing the vitelline membrane, fertilization is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation. The fertilized egg cell is known as the zygote, cell division with no significant growth, producing a cluster of cells that is the same size as the original zygote, is called cleavage. At least four initial cell divisions occur, resulting in a ball of at least sixteen cells called the morula. The different cells derived from cleavage, up to the stage, are called blastomeres. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic or meroblastic, on the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk, i. e. birds and reptiles. Because cleavage is impeded in the pole, there is a very uneven distribution and size of cells, being more numerous. In holoblastic eggs the first cleavage always occurs along the axis of the egg. Fluid collects between the trophoblast and the part of the inner cell-mass, and thus the morula is converted into a vesicle. After the 7th cleavage has produced 128 cells, the embryo is called a blastula, the blastula is usually a spherical layer of cells surrounding a fluid-filled or yolk-filled cavity. Mammals at this form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula. The blastocyst must not be confused with the blastula, even though they are similar in structure, their cells have different fates. On the deep surface of the cell mass, a layer of flattened cells, called the endoderm, is differentiated and quickly assumes the form of a small sac
5.
Multicellular organism
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Multicellular organisms are organisms that consist of more than one cell, in contrast to unicellular organisms. Multicellular organisms arise in different ways, for example by cell division or by aggregation of many single cells. Colonial organisms are the result of identical individuals joining together to form a colony. Multicellularity has evolved independently at least 46 times, including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, however, complex multicellular organisms evolved only in six eukaryotic groups, animals, fungi, brown algae, red algae, green algae, and land plants. It evolved repeatedly for Chloroplastida, once or twice for animals, once for brown algae, the first evidence of multicellularity is from cyanobacteria-like organisms that lived 3–3.5 billion years ago. To reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells, animals have evolved a considerable diversity of cell types in a multicellular body, compared with 10–20 in plants and fungi. Loss of multicellularity occurred in some groups, fungi are predominantly multicellular, though early diverging lineages are largely unicellular and there have been numerous reversions to unicellularity across fungi. It may also have occurred in some red algae, but it is possible that they are primitively unicellular), loss of multicellularity is also considered probable in some green algae. In other groups, generally parasites, a reduction of multicellularity occurred, multicellular organisms, especially long-living animals, face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development. Changes in tissue morphology can be observed during this process, cancer in animals has often been described as a loss of multicellularity. There is a discussion about the possibility of existence of cancer in other organisms or even in protozoa. For example, plant galls have been characterized as tumors but some argue that plants do not develop cancer. In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a cell line evolved. However, Weismannist development is relatively rare, as part of species have the capacity for somatic embryogenesis. One hypothesis for the origin of multicellularity is that a group of function-specific cells aggregated into a mass called a grex. This is essentially what slime molds do, another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte. A membrane would then form around each nucleus, thereby resulting in a group of connected cells in one organism and this is what plant and animal embryos do as well as colonial choanoflagellates. Because the first multicellular organisms were simple, soft organisms lacking bone, shell or other body parts
6.
Ploidy
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Ploidy is the number of sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Cells are described according to the number of present, monoploid, diploid, triploid, tetraploid, pentaploid, hexaploid, heptaploid or septaploid. The generic term polyploid is used to describe cells with three or more sets of chromosomes, humans are diploid organisms, carrying two complete sets of chromosomes, one set of 23 chromosomes from their father and one set of 23 chromosomes from their mother. The two sets combined provide a complement of 46 chromosomes. This total number of chromosomes is called the chromosome number, when a species has a varying chromosome number, e. g. a diploid and tetraploid form, the chromosome number is called diploid number in the diploid form, and tetraploid number in the tetraploid form. The number of found in a single complete set of chromosomes is called the monoploid number. The haploid number is unique to gametes, and refers to the number of chromosomes found in a gamete. The haploid number for humans is 23, and the monoploid number equals 46 divided by the level of 2. When a human germ cell undergoes meiosis the two sets of 23 chromosomes are split in half to form gametes, after fusion of a male and a female gamete both containing 1 set of 23 chromosomes, the resulting zygote has 46 chromosomes,2 sets of 23 chromosomes. The common potato is an example of an organism, carrying four sets of chromosomes. The potato plant inherits two sets of 12 chromosomes from the parent, and two sets of 12 chromosomes from the ovule parent. The four sets combined provide a complement of 48 chromosomes. The monoploid number equals the number divided by the ploidy level,48 chromosomes in total divided by a ploidy level of 4 equals a monoploid number of 12. Because the chromosome number is reduced only by the specialized process of meiosis. However, in many situations somatic cells double their number by means of endoreduplication as an aspect of cellular differentiation. When a germ cell with an number of chromosomes undergoes meiosis. Triploid organisms for instance are usually sterile, because of this, triploidy is a common way of making seedless fruit such as bananas and watermelons. If the fertilization of human gametes results in 3 sets of chromosomes the condition is called triploid syndrome, the term ploidy is a back-formation from haploidy and diploidy
7.
Eukaryote
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A eukaryote is any organism whose cells contain a nucleus and other organelles enclosed within membranes. Eukaryotes belong to the taxon Eukarya or Eukaryota, the presence of a nucleus gives eukaryotes their name, which comes from the Greek εὖ and κάρυον. Eukaryotic cells also contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, plants and algae contain chloroplasts. Eukaryotic organisms may be unicellular or multicellular, only eukaryotes form multicellular organisms consisting of many kinds of tissue made up of different cell types. Eukaryotes can reproduce asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two identical cells. In meiosis, DNA replication is followed by two rounds of division to produce four daughter cells each with half the number of chromosomes as the original parent cell. These act as sex cells resulting from genetic recombination during meiosis, the domain Eukaryota appears to be monophyletic, and so makes up one of the three domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features, eukaryotes represent a tiny minority of all living things. However, due to their larger size, eukaryotes collective worldwide biomass is estimated at about equal to that of prokaryotes. Eukaryotes first developed approximately 1. 6–2.1 billion years ago, in 1905 and 1910, the Russian biologist Konstantin Mereschkowsky argued three things about the origin of nucleated cells. Firstly, plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, secondly, the host had earlier in evolution formed by symbiosis between an amoeba-like host and a bacteria-like cell that formed the nucleus. Thirdly, plants inherited photosynthesis from cyanobacteria, the split between the prokaryotes and eukaryotes was introduced in the 1960s. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton, the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1938 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes. However he mentioned this in one paragraph, and the idea was effectively ignored until Chattons statement was rediscovered by Stanier. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in cells in her paper. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA and this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles, mitochondria and chloroplasts
8.
Organism
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In biology, an organism is any contiguous living system, such as an animal, plant, fungus, protist, archaeon, or bacterium. All known types of organisms are capable of some degree of response to stimuli, reproduction, growth and development and homeostasis. An organism consists of one or more cells, when it has one cell it is known as an organism. Most unicellular organisms are of microscopic scale and are thus described as microorganisms. Humans are multicellular organisms composed of trillions of cells grouped into specialized tissues. An organism may be either a prokaryote or a eukaryote, prokaryotes are represented by two separate domains—bacteria and archaea. Eukaryotic organisms are characterized by the presence of a cell nucleus. Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes, estimates on the number of Earths current species range from 10 million to 14 million, of which only about 1.2 million have been documented. More than 99% of all species, amounting to five billion species. In 2016, a set of 355 genes from the last universal ancestor of all living organisms living was identified. The term organism first appeared in the English language in 1703 and it is directly related to the term organization. There is a tradition of defining organisms as self-organizing beings. An organism may be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. Dictionary definitions can be broad, using such as any living structure, such as a plant, animal, fungus or bacterium, capable of growth. Many definitions exclude viruses and possible man-made non-organic life forms, as viruses are dependent on the machinery of a host cell for reproduction. A superorganism is an organism consisting of individuals working together as a single functional or social unit. There has been controversy about the best way to define the organism, several contributions are responses to the suggestion that the category of organism may well not be adequate in biology. Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction and this controversy is problematic because some cellular organisms are also incapable of independent survival and live as obligatory intracellular parasites
9.
Zygote
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A zygote, is an eukaryotic cell formed by a fertilization event between two gametes. The zygotes genome is a combination of the DNA in each gamete, in multicellular organisms, the zygote is the earliest developmental stage. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring, oscar Hertwig and Richard Hertwig made some of the first discoveries on animal zygote formation. In fungi, the fusion of haploid cells is called karyogamy. The result of karyogamy is a diploid cell called a zygote or zygospore and this cell may then enter meiosis or mitosis depending on the life cycle of the species. In plants, the zygote may be polyploid if fertilization occurs between meiotically unreduced gametes, in land plants, the zygote is formed within a chamber called the archegonium. In seedless plants, the archegonium is usually flask-shaped, with a long hollow neck through which the cell enters. As the zygote divides and grows, it does so inside the archegonium, in human fertilization, a release ovum and a haploid sperm cell —combine to form a single 2n diploid cell called the zygote. The other product of meiosis is the polar body with only chromosomes. In the fertilized daughter, DNA is then replicated in the two separate pronuclei derived from the sperm and ovum, making the zygotes chromosome number temporarily 4n diploid. After approximately 30 hours from the time of fertilization, fusion of the pronuclei, between the stages of fertilization and implantation, the developing human is a preimplantation conceptus. However, the National Institute of Health has made the determination that the classification of pre-implantation embryo is still correct. After fertilization, the travels down the oviduct towards the uterus while continuing to divide mitotically without actually increasing in size. After four divisions, the conceptus consists of 16 blastomeres, through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation. When the blastocyst hatches from the zona pellucida, it can implant in the lining of the uterus. The human zygote has been edited in experiments designed to cure inherited diseases. In the amoeba, reproduction occurs by division of the parent cell
10.
Fertilisation
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The cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation. In 1784, Spallanzani established the need of interaction between the females ovum and males sperm to form a zygote, oscar Hertwig, in Germany, described the fusion of nuclei of spermatozoa and of ova from sea urchin. In flowering plants there are two sperm from each pollen grain, in seed plants, after pollination, a pollen grain germinates, and a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle. The sperm are transferred from the pollen through the pollen tube, bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called non-vascular plants. Some bryophytes do have specialised tissues for the transport of water, however, since these do not contain lignin, a fern is a member of a group of roughly 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular and they have stems and leaves, like other vascular plants. Most ferns have what are called fiddleheads that expand into fronds, the gymnosperms are a group of seed-producing plants that includes conifers, Cycads, Ginkgo, and Gnetales. The term gymnosperm comes from the Greek composite word γυμνόσπερμος, meaning naked seeds and their naked condition stands in contrast to the seeds and ovules of flowering plants, which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves, often modified to form cones, the pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the style and curls to the bottom of the ovary, then near the receptacle, it breaks through the ovule through the micropyle, after being fertilised, the ovary starts to swell and develop into the fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule, the growth of the pollen tube is controlled by the vegetative cytoplasm. The sperms are interconnected and dimorphic, the one, in a number of plants, is also linked to the tube nucleus and the interconnected sperm. Double fertilisation is the process in angiosperms in which two sperm from each pollen tube fertilise two cells in a gametophyte that is inside an ovule. After the pollen tube enters the gametophyte, the tube nucleus disintegrates. This is the point when fertilisation actually occurs, pollination and fertilisation are two separate processes, the nucleus of the other sperm cell fuses with two haploid polar nuclei in the centre of the gametophyte. This triploid cell divides through mitosis and forms the endosperm, a nutrient-rich tissue, the two central-cell maternal nuclei that contribute to the endosperm arise by mitosis from the single meiotic product that also gave rise to the egg. Therefore, maternal contribution to the constitution of the triploid endosperm is double that of the embryo
11.
Egg cell
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The egg cell, or ovum, is the female reproductive cell in oogamous organisms. The egg cell is not capable of active movement. When egg and sperm fuse, a cell is formed. Karl Ernst von Baer discovered the mammalian ovum in 1827, the fusion of spermatozoa with ova was observed by Oskar Hertwig in 1876. In animals, egg cells are known as ova. The term ovule is used for the young ovum of an animal, in vertebrates, ova are produced by female gonads called ovaries a number of ova are present at birth in mammals and mature via oogenesis. White et al. disproved the longstanding dogma that all of the ova are produced before birth, the team from the Vincent Center for Reproductive Biology, Massachusetts, Boston could prove that oocyte fromation takes place in ovaries of reproductive-age women. This report challenged a fundamental belief, held since the 1950s, in all mammals the ovum is fertilized inside the female body. The human ova grow from primitive germ cells that are embedded in the substance of the ovaries, each of them divides repeatedly to give secretions of the uterine glands, ultimately forming a blastocyst. The ovum is one of the largest cells in the human body, the human ovum measures approximately 0.12 mm in diameter. Ooplasm is the yolk of the ovum, a substance at its center, which contains its nucleus, named the germinal vesicle. Mammalian ova contain only an amount of the nutritive yolk. In contrast, bird eggs contain enough to supply the chick with nutriment throughout the period of incubation. In the oviparous animals the ova develop protective layers and pass through the oviduct to the outside of the body and they are fertilized by male sperm either inside the female body, or outside. After fertilization, an embryo develops, nourished by nutrients contained in the egg and it then hatches from the egg, outside the mothers body. See egg for a discussion of eggs of oviparous animals, the egg cells cytoplasm and mitochondria are the sole means the egg is able to reproduce by mitosis and eventually form a blastocyst after fertilization. Some fish, reptiles and many use this technique. Nearly all land plants have alternating diploid and haploid generations, gametes are produced by the gametophyte, which is the haploid generation
12.
Sperm
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Sperm is the male reproductive cell and is derived from the Greek word sperma. In the types of reproduction known as anisogamy and its subtype oogamy. A uniflagellar sperm cell that is motile is referred to as a spermatozoon, Sperm cells cannot divide and have a limited life span, but after fusion with egg cells during fertilization, a new organism begins developing, starting as a totipotent zygote. The human sperm cell is haploid, so that its 23 chromosomes can join the 23 chromosomes of the egg to form a diploid cell. In mammals, sperm develops in the testicles and is released from the penis, the mammalian sperm cell consists of a head, neck, a midpiece and a tail. The head contains the nucleus with densely coiled chromatin fibres, surrounded anteriorly by an acrosome, the neck contains the sperm centriole. The midpiece has a central core with many mitochondria spiralled around it, used for ATP production for the journey through the female cervix, uterus. The tail or flagellum executes the lashing movements that propel the spermatocyte, although semen contains millions of sperm, the egg will admit only one. The other ones will die and be absorbed. The spermatozoa of animals are produced through spermatogenesis inside the gonads via meiotic division. The initial spermatozoon process takes around 70 days to complete, the spermatid stage is where the sperm develops the familiar tail. The next stage where it becomes fully mature takes around 60 days when it is called a spermatozoan, Sperm cells are carried out of the male body in a fluid known as semen. Human sperm cells can survive within the reproductive tract for more than 5 days post coitus. Semen is produced in the vesicles, prostate gland and urethral glands. In 2016 scientists at Nanjing Medical University claimed they had produced cells resembling mouse spermatids artificially from stem cells and they injected these spermatids into mouse eggs and produced pups. Sperm quantity and quality are the parameters in semen quality. Thus, in humans, it is a measure of fertility in a man, the genetic quality of sperm, as well as its volume and motility, all typically decrease with age. Human sperm cells are vulnerable to free radical attack and the generation of oxidative DNA damage
13.
DNA
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Deoxyribonucleic acid is a molecule that carries the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses. DNA and RNA are nucleic acids, alongside proteins, lipids and complex carbohydrates, most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. The two DNA strands are termed polynucleotides since they are composed of simpler units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases—cytosine, guanine, adenine, or thymine —a sugar called deoxyribose, and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. The nitrogenous bases of the two polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA. The total amount of related DNA base pairs on Earth is estimated at 5.0 x 1037, in comparison the total mass of the biosphere has been estimated to be as much as 4 trillion tons of carbon. The DNA backbone is resistant to cleavage, and both strands of the double-stranded structure store the same biological information and this information is replicated as and when the two strands separate. A large part of DNA is non-coding, meaning that these sections do not serve as patterns for protein sequences, the two strands of DNA run in opposite directions to each other and are thus antiparallel. Attached to each sugar is one of four types of nucleobases and it is the sequence of these four nucleobases along the backbone that encodes biological information. RNA strands are created using DNA strands as a template in a process called transcription, under the genetic code, these RNA strands are translated to specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells DNA is organized into structures called chromosomes. During cell division these chromosomes are duplicated in the process of DNA replication, eukaryotic organisms store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts. In contrast prokaryotes store their DNA only in the cytoplasm, within the eukaryotic chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed, DNA was first isolated by Friedrich Miescher in 1869. DNA is used by researchers as a tool to explore physical laws and theories, such as the ergodic theorem. The unique material properties of DNA have made it an attractive molecule for material scientists and engineers interested in micro-, among notable advances in this field are DNA origami and DNA-based hybrid materials. DNA is a polymer made from repeating units called nucleotides
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Plant
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Plants are mainly multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. The term is generally limited to the green plants, which form an unranked clade Viridiplantae. This includes the plants, conifers and other gymnosperms, ferns, clubmosses, hornworts, liverworts, mosses and the green algae. Green plants have cell walls containing cellulose and obtain most of their energy from sunlight via photosynthesis by primary chloroplasts and their chloroplasts contain chlorophylls a and b, which gives them their green color. Some plants are parasitic and have lost the ability to produce amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although reproduction is also common. There are about 300–315 thousand species of plants, of which the great majority, green plants provide most of the worlds molecular oxygen and are the basis of most of Earths ecologies, especially on land. Plants that produce grains, fruits and vegetables form humankinds basic foodstuffs, Plants play many roles in culture. They are used as ornaments and, until recently and in variety, they have served as the source of most medicines. The scientific study of plants is known as botany, a branch of biology, Plants are one of the two groups into which all living things were traditionally divided, the other is animals. The division goes back at least as far as Aristotle, who distinguished between plants, which generally do not move, and animals, which often are mobile to catch their food. Much later, when Linnaeus created the basis of the system of scientific classification. Since then, it has become clear that the plant kingdom as originally defined included several unrelated groups, however, these organisms are still often considered plants, particularly in popular contexts. When the name Plantae or plant is applied to a group of organisms or taxon. The evolutionary history of plants is not yet settled. Those which have been called plants are in bold, the way in which the groups of green algae are combined and named varies considerably between authors. Algae comprise several different groups of organisms which produce energy through photosynthesis, most conspicuous among the algae are the seaweeds, multicellular algae that may roughly resemble land plants, but are classified among the brown, red and green algae. Each of these groups also includes various microscopic and single-celled organisms
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Animal
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Animals are multicellular, eukaryotic organisms of the kingdom Animalia. The animal kingdom emerged as a clade within Apoikozoa as the group to the choanoflagellates. Animals are motile, meaning they can move spontaneously and independently at some point in their lives and their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later in their lives. All animals are heterotrophs, they must ingest other organisms or their products for sustenance, most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. Animals can be divided broadly into vertebrates and invertebrates, vertebrates have a backbone or spine, and amount to less than five percent of all described animal species. They include fish, amphibians, reptiles, birds and mammals, the remaining animals are the invertebrates, which lack a backbone. These include molluscs, arthropods, annelids, nematodes, flatworms, cnidarians, ctenophores, the study of animals is called zoology. The word animal comes from the Latin animalis, meaning having breath, the biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects, and humans. Aristotle divided the world between animals and plants, and this was followed by Carl Linnaeus, in the first hierarchical classification. In Linnaeuss original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, in 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms, Metazoa and Protozoa. The protozoa were later moved to the kingdom Protista, leaving only the metazoa, thus Metazoa is now considered a synonym of Animalia. Animals have several characteristics that set apart from other living things. Animals are eukaryotic and multicellular, which separates them from bacteria and they are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae. They are also distinguished from plants, algae, and fungi by lacking cell walls. All animals are motile, if only at life stages. In most animals, embryos pass through a stage, which is a characteristic exclusive to animals. With a few exceptions, most notably the sponges and Placozoa and these include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals
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Protist
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Protist is an informal term for any eukaryotic organism that is not an animal, plant or fungus. The protists do not form a group, or clade. Besides their relatively simple levels of organization, protists do not necessarily have much in common, others use the term protist more broadly, to encompass both microbial eukaryotes and macroscopic organisms that do not fit into the other traditional kingdoms. In cladistic systems, there are no equivalents to the taxa Protista or Protoctista, in cladistic classification, the contents of Protista are distributed among various supergroups and Protista, Protoctista and Protozoa are considered obsolete. However, the term protist continues to be used informally as a term for eukaryotic microorganisms. For example, the phrase protist pathogen may be used to denote any disease-causing microbe which is not bacteria, virus, the term protista was first used by Ernst Haeckel in 1866. Some protists, sometimes called ambiregnal protists, have considered to be both protozoa and algae or fungi, and names for these have been published under either or both of the ICN and the ICZN. Conflicts, such as these – for example the dual-classification of Euglenids and Dinobryons and these traditional subdivisions, largely based on superficial commonalities, have been replaced by classifications based on phylogenetics. Molecular analyses in modern taxonomy have been used to redistribute former members of this group into diverse, however, the older terms are still used as informal names to describe the morphology and ecology of various protists. For example, the term protozoa is used to refer to species of protists that do not form filaments. Among the pioneers in the study of the protists, which were almost ignored by Linnaeus except for some genera were Leeuwenhoek, O. F. Müller, C. G. Ehrenberg, the first groups used to classify microscopic organism were the Animalcules and the Infusoria. In 1817, the German naturalist Georg August Goldfuss introduced the word Protozoa to refer to such as ciliates. After the cell theory of Schwann and Schleiden, this group was modified in 1848 by Carl von Siebold to include only animal-like unicellular organisms, such as foraminifera and amoebae. He defined the Protoctista as a kingdom of nature, in addition to the then-traditional kingdoms of plants. The kingdom of minerals was later removed from taxonomy in 1866 by Ernst Haeckel, leaving plants, animals, in 1938, Herbert Copeland resurrected Hoggs label, arguing that Haeckels term Protista included anucleated microbes such as bacteria, which the term Protoctista did not. In contrast, Copelands term included nucleated eukaryotes such as diatoms, green algae and this classification was the basis for Whittakers later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life. The kingdom Protista was later modified to separate prokaryotes into the kingdom of Monera. Many systematists today do not treat Protista as a formal taxon, some systematists judge paraphyletic taxa acceptable, and use Protista in this sense as a formal taxon
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Mitosis
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In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. Mitosis and cytokinesis together define the phase of an animal cell cycle—the division of the mother cell into two daughter cells genetically identical to each other. The process of mitosis is divided into stages corresponding to the completion of one set of activities and these stages are prophase, prometaphase, metaphase, anaphase, and telophase. During mitosis, the chromosomes, which have already duplicated, condense, the result is two genetically identical daughter nuclei. The rest of the cell may then continue to divide by cytokinesis to produce two daughter cells, producing three or more daughter cells instead of normal two is a mitotic error called tripolar mitosis or multipolar mitosis. Other errors during mitosis can induce apoptosis or cause mutations, certain types of cancer can arise from such mutations. Mitosis occurs only in eukaryotic cells, prokaryotic cells, which lack a nucleus, divide by a different process called binary fission. Most animal cells undergo a change, known as mitotic cell rounding. Most human cells are produced by cell division. Important exceptions include the gametes – sperm and egg cells – which are produced by meiosis, german zoologist Otto Bütschli might have claimed the discovery of the process presently known as mitosis, a term coined by Walther Flemming in 1882. Mitosis was discovered in frog, rabbit, and cat cornea cells in 1873, the term is derived from the Greek word μίτος mitos warp thread. The primary result of mitosis and cytokinesis is the transfer of a parent cells genome into two daughter cells, the genome is composed of a number of chromosomes—complexes of tightly coiled DNA that contain genetic information vital for proper cell function. Because each resultant daughter cell should be identical to the parent cell. This occurs during the S phase of interphase, chromosome duplication results in two identical sister chromatids bound together by cohesin proteins at the centromere. When mitosis begins, the chromosomes condense and become visible, in some eukaryotes, for example animals, the nuclear envelope, which segregates the DNA from the cytoplasm, disintegrates into small vesicles. The nucleolus, which makes ribosomes in the cell, also disappears, microtubules project from opposite ends of the cell, attach to the centromeres, and align the chromosomes centrally within the cell. The microtubules then contract to pull the chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes, as the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell and condense maximally in late anaphase
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Fetus
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A fetus is a stage in the prenatal development of viviparous organisms. In human development, a fetus or foetus is a human between the embryonic state and birth. The fetal stage of development tends to be taken as beginning at the age of eleven weeks. In biological terms, however, prenatal development is a continuum, the use of the term fetus generally implies that an embryo has developed to the point of being recognizable as a human, this is the point usually taken to be the ninth week after fertilization. A fetus is also characterized by the presence of all the body organs, though they will not yet be fully developed and functional. The word fetus is from the Latin fētus, the British, Irish, and Commonwealth spelling is foetus, which has been in use since at least 1594. It arose as a hypercorrection based on an incorrect etymology that may have originated with an error by Isidore of Seville in AD620 and this spelling is the most common in most Commonwealth nations, except in the medical literature, where fetus is used. The etymologically accurate original spelling fetus is used in Canada and the United States, in addition, fetus is now the standard English spelling throughout the world in medical journals. The spelling faetus was used historically, in humans, the fetal stage commences at the beginning of the ninth week. At the start of the stage, the fetus is typically about 30 millimetres in length from crown to rump. The head makes up half of the fetus size. Breathing-like movement of the fetus is necessary for stimulation of lung development, the heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation. The genitalia of the starts to form and placenta becomes fully functional during week 9. At this point in development, uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop. A woman pregnant for the first time, typically feels fetal movements at about 21 weeks, whereas a woman who has given birth at least once, by the end of the fifth month, the fetus is about 20 cm long. The amount of body fat rapidly increases, thalamic brain connections, which mediate sensory input, form. Bones are fully developed, but are soft and pliable. Iron, calcium, and phosphorus become more abundant, fingernails reach the end of the fingertips
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Medieval Latin
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Despite the clerical origin of many of its authors, medieval Latin should not be confused with Ecclesiastical Latin. There is no consensus on the exact boundary where Late Latin ends. Medieval Latin had a vocabulary, which freely borrowed from other sources. Greek provided much of the vocabulary of Christianity. The various Germanic languages spoken by the Germanic tribes, who invaded southern Europe, were major sources of new words. Germanic leaders became the rulers of parts of the Roman Empire that they conquered, other more ordinary words were replaced by coinages from Vulgar Latin or Germanic sources because the classical words had fallen into disuse. Latin was also spread to such as Ireland and Germany. Works written in the lands, where Latin was a language with no relation to the local vernacular, also influenced the vocabulary. English words like abstract, subject, communicate, matter, probable, the high point of the development of medieval Latin as a literary language came with the Carolingian renaissance, a rebirth of learning kindled under the patronage of Charlemagne, king of the Franks. On the other hand, strictly speaking there was no form of medieval Latin. Every Latin author in the period spoke Latin as a second language, with varying degrees of fluency, and syntax, grammar. For instance, rather than following the classical Latin practice of placing the verb at the end. Unlike classical Latin, where esse was the auxiliary verb, medieval Latin writers might use habere as an auxiliary, similar to constructions in Germanic. The accusative and infinitive construction in classical Latin was often replaced by a clause introduced by quod or quia. This is almost identical, for example, to the use of que in similar constructions in French. In every age from the late 8th century onwards, there were learned writers who were familiar enough with classical syntax to be aware that these forms and usages were wrong, however the use of quod to introduce subordinate clauses was especially pervasive and is found at all levels. That resulted in two features of Medieval Latin compared with Classical Latin. First, many attempted to show off their knowledge of Classical Latin by using rare or archaic constructions
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Greek language
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Greek is an independent branch of the Indo-European family of languages, native to Greece and other parts of the Eastern Mediterranean. It has the longest documented history of any living language, spanning 34 centuries of written records and its writing system has been the Greek alphabet for the major part of its history, other systems, such as Linear B and the Cypriot syllabary, were used previously. The alphabet arose from the Phoenician script and was in turn the basis of the Latin, Cyrillic, Armenian, Coptic, Gothic and many other writing systems. Together with the Latin texts and traditions of the Roman world, during antiquity, Greek was a widely spoken lingua franca in the Mediterranean world and many places beyond. It would eventually become the official parlance of the Byzantine Empire, the language is spoken by at least 13.2 million people today in Greece, Cyprus, Italy, Albania, Turkey, and the Greek diaspora. Greek roots are used to coin new words for other languages, Greek. Greek has been spoken in the Balkan peninsula since around the 3rd millennium BC, the earliest written evidence is a Linear B clay tablet found in Messenia that dates to between 1450 and 1350 BC, making Greek the worlds oldest recorded living language. Among the Indo-European languages, its date of earliest written attestation is matched only by the now extinct Anatolian languages, the Greek language is conventionally divided into the following periods, Proto-Greek, the unrecorded but assumed last ancestor of all known varieties of Greek. The unity of Proto-Greek would have ended as Hellenic migrants entered the Greek peninsula sometime in the Neolithic era or the Bronze Age, Mycenaean Greek, the language of the Mycenaean civilisation. It is recorded in the Linear B script on tablets dating from the 15th century BC onwards, Ancient Greek, in its various dialects, the language of the Archaic and Classical periods of the ancient Greek civilisation. It was widely known throughout the Roman Empire, after the Roman conquest of Greece, an unofficial bilingualism of Greek and Latin was established in the city of Rome and Koine Greek became a first or second language in the Roman Empire. The origin of Christianity can also be traced through Koine Greek, Medieval Greek, also known as Byzantine Greek, the continuation of Koine Greek in Byzantine Greece, up to the demise of the Byzantine Empire in the 15th century. Much of the written Greek that was used as the language of the Byzantine Empire was an eclectic middle-ground variety based on the tradition of written Koine. Modern Greek, Stemming from Medieval Greek, Modern Greek usages can be traced in the Byzantine period and it is the language used by the modern Greeks, and, apart from Standard Modern Greek, there are several dialects of it. In the modern era, the Greek language entered a state of diglossia, the historical unity and continuing identity between the various stages of the Greek language is often emphasised. Greek speakers today still tend to regard literary works of ancient Greek as part of their own rather than a foreign language and it is also often stated that the historical changes have been relatively slight compared with some other languages. According to one estimation, Homeric Greek is probably closer to demotic than 12-century Middle English is to modern spoken English, Greek is spoken by about 13 million people, mainly in Greece, Albania and Cyprus, but also worldwide by the large Greek diaspora. Greek is the language of Greece, where it is spoken by almost the entire population
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Latinisation of names
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Latinisation is the practice of rendering a non-Latin name in a Latin style. It is commonly found with personal names, with toponyms. It goes further than romanisation, which is the transliteration of a word to the Latin alphabet from another script and this was often done in the classical era for much the same reason as English-speaking cultures produce English versions of some foreign names. In the case of names in the post-Roman era this may be done to emulate Latin authors. In a scientific context, the purpose of Latinisation may be to produce a name which is internationally consistent. Humanist names, assumed by Renaissance humanists, were very largely Latinised names, Latinisation in humanist names may consist of translation from vernacular European languages, sometimes involving a playful element of punning. Such names could be a cover for social origins. Latinisation is a practice for scientific names. For example, Livistona, the name of a genus of trees, is a Latinisation of Livingstone. In English, place names appear in Latinised form. This is a result of many text books mentioning the places being written in Latin. Because of this, the English language often uses Latinised forms of place names instead of anglicised forms or the original names. Examples of Latinised names for countries or regions are, Estonia Ingria Livonia During the age of the Roman Empire, additionally, Latinised versions of Greek substantives, particularly proper nouns, could easily be declined by Latin speakers with minimal modification of the original word. During the medieval period, following the collapse of the Empire in Western Europe, in the early medieval period, most European scholars were priests and most educated people spoke Latin, and as a result, Latin became firmly established as the scholarly language for the West. Though during modern times Europe has largely abandoned Latin as a scholarly language, by tradition, it is still common in some fields to name new discoveries in Latin. Romanization, conversion of a text in Latin letters Nicolson, Dan H, orthography of Names and Epithets, Latinization of Personal Names
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Tadpole
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A tadpole is the larval stage in the life cycle of an amphibian, particularly that of a frog or toad. They are usually wholly aquatic, though some species have tadpoles that are terrestrial, when first hatched from the egg they have a more or less globular body, a laterally compressed tail and internal or external gills. As they grow they undergo metamorphosis, during which process they grow limbs, develop lungs, most tadpoles are herbivorous and during metamorphosis the mouth and internal organs are rearranged to prepare for an adult carnivorous lifestyle. Having no hard parts, it might be expected that fossil tadpoles would not exist, however, traces of biofilms have been preserved and fossil tadpoles have been found dating back to the Miocene. Tadpoles are eaten in parts of the world and are mentioned in folk tales. The name tadpole is from Middle English taddepol, made up of the elements tadde, toad, similarly, pollywog / polliwog is from Middle English polwygle, made up of the same pol, head, and wiglen, to wiggle. Tadpoles are young amphibians that live in the water, though a few tadpoles are semi-terrestrial and terrestrial. During the tadpole stage of the life cycle, most respire by means of autonomous external or internal gills. They do not usually have arms or legs until the transition to adulthood, as a tadpole matures, it most commonly metamorphosizes by gradually growing limbs and then outwardly absorbing its tail by apoptosis. Lungs develop around the time of leg development, and tadpoles late in development will often be found near the surface of the water, where they breathe air. During the final stages of external metamorphosis, the mouth changes from a small. The intestines shorten to accommodate the new diet, most tadpoles are herbivorous, subsisting on algae and plants. Some species are omnivorous, eating detritus and, when available, tadpoles vary greatly in size, both during their development and between species. For example, in a family, Megophryidae, length of late-stage tadpoles varies between 33 millimetres and 106 millimetres. The tadpoles of Pseudis paradoxa grow to 25 centimetres, the largest of any frog, despite their soft-bodied nature and lack of mineralised hard parts, fossil tadpoles have been recovered from Upper Miocene strata. They are preserved by virtue of biofilms, with more robust structures preserved as a carbon film, in Miocene fossils from Libros, Spain, the brain case is preserved in calcium carbonate, and the nerve cord in calcium phosphate. Other parts of the tadpoles bodies exist as organic remains and bacterial biofilms, tadpole remains with telltale external gills are also known from several of the Labyrinthodont groups. Some tadpoles are used as food, tadpoles of megophryid frog Oreolalax rhodostigmatus are particularly large, more than 10 cm in length, and are collected for human consumption in China
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Blastula
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The blastula is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoele formed during an early stage of embryonic development in animals. Embryo development begins with a sperm fertilizing an egg to become a zygote which undergoes many cleavages to develop into a ball of cells called a morula, only when the blastocoele is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the layers of the embryo form. A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, in mammals the blastula is referred to as a blastocyst. The blastocyst contains an embryoblast that will give rise to the definitive structures of the fetus, and the trophoblast. During the blastula stage of development, a significant amount of activity occurs within the embryo to establish cell polarity, cell specification, axis formation. Many of the interactions between blastomeres are dependent on expression, particularly E-cadherin in mammals and EP-cadherin in amphibians. The study of the blastula and of cell specification has many implications on the field of stem cell research as well as the improvement of fertility treatments. Embryonic stem cells are a field which, though controversial, have potential for treating disease. In Xenopus, blastomeres behave as pluripotent stem cells which can migrate down several pathways, by manipulating the cell signals during the blastula stage of development, various tissues can be formed. This potential can be instrumental in regenerative medicine for disease and injury cases, in vitro fertilisation involves implantation of a blastula into a mother’s uterus. Blastula cell implantation could potentially serve to eliminate infertility, the blastula stage of early embryo development begins with the appearance of the blastocoele. The origin of the blastocoele in Xenopus has been shown to be from the first cleavage furrow, prior to this transition, cleavage occurs with only the synthesis and mitosis phases of the cell cycle. This transition begins the growth in size of the organism, the mid-blastula transition is also characterised by a marked increase in transcription of new, non-maternal mRNA transcribed from the genome of the organism. Large amounts of the maternal mRNA are destroyed at this point and these two processes shift the control of the embryo from the maternal mRNA to the nuclei. A blastula is a sphere of cells surrounding a blastocoele, the blastocoele is a fluid filled cavity which contains amino acids, proteins, growth factors, sugars, ions and other components which are necessary for cellular differentiation. The blastocoele also allows blastomeres to move during the process of gastrulation, in Xenopus embryos, the blastula is composed of three different regions. The animal cap forms the roof of the blastocoele and goes on primarily to form ectodermal derivatives, the equatorial or marginal zone, which compose the walls of the blastocoel differentiate primarily into mesodermal tissue
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Gastrulation
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Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a multilayered structure known as the gastrula. In triploblastic organisms the gastrula is trilaminar and these three germ layers are known as the ectoderm, mesoderm, and endoderm. In diploblastic organisms, such as Cnidaria and Ctenophora, the gastrula has only ectoderm and endoderm, Gastrulation takes place after cleavage and the formation of the blastula. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers, each layer gives rise to specific tissues and organs in the developing embryo. The ectoderm gives rise to epidermis, the system. The endoderm gives rise to the epithelium of the system and respiratory system. The mesoderm gives rise to many types such as muscle, bone. In vertebrates, mesoderm derivatives include the notochord, the heart, blood and blood vessels, the cartilage of the ribs and vertebrae, and the dermis. Following gastrulation, cells in the body are organized into sheets of connected cells, or as a mesh of isolated cells. The molecular mechanism and timing of gastrulation is different in different organisms, the terms gastrula and gastrulation were coined by Ernst Haeckel, in his 1872 work Biology of Calcareous Sponges. Gastrulation is highly variable across the kingdom but has underlying similarities. Gastrulation has been studied in animals, but some models have been used for longer than others. Furthermore, it is easier to study development in animals that develop outside the mother, Protostome derives from the Greek word protostoma meaning first mouth whereas Deuterostomes etymology is second mouth from the words second and mouth. In deuterostome development, the blastopore becomes the animals anus, cleavage Protostomes have what is known as spiral cleavage which is determinate, this meaning that the fate of the cells is determined as they are formed. Deuterostomes have what is known as radial cleavage that is indeterminate, sea urchins Euechinoidea have been an important model system in developmental biology since the 19th century. Their gastrulation is often considered the archetype for invertebrate deuterostomes, sea urchins exhibit highly stereotyped cleavage patterns and cell fates. Maternally deposited mRNAs establish the center of the sea urchin embryo. Canonical Wnt and Delta-Notch signaling progressively segregate progressive endoderm and mesoderm, in Euechinoids the first cells to internalize are the primary mesenchyme cells, which have a skeletogenic fate, which ingress during the blastula stage
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Organogenesis
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In animal development, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. This process can be studied using both embryos and organoids, internal organs initiate development in humans within the 3rd to 8th weeks in utero. The germ layers in organogenesis differ by three processes, folds, splits, and condensation, developing early during this stage in chordate animals are the notochord, which induces the formation of the neural plate, and ultimately the neural tube. Vertebrate animals all differentiate from the gastrula the same way, vertebrates develop a neural crest that differentiates into many structures, including some bones, muscles, and components of the peripheral nervous system. The coelom of the forms from a split of the mesoderm along the somite axis. In plants, organogenesis, which is simply the process of forming new organs, in the shoot, the shoot apical meristems regularly produce new lateral organs and lateral branches. In the root, new lateral roots form from weakly differentiated internal tissue, in vitro and in response to specific cocktails of hormones, most plant tissues can de-differentiate and form a mass of dividing totipotent stem cells called a callus. De novo organogenesis can then occur from those cells, the type of organ that is formed depends on the relative concentrations of the hormones in the medium. Organoid Ectoderm Embryogenesis Endoderm Eye development Gastrulation Germ layer Germ line development Gonadogenesis Heart development Histogenesis Limb development Mesoderm Morphogenesis Noogenesis
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Blastocoel
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A blastocoel is also termed the blastocyst cavity is the name given to the fluid-filled cavity of the blastula that results from cleavage of the oocyte after fertilization. It forms during embryogenesis, as what has been termed a Third Stage after the single-celled fertilized oocyte has divided into 16-32 cells, the fluid-filled cavity forms in the animal hemisphere of the frog. However, the formation of the blastocoel has been traced back to the very first cleavage furrow. It was demonstrated in the embryo that the first cleavage furrow widens in the animal hemisphere creating a small intercellular cavity that is sealed off via tight junctions. As cleavage continues, the cavity expands to become the developed blastocoel, the blastocoel is a crucial component of amphibian embryo development. It permits cell migration during gastrulation and prevents the cells beneath the blastocoel from interacting prematurely with the cells above the blastocoel. For instance, the blastocoel prevents the vegetal cells destined to become endoderm from coming in contact with cells in the ectoderm fated to give rise to the skin. The blastocoel can be damaged and abolished if the adhesion between blastomeres, provided by cell adhesion molecules like EP-cadherin, is destroyed as mRNA by oligonucleotides, if the mRNA is destroyed, then there’s no EP-cadherin, little to no blastomere adhesion and the blastocoel is non-existent. During the next stage of development, amphibian gastrulation, the blastocoel is displaced by the formation of the archenteron. At the end of gastrulation, the blastocoel has been obliterated, at the 120- cell stage, the sea urchin embryo is considered a blastula because of its developed blastocoel, which every embryonic cell surrounds and touches. Every cell is in contact with the fluid of the blastocoel on the inside. The loosely connected blastomeres are now tightly connected because of tight junctions that create a seamless epithelium that completely encircles the blastocoel, even as the blastomeres continue to divide, the blastula remains one-cell thick and thins out as the embryo expands outward. This is accomplished in part due to the influx of water that expands the blastocoel, at this point, the cells have become specified and are ciliated on the opposite side of the blastocoel. The vegetal plate and animal hemisphere develop and secrete an enzyme that digests the fertilization envelope. Important to the sea urchin blastula is the ingression of the primary mesenchyme and these cells then dissociate and ingress into the blastocoel and are called the primary mesenchyme. The cells move randomly along the inside of the blastocoel, until they become localized in the region of the blastocoel. After fertilization, the cells, called blastomeres, undergo rotational cleavage until they are at the 16-cell stage called the morula. The morula has a group of internal cells surrounded by a larger group of external cells
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Eutheria
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Eutheria is one of two mammalian clades with extant members that diverged in the Early Cretaceous or perhaps the Late Jurassic. Except for the Virginia opossum, from North America, which is a metatherian, all post-Miocene mammals indigenous to Europe, Africa, Asia, extant eutherians, their last common ancestor, and all extinct descendants of that ancestor are members of Placentalia. Eutherians are distinguished from noneutherians by various phenotypic traits of the feet, ankles, jaws, all extant eutherians lack epipubic bones, which are present in all other living mammals. This allows for expansion of the abdomen during pregnancy, the oldest known eutherian species is Juramaia sinensis, dated at 160 million years ago from the Jurassic in China. Eutheria was named in 1872 by Theodore Gill, in 1880 Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia, distinguishing features are, an enlarged malleolus at the bottom of the tibia, the larger of the two shin bones. Various features of jaws and teeth, eutheria contains several extinct genera as well as larger groups, many with complicated taxonomic histories still not fully understood. Members of the Adapisoriculidae, Cimolesta and Leptictida have been placed within the out-dated placental group Insectivora. However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, the weakly favoured cladogram favours Boreoeuthearia as a basal Eutherian clade as sister to the Atlantogenata
28.
Bird
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Birds, a subgroup of Reptiles, are the last living examples of Dinosaurs. Birds live worldwide and range in size from the 5 cm bee hummingbird to the 2.75 m ostrich. They rank as the class of tetrapods with the most living species, at ten thousand. Birds are the closest living relatives of crocodilians, the fossil record indicates that birds evolved from feathered ancestors within the theropod group of saurischian dinosaurs. True birds first appeared during the Cretaceous period, around 100 million years ago, Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Birds have wings which are more or less developed depending on the species, the digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have evolved for swimming. Many species annually migrate great distances, Birds are social, communicating with visual signals, calls, and bird songs, and participating in such social behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of species are socially monogamous, usually for one breeding season at a time, sometimes for years. Other species have breeding systems that are polygynous or, rarely, Birds produce offspring by laying eggs which are fertilised through sexual reproduction. They are usually laid in a nest and incubated by the parents, most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilised, songbirds, parrots, and other species are popular as pets. Guano is harvested for use as a fertiliser, Birds prominently figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century, human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry, the first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae. Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use, Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda, Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria
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Germ layer
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A germ layer is a primary layer of cells that form during embryogenesis. The three germ layers in vertebrates are particularly pronounced, however, all eumetazoans, produce two or three germ layers. Animals with radial symmetry, like cnidarians, produce two germ layers making them diploblastic, animals with bilateral symmetry produce a third layer, between these two layers. Germ layers eventually give rise to all of an animal’s tissues, caspar Friedrich Wolff observed organization of the early embryo in leaf-like layers. In 1817, Heinz Christian Pander discovered three primordial germ layers while studying chick embryos, among animals, sponges show the simplest organization, having a single germ layer. Although they have differentiated cells, they lack true tissue coordination, diploblastic animals, Cnidaria and Ctenophora, show an increase in complexity, having two germ layers, the endoderm and ectoderm. Diploblastic animals are organized into recognisable tissues, all higher animals are triploblastic, possessing a mesoderm in addition to the germ layers found in Diploblasts. Fertilization leads to the formation of a zygote, during the next stage, cleavage, mitotic cell divisions transform the zygote into a hollow ball of cells, a blastula. This early embryonic form undergoes gastrulation, forming a gastrula with either two or three layers, in all vertebrates, these progenitor cells differentiate into all adult tissues and organs. In humans, after three days, the zygote forms a solid mass of cells by mitotic division, called a morula. This then changes to a blastocyst, consisting of a layer called a trophoblast. Filled with uterine fluid, the blastocyst breaks out of the zona pellucida, the inner cell mass initially has two layers, the hypoblast and epiblast. At the end of the week, a primitive streak appears. The top layer is now called the ectoderm, the endoderm is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the layer of the gastrula. The endoderm consists at first of flattened cells, which subsequently become columnar and it forms the epithelial lining of the whole of the digestive tube except part of the mouth and pharynx and the terminal part of the rectum. The endoderm forms, the stomach, the colon, the liver, the pancreas, the bladder, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid. The mesoderm germ layer forms in the embryos of triploblastic animals, during gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm
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Vertebrate
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Vertebrates /ˈvɜːrtᵻbrᵻts/ comprise all species of animals within the subphylum Vertebrata /-ɑː/. Vertebrates represent the majority of the phylum Chordata, with currently about 66,000 species described. Vertebrates include the fish and the jawed vertebrates, which include the cartilaginous fish. A bony fish known as the lobe-finned fishes is included with tetrapods, which are further divided into amphibians, reptiles, mammals. Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as 7.7 mm, to the blue whale, vertebrates make up less than five percent of all described animal species, the rest are invertebrates, which lack vertebral columns. The vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution, though their closest living relatives, hagfish do, however, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as Craniata when discussing morphology, molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys, and so also are vertebrates in a monophyletic sense. Others consider them a group of vertebrates in the common taxon of craniata. The word origin of vertebrate derives from the Latin word vertebratus, the Proto-Indo-European language origins are still unclear. Vertebrate is derived from the vertebra, which refers to any of the bones or segments of the spinal column. All vertebrates are built along the basic body plan, a stiff rod running through the length of the animal, with a hollow tube of nervous tissue above it. In all vertebrates, the mouth is found at, or right below, the remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut. However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeon, jawed vertebrates are typified by paired appendages, but this trait is not required in order for an animal to be a vertebrate. All basal vertebrates breathe with gills, the gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. Each gill is supported by a cartilagenous or bony gill arch, the bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches than this, as some of their relatives have more than 50 pairs of gills. In amphibians and some primitive fishes, the larvae bear external gills. These are reduced in adulthood, their function taken over by the gills proper in fishes, some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods
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Neural crest
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After gastrulation, neural crest cells are specified at the border of the neural plate and the non-neural ectoderm. During neurulation, the borders of the plate, also known as the neural folds. The emergence of neural crest was important in vertebrate evolution because many of its derivatives are defining features of the vertebrate clade. Understanding the molecular mechanisms of neural crest formation is important for our knowledge of disease because of its contributions to multiple cell lineages. Abnormalities in neural crest development cause neurocristopathies, which include such as frontonasal dysplasia, Waardenburg-Shah syndrome. Therefore, defining the mechanisms of neural crest development may reveal key insights into vertebrate evolution, neural crest was first described in the chick embryo by Wilhelm His in 1868 as the cord in between because of its origin between the neural plate and non-neural ectoderm. He named the tissue ganglionic crest since its final destination was each lateral side of the tube where it differentiated into spinal ganglia. During the first half of the 20th century the majority of research on neural crest was done using amphibian embryos which was reviewed by Hörstadius in a well known monograph. Cell labeling techniques advanced the field of neural crest because they allowed researchers to visualize the migration of the tissue throughout the developing embryos, in the 1960s Weston and Chibon utilized radioisotopic labeling of the nucleus with tritiated thymidine in chick and amphibian embryo respectively. However, this suffers from drawbacks of stability, since every time the labeled cell divides the signal is diluted. Modern cell labeling techniques such as rhodamine-lysinated dextran and the vital dye diI have also developed to transiently mark neural crest lineages. The quail-chick marking system, devised by Nicole Le Douarin in 1969, was another technique used to track neural crest cells. Chimeras, generated through transplantation, enabled researchers to distinguish neural crest cells of one species from the tissue of another species. With this technique, generations of scientists were able to reliably mark, a molecular cascade of events is involved in establishing the migratory and multipotent characteristics of neural crest cells. This gene-regulatory network can be subdivided into the following four sub-networks described below, Wnt signaling has been demonstrated in neural crest induction in several species through gain-of-function and loss-of-function experiments. The current role of BMP in neural crest formation is associated with the induction of the neural plate, BMP antagonists diffusing from the ectoderm generates a gradient of BMP activity. In this manner, the neural crest lineage forms from intermediate levels of BMP signaling required for the development of the neural plate, Fgf from the paraxial mesoderm has been suggested as a source of neural crest inductive signal. Researchers have demonstrated that the expression of dominate-negative Fgf receptor in ectoderm explants blocks neural crest induction when recombined with paraxial mesoderm and our current understanding of the role of BMP, Wnt, and Fgf pathways on neural crest specifier expression remains incomplete
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Natural competence
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In microbiology, genetics, cell biology, and molecular biology, competence is the ability of a cell to take up extracellular DNA from its environment in the process called transformation. This article primarily deals with natural competence in bacteria, although information about artificial competence is also provided, in 1944 Oswald Avery, Colin MacLeod, and Maclyn McCarty demonstrated that this transforming factor was pure DNA. This was the first compelling evidence that DNA carries the information of the cell. Since then, natural competence has been studied in a number of different bacteria, particularly Bacillus subtilis, Streptococcus pneumoniae, Neisseria gonorrhoeae, areas of active research include the mechanisms of DNA transport, the regulation of competence in different bacteria, and the evolutionary function of competence. In the natural world DNA usually becomes available by death and lysis of other cells, during uptake, DNA is transported across the cell membrane, and the cell wall if one is present. Once the DNA is inside the cell it may be degraded to nucleotides, alternatively it may be recombined into the cell’s genome by its DNA repair enzymes. If this recombination changes the cell’s genotype the cell is said to have been transformed, artificial competence and transformation are used as research tools in many organisms. In almost all naturally competent bacteria components of extracellular filaments called type 4 pili bind extracellular double stranded DNA, the DNA is then translocated across the membrane through multi-component protein complexes driven by the degradation of one strand of the DNA. Single stranded DNA in the cell is bound by a protein, DprA, which loads the DNA onto RecA. In laboratory cultures, natural competence is usually tightly regulated and often triggered by nutritional shortages or adverse conditions, transcription factors have been discovered which regulate competence, an example is sxy which has been found to be regulated in turn by a 5 non-coding RNA element. In bacteria capable of forming spores, conditions inducing sporulation often overlap with those inducing competence, thus cultures or colonies containing sporulating cells often also contain competent cells. Recent research by Süel et al. has identified a core module of genes which can explain entry into. Neisserial genomes contain thousands of copies of the preferred sequence GCCGTCTGAA, a secondary suggestion has also been made, noting the occasional advantage of horizontal gene transfer. Arguments to support genetic diversity as the evolutionary function of sex are given by Barton and Charleworth. and by Otto. However, the difficulties associated with the evolution of sex suggest that sex for genetic diversity is problematic. However, since bacteria tend to grow in clones, the DNA available for transformation would generally have the same genotype as that of the recipient cells, thus, there is always a high cost in protein expression without, in general, an increase in diversity. Some naturally competent bacteria also secrete nucleases into their surroundings, the energetics of DNA uptake are not understood in any system, so it is difficult to compare the efficiency of nuclease secretion to that of DNA uptake and internal degradation. In principle the cost of production and the uncertainty of nucleotide recovery must be balanced against the energy needed to synthesize the uptake machinery
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Neurogenesis
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Neurogenesis is the process by which neurons are generated from neural stem cells and progenitor cells. Through precise genetic mechanisms of cell fate determination, many different varieties of excitatory and inhibitory neurons are generated from different kinds of stem cells. Neurogenesis occurs during embryogenesis in all animals and is responsible for producing all the neurons of the organism, prior to the period of neurogenesis, neural stem cells first multiply until the correct number of progenitor cells is achieved. The process of neurogenesis then involves a cell division of the parent neural stem cell. The molecular and genetic factors influencing neurogenesis notably include the Notch pathway, all neurons are thus post-mitotic, and most neurons of the human central nervous system live the lifetime of the individual. In mammals, adult neurogenesis has been shown to occur in three places of the brain, the dentate gyrus of the hippocampus, subventricular zone. In some vertebrates, regenerative neurogenesis has also shown to occur. Likewise, many antidepressants have been shown to increase the rate of neurogenesis within the hippocampus
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Brain
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The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head, usually close to the organs for senses such as vision. The brain is the most complex organ in a vertebrates body, in a human, the cerebral cortex contains approximately 15–33 billion neurons, each connected by synapses to several thousand other neurons. Physiologically, the function of the brain is to exert centralized control over the other organs of the body, the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment, the operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. This article compares the properties of brains across the range of animal species. It deals with the human brain insofar as it shares the properties of other brains, the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because more can be said about them in a human context. The most important is brain disease and the effects of brain damage, the shape and size of the brain varies greatly between species, and identifying common features is often difficult. Nevertheless, there are a number of principles of architecture that apply across a wide range of species. Some aspects of structure are common to almost the entire range of animal species, others distinguish advanced brains from more primitive ones. The simplest way to gain information about brain anatomy is by visual inspection, Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, and then sliced apart for examination of the interior. Visually, the interior of the consists of areas of so-called grey matter, with a dark color, separated by areas of white matter. Further information can be gained by staining slices of tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations. It is also possible to examine the microstructure of brain tissue using a microscope, the brains of all species are composed primarily of two broad classes of cells, neurons and glial cells. Glial cells come in types, and perform a number of critical functions, including structural support, metabolic support, insulation. Neurons, however, are considered the most important cells in the brain. The property that makes neurons unique is their ability to send signals to target cells over long distances
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Spinal cord
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The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. The brain and spinal cord make up the central nervous system. The spinal cord begins at the bone and extends down to the space between the first and second lumbar vertebrae, it does not extend the entire length of the vertebral column. It is around 45 cm in men and around 43 cm long in women, also, the spinal cord has a varying width, ranging from 13 mm thick in the cervical and lumbar regions to 6.4 mm thick in the thoracic area. The enclosing bony vertebral column protects the relatively shorter spinal cord, the spinal cord is the main pathway for information connecting the brain and peripheral nervous system. The length of the cord is much shorter as compared to the length of the vertebral column. The human spinal cord extends from the magnum and continues through to the conus medullaris near the second lumbar vertebra. It is about 45 cm long in men and around 43 cm in women, ovoid-shaped, the cervical enlargement, located from C5 to T1 spinal segments, is where sensory input comes from and motor output goes to the arms. The lumbar enlargement, located between L1 and S3 spinal segments, handles input and motor output coming from and going to the legs. The spinal cord is continuous with the portion of the medulla. It does not run the length of the vertebral column in adults. It is made of 31 segments that contain sensory nerve root. The first cervical segment is very small and may be absent in most individuals. Nerve roots merge into 31 bilaterally symmetric pairs of spinal nerves, the peripheral nervous system is made up of these spinal roots, nerves, and ganglia. Dorsal root ganglia hold the bodies of dorsal root ganglia in the spinal ganglia related with that spinal segment. Ventral roots only have efferent fibers that arise from motor neurons whose cell bodies are found in the ventral horns of the spinal cord. The spinal cord are protected by three layers of tissue or membranes called meninges, that surround the canal, the dura mater is the outermost layer, and it forms a tough protective coating. Between the dura mater and the bone of the vertebrae is a space called the epidural space
. PMID 22639618.
