Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids; the two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. Each nucleotide is composed of one of four nitrogen-containing nucleobases, a sugar called deoxyribose, 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 separate polynucleotide strands are bound together, according to base pairing rules, with hydrogen bonds to make double-stranded DNA; the complementary nitrogenous bases are divided into two groups and purines. In DNA, the pyrimidines are cytosine. Both strands of double-stranded DNA store the same biological information.
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 are thus antiparallel. Attached to each sugar is one of four types of nucleobases, it is the sequence of these four nucleobases along the backbone. RNA strands are created using DNA strands as a template in a process called transcription. Under the genetic code, these RNA strands specify the sequence of amino acids within proteins in a process called translation. Within eukaryotic cells, DNA is organized into long structures called chromosomes. Before typical cell division, these chromosomes are duplicated in the process of DNA replication, providing a complete set of chromosomes for each daughter cell. Eukaryotic organisms store most of their DNA inside the cell nucleus as nuclear DNA, some in the mitochondria as mitochondrial DNA, or in chloroplasts as chloroplast DNA. In contrast, prokaryotes store their DNA only in circular chromosomes.
Within eukaryotic chromosomes, chromatin proteins, such as histones and organize DNA. These compacting 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, its molecular structure was first identified by Francis Crick and James Watson at the Cavendish Laboratory within the University of Cambridge in 1953, whose model-building efforts were guided by X-ray diffraction data acquired by Raymond Gosling, a post-graduate student of Rosalind Franklin. DNA is used by researchers as a molecular tool to explore physical laws and theories, such as the ergodic theorem and the theory of elasticity; the unique material properties of DNA have made it an attractive molecule for material scientists and engineers interested in micro- and nano-fabrication. Among notable advances in this field are DNA origami and DNA-based hybrid materials. DNA is a long polymer made from repeating units called nucleotides.
The structure of DNA is dynamic along its length, being capable of coiling into tight loops and other shapes. In all species it is composed of two helical chains, bound to each other by hydrogen bonds. Both chains are coiled around the same axis, have the same pitch of 34 angstroms; the pair of chains has a radius of 10 angstroms. According to another study, when measured in a different solution, the DNA chain measured 22 to 26 angstroms wide, one nucleotide unit measured 3.3 Å long. Although each individual nucleotide is small, a DNA polymer can be large and contain hundreds of millions, such as in chromosome 1. Chromosome 1 is the largest human chromosome with 220 million base pairs, would be 85 mm long if straightened. DNA does not exist as a single strand, but instead as a pair of strands that are held together; these two long strands coil in the shape of a double helix. The nucleotide contains both a segment of the backbone of a nucleobase. A nucleobase linked to a sugar is called a nucleoside, a base linked to a sugar and to one or more phosphate groups is called a nucleotide.
A biopolymer comprising multiple linked nucleotides is called a polynucleotide. The backbone of the DNA strand is made from alternating sugar residues; the sugar in DNA is 2-deoxyribose, a pentose sugar. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings; these are known as the 3′-end, 5′-end carbons, the prime symbol being used to distinguish these carbon atoms from those of the base to which the deoxyribose forms a glycosidic bond. When imagining DNA, each phosphoryl is considered to "belong" to the nucleotide whose 5′ carbon forms a bond therewith. Any DNA strand therefore has one end at which there is a phosphoryl attached to the 5′ carbon of a ribose and another end a
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. A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds the blastocoele. 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, the trophoblast, which goes on to form the extra-embryonic tissues. During the blastula stage of development, a significant amount of activity occurs within the early embryo to establish cell polarity, cell specification, axis formation, regulate gene expression.
In many animals such as Drosophila and Xenopus, the mid blastula transition is a crucial step in development during which the maternal mRNA is degraded and control over development is passed to the embryo. Many of the interactions between blastomeres are dependent on cadherin expression 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 continued improvement of fertility treatments. Embryonic stem cells are a field which, though controversial, have tremendous potential for treating disease. In Xenopus, blastomeres behave as pluripotent stem cells which can migrate down several pathways, depending on cell signaling. 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 injury cases. In vitro fertilisation involves implantation of a blastula into a mother's uterus.
Blastula cell implantation could serve to eliminate infertility. The blastula stage of early embryo development begins with the appearance of the blastocoel; the origin of the blastocoele in Xenopus has been shown to be from the first cleavage furrow, widened and sealed with tight junctions to create a cavity. In many organisms the development of the embryo up to this point and for the early part of the blastula stage is controlled by maternal mRNA, so called because it was produced in the egg prior to fertilization and is therefore from the mother. In many organisms including Xenopus and Drosophila, the mid-blastula transition occurs after a particular number of cell divisions for a given species, is defined by the ending of the synchronous cell division cycles of the early blastula development, the lengthening of the cell cycles by the addition of the G1 and G2 phases. Prior to this transition, cleavage occurs with only the synthesis and mitosis phases of the cell cycle; the addition of the two growth phases into the cell cycle allows for the cells to increase in size, as up to this point the blastomeres undergo reductive divisions in which the overall size of the embryo does not increase, but more cells are created.
This transition begins the growth in size of the organism. The mid-blastula transition is characterized 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, either by proteins such as SMAUG in Drosophila or by microRNA; 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, growth factors, sugars and other components which are necessary for cellular differentiation. The blastocoele 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 to form ectodermal derivatives. The equatorial or marginal zone, which compose the walls of the blastocoel differentiate into mesodermal tissue.
The vegetal mass is composed of the blastocoel floor and develops into endodermal tissue. In the mammalian blastocyst there are three lineages that give rise to tissue development; the epiblast gives rise to the fetus itself while the trophoblast develops into part of the placenta and the primitive endoderm becomes the yolk sac. In mouse embryo, blastocoele formation begins at the 32-cell stage. During this process, water enters the embryo, aided by an osmotic gradient, the result of Na+/K+ ATPases that produce a high Na+ gradient on the basolateral side of the trophectoderm; this movement of water is facilitated by aquaporins. A seal is created by tight junctions of the epithelial cells. Tight junctions are important in embryo development. In the blastula, these cadherin mediated cell interactions are essential to development of epithelium which are most important to paracellular transport, maintenance of cell polarity and the creation of a permeability seal to regulate blastocoel formation; these tight junctions arise after the polarity of epithelial cells is established which sets the foundation for further development and specification.
Within the blastula, inner blastomeres are non-polar while epithelial cells demonstrate polarity. Mammalian embryos undergo compaction around the 8-cell stage where E-cadherins as well
Cell potency is a cell's ability to differentiate into other cell types. The more cell types. Potency is described as the gene activation potential within a cell, which like a continuum, begins with totipotency to designate a cell with the most differentiation potential, multipotency and unipotency. Totipotency is the ability of a single cell to divide and produce all of the differentiated cells in an organism. Spores and zygotes are examples of totipotent cells. In the spectrum of cell potency, totipotency represents the cell with the greatest differentiation potential, being able to differentiate into any embryonic cell, as well as extraembryonic cells. In contrast, pluripotent cells can only differentiate into embryonic cells, it is possible for a differentiated cell to return to a state of totipotency. This conversion to totipotency is complex, not understood and the subject of recent research. Research in 2011 has shown that cells may differentiate not into a totipotent cell, but instead into a "complex cellular variation" of totipotency.
Stem cells resembling totipotent blastomeres from 2-cell stage embryos can arise spontaneously in mouse embryonic stem cell cultures and can be induced to arise more in vitro through down-regulation of the chromatin assembly activity of CAF-1. The human development model is one. Human development begins when a sperm fertilizes an egg and the resulting fertilized egg creates a single totipotent cell, a zygote. In the first hours after fertilization, this zygote divides into identical totipotent cells, which can develop into any of the three germ layers of a human, or into cells of the placenta. After reaching a 16-cell stage, the totipotent cells of the morula differentiate into cells that will become either the blastocyst's Inner cell mass or the outer trophoblasts. Four days after fertilization and after several cycles of cell division, these totipotent cells begin to specialize; the inner cell mass, the source of embryonic stem cells, becomes pluripotent. Research on Caenorhabditis elegans suggests that multiple mechanisms including RNA regulation may play a role in maintaining totipotency at different stages of development in some species.
Work with zebrafish and mammals suggest a further interplay between miRNA and RNA-binding proteins in determining development differences. In mouse primordial germ cells, genome-wide reprogramming leading to totipotency involves erasure of epigenetic imprints. Reprogramming is facilitated by active DNA demethylation involving the DNA base excision repair enzymatic pathway; this pathway entails erasure of CpG methylation in primordial germ cells via the initial conversion of 5mC to 5-hydroxymethylcytosine, a reaction driven by high levels of the ten-eleven dioxygenase enzymes TET-1 and TET-2. In cell biology, pluripotency refers to a stem cell that has the potential to differentiate into any of the three germ layers: endoderm, mesoderm, or ectoderm. However, cell pluripotency is a continuum, ranging from the pluripotent cell that can form every cell of the embryo proper, e.g. embryonic stem cells and iPSCs, to the incompletely or pluripotent cell that can form cells of all three germ layers but that may not exhibit all the characteristics of pluripotent cells.
Induced pluripotent stem cells abbreviated as iPS cells or iPSCs, are a type of pluripotent stem cell artificially derived from a non-pluripotent cell an adult somatic cell, by inducing a "forced" expression of certain genes and transcription factors. These transcription factors play a key role in determining the state of these cells and highlights the fact that these somatic cells do preserve the same genetic information as early embryonic cells; the ability to induce cells into a pluripotent state was pioneered in 2006 using mouse fibroblasts and four transcription factors, Oct4, Sox2, Klf4 and c-Myc. This was followed in 2007 by the successful induction of human iPSCs derived from human dermal fibroblasts using methods similar to those used for the induction of mouse cells; these induced cells exhibit similar traits to those of embryonic stem cells but do not require the use of embryos. Some of the similarities between ESCs and iPSCs include pluripotency, self-renewal ability, a trait that implies that they can divide and replicate indefinitely, gene expression.
Epigenetic factors are thought to be involved in the actual reprogramming of somatic cells in order to induce pluripotency. It has been theorized that certain epigenetic factors might work to clear the original somatic epigenetic marks in order to acquire the new epigenetic marks that are part of achieving a pluripotent state. Chromatin is reorganized in iPSCs and becomes like that found in ESCs in that it is less condensed and therefore more accessible. Euchromatin modifications are common, consistent with the state of euchromatin found in ESCs. Due to their great similarity to ESCs, iPSCs have been of great interest to the medical and research community. IPSCs could have the same therapeutic implications and applications as ESCs but without the controversial use of embryos
The placenta is a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, gas exchange via the mother's blood supply. Placentas are a defining characteristic of placental mammals, but are found in marsupials and some non-mammals with varying levels of development; the placenta functions as a fetomaternal organ with two components: the fetal placenta, which develops from the same blastocyst that forms the fetus, the maternal placenta, which develops from the maternal uterine tissue. It metabolizes a number of substances and can release metabolic products into maternal or fetal circulations; the placenta is expelled from the body upon birth of the fetus. The word placenta comes from the Latin word for a type of cake, from Greek πλακόεντα/πλακοῦντα plakóenta/plakoúnta, accusative of πλακόεις/πλακούς plakóeis/plakoús, "flat, slab-like", in reference to its round, flat appearance in humans; the classical plural is placentae, but the form placentas is common in modern English and has the wider currency at present.
Placental mammals, such as humans, have a chorioallantoic placenta that forms from the chorion and allantois. In humans, the placenta averages 22 cm in length and 2–2.5 cm in thickness, with the center being the thickest, the edges being the thinnest. It weighs 500 grams, it has crimson color. It connects to the fetus by an umbilical cord of 55–60 cm in length, which contains two umbilical arteries and one umbilical vein; the umbilical cord inserts into the chorionic plate. Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells; this results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta has a disc shape, but size varies vastly between different mammalian species; the placenta takes a form in which it comprises several distinct parts connected by blood vessels. The parts, called lobes, may number two, four, or more.
Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, so on. If there is a discernible main lobe and auxiliary lobe, the latter is called a succenturiate placenta. Sometimes the blood vessels connecting the lobes get in the way of fetal presentation during labor, called vasa previa. About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta; some 350 of these genes are more expressed in the placenta and fewer than 100 genes are placenta specific. The corresponding specific proteins are expressed in trophoblasts and have functions related to female pregnancy. Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and the cancer testis antigen PAGE4 expressed in cytotrophoblasts, CSH1and KISS1 expressed in syncytiotrophoblasts, PAPPA2 and PRG2 expressed in extravillous trophoblasts; the placenta begins to develop upon implantation of the blastocyst into the maternal endometrium.
The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer; the syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development; the syncytiotrophoblast, thereby contributes to the barrier function of the placenta. The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14. In preparation for implantation of the blastocyst, the endometrium undergoes decidualization. Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter is increased; the increased diameter and straighter flow path both act to increase maternal blood flow to the placenta.
There is high pressure as the maternal blood fills intervillous space through these spiral arteries which bathe the fetal villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetal chorion, though no fluid is exchanged; as the pressure decreases between pulses, the deoxygenated blood flows back through the endometrial veins. Maternal blood flow is 600–700 ml/min at term; this begins at day 5 - day 12 Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries. Chorionic arteries, in turn, branch into cotyledon arteries. In the villi, these vessels branch to form an extensive arterio-capillary-venous system, bringing the fetal blood close to the maternal blood. Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation.
On the other hand, there is no neural vascular regulation, catecholamines have only little effect. The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and
Polyploidy is the state of a cell or organism having more than two paired sets of chromosomes. Most species whose cells have nuclei are diploid, meaning they have two sets of chromosomes—one set inherited from each parent. However, polyploidy is found in some organisms and is common in plants. In addition, polyploidy occurs in some tissues of animals that are otherwise diploid, such as human muscle tissues; this is known as endopolyploidy. Species whose cells do not have nuclei, that is, may be polyploid, as seen in the large bacterium Epulopiscium fishelsoni. Hence ploidy is defined with respect to a cell. Most eukaryotes produce haploid gametes by meiosis. A monoploid has only one set of chromosomes, the term is only applied to cells or organisms that are diploid. Males of bees and other Hymenoptera, for example, are monoploid. Unlike animals and multicellular algae have life cycles with two alternating multicellular generations; the gametophyte generation is haploid, produces gametes by mitosis, the sporophyte generation is diploid and produces spores by meiosis.
Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or over-represented are said to be aneuploid. Aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes. Polyploidy may occur due to abnormal cell division, either during mitosis, or during metaphase I in meiosis. In addition, it can be induced in plants and cell cultures by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well. Oryzalin will double the existing chromosome content. Polyploidy occurs in differentiated human tissues in the liver, heart muscle, bone marrow and the placenta, it occurs in the somatic cells of some animals, such as goldfish and salamanders, but is common among ferns and flowering plants, including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid, tetraploid with the common name of durum or macaroni wheat, hexaploid with the common name of bread wheat.
Many agriculturally important plants of the genus Brassica are tetraploids. Polyploidization is a mechanism of sympatric speciation because polyploids are unable to interbreed with their diploid ancestors. An example is the plant Erythranthe peregrina. Sequencing confirmed that this species originated from E. × robertsii, a sterile triploid hybrid between E. guttata and E. lutea, both of which have been introduced and naturalised in the United Kingdom. New populations of E. peregrina arose on the Scottish mainland and the Orkney Islands via genome duplication from local populations of E. × robertsii. Because of a rare genetic mutation, E. peregrina is not sterile. Polyploid types are labeled according to the number of chromosome sets in the nucleus; the letter x is used to represent the number of chromosomes in a single set. Triploid, for example sterile saffron crocus, or seedless watermelons common in the phylum Tardigrada tetraploid, for example Salmonidae fish, the cotton Gossypium hirsutum pentaploid, for example Kenai Birch hexaploid, for example wheat, kiwifruit heptaploid or septaploid octaploid or octoploid, for example Acipenser, dahlias decaploid, for example certain strawberries dodecaploid, for example the plants Celosia argentea and Spartina anglica or the amphibian Xenopus ruwenzoriensis.
Examples in animals are more common in non-vertebrates such as flatworms and brine shrimp. Within vertebrates, examples of stable polyploidy include many cyprinids; some fish have as many as 400 chromosomes. Polyploidy occurs in amphibians. Polyploid lizards are quite common, but are sterile and must reproduce by parthenogenesis. Polyploid mole salamanders are all female and reproduce by kleptogenesis, "stealing" spermatophores from diploid males of related species to trigger egg development but not incorporating the males' DNA into the offspring. While mammalian liver cells are polyploid, rare instances of polyploid mammals are known, but most result in prenatal death. An octodontid rodent of Argentina's harsh desert regions, known as the plains viscacha rat has been reported as an exception to this'rule'. However, careful analysis using chromosome paints shows that there are only two copies of each chromosome in T. barrerae, not the four expected if it were a tetraploid. This rodent kin to guinea pigs and chinchillas.
Its "new" diploid number is 102 and so its cells are twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the same family, whose 2n = 56, it was therefore surmised that an Octomys-like ancestor produced tetraploid offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents. Polyploidy was induced in fish by Har Swarup using a cold-shock treatment of the eggs close to the time o
Anatomical terminology is a form of scientific terminology used by anatomists and health professionals such as doctors. Anatomical terminology uses many unique terms and prefixes deriving from Ancient Greek and Latin; these terms can be confusing to those unfamiliar with them, but can be more precise, reducing ambiguity and errors. Since these anatomical terms are not used in everyday conversation, their meanings are less to change, less to be misinterpreted. To illustrate how inexact day-to-day language can be: a scar "above the wrist" could be located on the forearm two or three inches away from the hand or at the base of the hand. 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 same prefixes and suffixes are used to add meanings to different roots; the root of a term refers to an organ or tissue. For example, the Latin names of structures such as musculus biceps brachii can be split up and refer to, musculus for muscle, biceps for "two-headed", brachii as in the brachial region of the arm.
The first word describes what is being spoken about, the second describes it, the third points to location. 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, such as the midclavicular line from the centre of the clavicle. The cephalon or cephalic region refers to the head; this area is further differentiated into the cranium, frons, auris, nasus and mentum. The neck area is called 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, is used when describing the skull.
Different terminology is used in the arms, in part to reduce ambiguity as to what the "front", "back", "inner" and "outer" surfaces are. For this reason, the terms below are used: Radial referring to the radius bone, seen laterally in the standard anatomical position. 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, other structures such as the eye. International morphological terminology is used by the colleges of medicine and dentistry and other areas of the health sciences, it facilitates communication and exchanges between scientists from different countries of the world and it is used daily in the fields of research 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, it is functional.
It covers the gross anatomy and the microscopic of living beings. It involves the anatomy of the adult, it includes comparative anatomy between different species. The vocabulary is extensive and complex, requires a systematic presentation. Within the international field, a group of experts reviews and discusses the morphological terms of the structures of the human body, forming today's Terminology Committee from the International Federation of Associations of Anatomists, it deals with the anatomical and embryologic terminology. In the Latin American field, there are meetings called Iberian Latin American Symposium Terminology, where a group of experts of the Pan American Association of Anatomy that speak Spanish and Portuguese and studies the international morphological terminology; the current international standard for human anatomical terminology is based on the Terminologia Anatomica. It was developed by the Federative Committee on Anatomical Terminology and the International Federation of Associations of Anatomists and was released in 1998.
It supersedes Nomina Anatomica. Terminologia Anatomica contains terminology for about 7500 human gross anatomical structures. For microanatomy, known as histology, a similar standard exists in Terminologia Histologica, for embryology, the study of development, a standard exists in Terminologia Embryologica; these standards specify accepted names that can be used to refer to histological and embryological structures in journal articles and other areas. As of September 2016, two sections of the Terminologia Anatomica, including central nervous system and peripheral nervous system, were merged to form the Terminologia Neuroanatomica; the Terminologia Anatomica has been perceived with a considerable criticism regarding its content including coverage and spelling mistakes and errors. Anatomical terminology is chosen to highlight the relative location of body structures. For instance, an anatomist might describe one band of tissue as "inferior to" another or a physician might describe a tumor as "superficial to" a deeper body structure.
Anatomical terms used to describe location
Fertilisation or fertilization known as generative fertilisation, pollination, fecundation and impregnation, is the fusion of gametes to initiate the development of a new individual organism or offspring. This cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation. In Antiquity, Aristotle conceived the formation of new individuals through fusion of male and female fluids, with form and function emerging in a mode called by him as epigenetic. In 1784, Spallanzani established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs. In 1827, von Baer observed a therian mammalian egg for the first time. Oscar Hertwig, in Germany, described the fusion of ova from sea urchin; the evolution of fertilisation is related to the origin of meiosis, as both are part of sexual reproduction, originated in eukaryotes.
There are two conflicting theories on how the couple meiosis–fertilisation arose. One is; the other is. The gametes that participate in fertilisation of plants are the pollen, the egg cell. Various families of plants have differing methods. In Bryophyte land plants, fertilisation takes place within the archegonium. In flowering plants a second fertilisation event involves another sperm cell and the central cell, a second female gamete. In flowering plants there are two sperm from each pollen grain. In seed plants, after pollination, a pollen grain germinates, 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 to the ovule. Pollen tube growth Unlike animal sperm, motile, plant sperm is immotile and relies on the pollen tube to carry it to the ovule where the sperm is released; the pollen tube penetrates the stigma and elongates through the extracellular matrix of the style before reaching the ovary.
Near the receptacle, it breaks through the ovule through the micropyle and the pollen tube "bursts" into the embryo sac, releasing sperm. The growth of the pollen tube has been believed to depend on chemical cues from the pistil, however these mechanisms were poorly understood until 1995. Work done on tobacco plants revealed a family of glycoproteins called TTS proteins that enhanced growth of pollen tubes. Pollen tubes in a sugar free pollen germination medium and a medium with purified TTS proteins both grew. However, in the TTS medium, the tubes grew at a rate 3x that of the sugar-free medium. TTS proteins were placed on various locations of semi in vevo pollinated pistils, pollen tubes were observed to extend toward the proteins. Transgenic plants lacking the ability to produce TTS proteins exhibited slower pollen tube growth and reduced fertility. Rupture of pollen tube The rupture of the pollen tube to release sperm in Arabidopsis has been shown to depend on a signal from the female gametophyte.
Specific proteins called FER protein kinases present in the ovule control the production of reactive derivatives of oxygen called reactive oxygen species. ROS levels have been shown via GFP to be at their highest during floral stages when the ovule is the most receptive to pollen tubes, lowest during times of development and following fertilization. High amounts of ROS activate Calcium ion channels in the pollen tube, causing these channels to take up Calcium ions in large amounts; this increased uptake of calcium causes the pollen tube to rupture, release its sperm into the ovule. Pistil feeding assays in which plants were fed diphenyl iodonium chloride suppressed ROS concentrations in Arabidopsis, which in turn prevented pollen tube rupture. 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. A fern is a member of a group of 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers.
They differ from mosses by being vascular. They leaves, like other vascular plants. Most ferns have what are called fiddleheads that expand into fronds, which are each delicately divided; the gymnosperms are a group of seed producing plants that includes conifers, Cycads and Gnetales. The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος, meaning "naked seeds", after the unenclosed condition of their seeds, 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 modified to form cones, or at the end of short stalks as in Ginkgo. After being fertilised, the ovary starts to 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. Hydrolytic enzymes are secreted by the pollen tube that digest the female tissue as the tube grows down the stigma and style