Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of'position', ensuring that the correct structures form in the correct places of the body. For example, Hox genes in insects specify which appendages form on a segment, Hox genes in vertebrates specify the types and shape of vertebrae that will form. In segmented animals, Hox proteins thus confer segmental or positional identity, but do not form the actual segments themselves. An analogy for the Hox genes can be made to the role of a play director that calls which scene the actors should carry out next. If the play director calls the scenes in the wrong order, the overall play will be presented in the wrong order. Mutations in the Hox genes can result in body parts and limbs in the wrong place along the body. Like a play director, the Hox genes do not act in the play or participate in limb formation themselves.
The protein product of each Hox gene is a transcription factor. Each Hox gene contains a well-conserved DNA sequence known as the homeobox, of which the term "Hox" was a contraction. However, in current usage the term Hox is no longer equivalent to homeobox, because Hox genes are not the only genes to possess a homeobox sequence: humans have over 200 homeobox genes of which 39 are Hox genes. Hox genes are thus a subset of the homeobox transcription factor genes. In many animals, the organization of the Hox genes in the chromosome is the same as the order of their expression along the anterior-posterior axis of the developing animal, are thus said to display colinearity; the products of Hox genes are Hox proteins. Hox proteins are a subset of transcription factors, which are proteins that are capable of binding to specific nucleotide sequences on DNA called enhancers through which they either activate or repress hundreds of other genes; the same Hox protein can act as a repressor at an activator at another.
The ability of Hox proteins to bind DNA is conferred by a part of the protein referred to as the homeodomain. The homeodomain is a 60-amino-acid-long DNA-binding domain; this amino acid sequence folds into a "helix-turn-helix" motif, stabilized by a third helix. The consensus polypeptide chain is:. Hox proteins act in partnership with co-factors, such as PBC and Meis proteins encoded by different types of homeobox gene. Helix 1 Helix 2 Helix 3/4 ______________ __________ _________________ RRRKRTAYTRYQLLELEKEFLFNRYLTRRRRIELAHSLNLTERHIKIWFQNRRMKWKKEN....|....|....|....|....|....|....|....|....|....|....|....| 10 20 30 40 50 60 Homeobox genes, thus the homeodomain protein motif, are found in most eukaryotes. Hox genes, being a subset of homeobox genes, arose more in evolution within the animal kingdom or Metazoa. Within the animal kingdom, Hox genes are present across the bilateria, have been found in Cnidaria such as sea anemones; this implies. In bilateria, Hox genes are arranged in gene clusters, although there are many exceptions where the genes have been separated by chromosomal rearrangements.
Comparing homeodomain sequences between Hox proteins reveals greater similarity between species than within a species. In most bilaterian animals, Hox genes are expressed in staggered domains along the head-to-tail axis of the embryo, suggesting that their role in specifying position is a shared, ancient feature; the functional conservation of Hox proteins can be demonstrated by the fact that a fly can function to a large degree with a chicken Hox protein in place of its own. So, despite having a last common ancestor that lived over 550 million years ago, the chicken and fly version of the same Hox gene are similar enough to target the same downstream genes in flies. Drosophila melanogaster is an important model for understanding evolution; the general principles of Hox gene function and logic elucidated in flies will apply to all bilaterian organisms, including humans. Drosophila, like all insects, has eight Hox genes; these are clustered into two complexes, both of which are located on chromosome 3.
The Antennapedia complex consists of five genes: labial, deformed, sex combs reduced, Antennapedia. The Bithorax complex, named after the Ultrabithorax gene, consists of the remaining three genes: Ultrabithorax, abdominal-A and abdominal-B; the lab gene is the most anteriorly expressed gene. It is expressed in the head in the intercalary segment, in the midgut. Loss of function of lab results in the failure of the Drosophila embryo to internalize the mouth and head structures that develop on the outside of its body. Failure of head involution deletes the salivary glands and pharynx; the lab gene was so named because it disrupted the labial appendage.
Human embryonic development
Human embryonic development, or human embryogenesis, refers to the development and formation of the human embryo. It is characterised by the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilisation occurs when the sperm cell enters and fuses with an egg cell; the genetic material of the sperm and egg combine to form a single cell called a zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development. Human embryology is the study of this development during the first eight weeks after fertilisation; the normal period of gestation is 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 in a process called cleavage.
A blastocyst is 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, the processes of neurulation and organogenesis follow. In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs; 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 other species among chordates. Fertilization takes place when the spermatozoon has entered the ovum and the two sets of genetic material carried by the gametes fuse together, resulting in the zygote; this takes place in the ampulla of one of the fallopian tubes. The zygote contains the combined genetic material carried by both the male and female gametes which consists of the 23 chromosomes from the nucleus of the ovum and the 23 chromosomes from the nucleus of the sperm.
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. Secondly there is an adhesive compatibility between the egg. With the sperm adhered to the ovum, the third process of acrosomal reaction takes place; the entry of the sperm causes calcium to be released. A parallel reaction takes place in the ovum called the zona reaction; this sees the release of cortical granules that release enzymes which digest sperm receptor proteins, thus preventing polyspermy. The granules fuse with the plasma membrane and modify the zona pellucida in such a way as to prevent further sperm entry; the beginning of the cleavage process is marked when the zygote divides through mitosis into two cells. This mitosis continues and the first two cells divide into four cells into eight cells and so on; each division takes from 12 to 24 hours.
The zygote is large compared to any other cell and undergoes cleavage without any overall increase in size. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases; the dividing cells, called blastomeres, are undifferentiated and aggregated into a sphere enclosed within the membrane of glycoproteins of the ovum. When eight blastomeres have formed they begin to develop gap junctions, enabling them to develop in an integrated way and co-ordinate their response to physiological signals and environmental cues; when the cells number around sixteen the solid sphere of cells within the zona pellucida is referred to as a morula At this stage the cells start to bind together in a process called compaction, cleavage continues as cellular differentiation. Cleavage itself is the first stage in the process of forming the blastocyst. Cells differentiate into an outer layer of an inner cell mass. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable.
They are still enclosed within the zona pellucida. This compaction serves to make the structure watertight, containing the fluid that the cells will secrete; the inner mass of cells differentiate to polarise at one end. They form gap junctions, which facilitate cellular communication; this polarisation leaves a cavity, the blastocoel, creating a structure, now termed the blastocyst. The trophoblasts secrete fluid into the blastocoel; the resulting increase in size of the blastocyst causes it to hatch through the zona pellucida, which disintegrates. The inner cell mass will give rise to the pre-embryo, the amnion, yolk sac and allantois, while the fetal part of the placenta will form from the outer trophoblast layer; the embryo plus its membranes is called the conceptus, by this stage the conceptus has reached the uterus. The zona pellucida disappears and the now exposed cells of the trophoblast allow the blastocyst to attach itself to the endometrium, where it will implant; the formation of the hypoblast and epiblast, which ar
Vertebrates comprise all species of animals within the subphylum Vertebrata. Vertebrates represent the overwhelming majority of the phylum Chordata, with about 69,276 species described. Vertebrates include the jawless fishes and jawed vertebrates, which include the cartilaginous fishes and the bony fishes; the bony fishes in turn, cladistically speaking include the tetrapods, which include amphibians, reptiles and mammals. Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as 7.7 mm, to the blue whale, at up to 33 m. Vertebrates make up less than five percent of all described animal species; the vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution, though their closest living relatives, the lampreys, do. Hagfish do, 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 related to lampreys, so are vertebrates in a monophyletic sense.
Others consider them a sister group of vertebrates in the common taxon of craniata. The word vertebrate derives from the Latin word vertebratus. Vertebrate is derived from the word vertebra, which refers to any of the bones or segments of the spinal column. All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal, with a hollow tube of nervous tissue above it and the gastrointestinal tract below. In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the anus opens to the exterior before the end of the body; the remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut. The defining characteristic of a vertebrate is the vertebral column, in which the notochord found in all chordates has been replaced by a segmented series of stiffer elements separated by mobile joints. However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeon and coelacanth.
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 chordate relatives have more than 50 pairs of gills. In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches; these are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish being irrevocably lost early in the evolution of tetrapods.
While the more derived vertebrates lack gills, the gill arches form during fetal development, form the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella and, in mammals, the malleus and incus. The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of neural crest cells; these are progenitors of stem cells, critical to coordinating the functions of cellular components. Neural crest cells migrate through the body from the nerve cord during development, initiate the formation of neural ganglia and structures such as the jaws and skull; the vertebrates are the only chordate group to exhibit cephalisation, the concentration of brain functions in the head. A slight swelling of the anterior end of the nerve cord is found in the lancelet, a chordate, though it lacks the eyes and other complex sense organs comparable to those of vertebrates.
Other chordates do not show any trends towards cephalisation. A peripheral nervous system branches out from the nerve cord to innervate the various systems; the front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon and rhombencephalon, further differentiated in the various vertebrate groups. Two laterally placed eyes form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss; the forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are paired, giving rise to hemispheres like the cerebral hemispheres in mammals; the resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of vesicles, is unique to vertebrates. All invertebrates with well-developed brains, such as insects and squids, have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth or gut.
Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are kept moist; the microscopic structure of a gill presents a large surface area to the external environment. Branchia is the zoologists' name for gills. With the exception of some aquatic insects, the filaments and lamellae contain blood or coelomic fluid, from which gases are exchanged through the thin walls; the blood carries oxygen to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water. Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks, insects and amphibians. Semiterrestrial marine animals such as crabs and mudskippers have gill chambers in which they store water, enabling them to use the dissolved oxygen when they are on land.
Galen observed that fish had multitudes of openings, big enough to admit gases, but too fine to give passage to water. Pliny the Elder held that fish respired by their gills, but observed that Aristotle was of another opinion; the word branchia comes from the Greek βράγχια, "gills", plural of βράγχιον. Many microscopic aquatic animals, some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, so can respire adequately without gills. However, more complex or more active aquatic organisms require a gill or gills. Many invertebrates, amphibians, use both the body surface and gills for gaseous exchange. Gills consist of thin filaments of tissue, branches, or slender, tufted processes that have a folded surface to increase surface area; the delicate nature of the gills is possible. The blood or other body fluid must be in intimate contact with the respiratory surface for ease of diffusion. A high surface area is crucial to the gas exchange of aquatic organisms, as water contains only a small fraction of the dissolved oxygen that air does.
A cubic meter of air contains about 250 grams of oxygen at STP. The concentration of oxygen in water is lower than in air and it diffuses more slowly. In fresh water, the dissolved oxygen content is 8 cm3/L compared to that of air, 210 cm3/L. Water is 100 times more viscous. Oxygen has a diffusion rate in air 10,000 times greater; the use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life. Rather than using lungs, "aseous exchange takes place across the surface of vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism; the density of the water prevents the gills from collapsing and lying on top of each other, what happens when a fish is taken out of water."Usually water is moved across the gills in one direction by the current, by the motion of the animal through the water, by the beating of cilia or other appendages, or by means of a pumping mechanism. In fish and some molluscs, the efficiency of the gills is enhanced by a countercurrent exchange mechanism in which the water passes over the gills in the opposite direction to the flow of blood through them.
This mechanism is efficient and as much as 90% of the dissolved oxygen in the water may be recovered. The gills of vertebrates develop in the walls of the pharynx, along a series of gill slits opening to the exterior. Most species employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other; the gills are composed of comb-like filaments, the gill lamellae, which help increase their surface area for oxygen exchange. When a fish breathes, it draws in a mouthful of water at regular intervals, it draws the sides of its throat together, forcing the water through the gill openings, so it passes over the gills to the outside. Fish gill slits may be the evolutionary ancestors of the tonsils, thymus glands, Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches; the gills of fish form a number of slits connecting the pharynx to the outside of the animal on either side of the fish behind the head.
There were many slits, but during evolution, the number reduced, modern fish have five pairs, never more than eight. Sharks and rays have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six pairs and the Broadnose sevengill shark being the only cartilaginous fish exceeding this number. Adjacent slits are separated by a cartilaginous gill arch from which projects a cartilaginous gill ray; this gill ray is the support for the sheet-like interbranchial septum, which the individual lamellae of the gills lie on either side of. The base of the arch may support gill rakers, projections into the pharyngeal cavity that help to prevent large pieces of debris from damaging the delicate gills. A smaller opening, the spiracle, lies in the back of the first gill slit; this bears a small pseudobranch that resembles a gill in structure, but only receives blood oxygenated by the true gills. The spiracle is thought to be homologous to the ear opening in higher vertebrates.
Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by swimming forward. In slow-moving or bottom-dwelling species among skates and rays, the spiracle may be enlarged, the fish breathes
A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are pronounced; some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as chordates produce a third layer, between these two layers. Making them triploblastic. Germ layers give rise to all of an animal’s tissues and organs through the process of organogenesis. 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. Between 1850 and 1855, Robert Remak had further refined the germ cell layer concept, stating that the external and middle layers form the epidermis, the gut, the intervening musculature and vasculature; the term "mesoderm" was introduced into English by Huxley in 1871, "ectoderm" and "endoderm" by Lankester in 1873. Among animals, sponges show the simplest organization. Although they have differentiated cells, they lack true tissue coordination.
Diploblastic animals 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. Triploblastic animals develop recognizable organs. Fertilization leads to the formation of a zygote. During the next stage, 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 organs. In the human embryo, after about three days, the zygote forms a solid mass of cells by mitotic division, called a morula; this changes to a blastocyst, consisting of an outer layer called a trophoblast, an inner cell mass called the embryoblast. Filled with uterine fluid, the blastocyst breaks out of the zona pellucida and undergoes implantation.
The inner cell mass has two layers: the hypoblast and epiblast. At the end of the second week, a primitive streak appears; the epiblast in this region moves towards the primitive streak, dives down into it, forms a new layer, called the endoderm, pushing the hypoblast out of the way The epiblast keeps moving and forms a second layer, the mesoderm. The top layer is now called the ectoderm; the endoderm is one of the germ layers formed during animal embryonic development. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm; the endoderm consists at first of flattened cells. It forms the epithelial lining of the whole of the digestive tract except part of the mouth and pharynx and the terminal part of the rectum, it forms the lining cells of all the glands which open into the digestive tract, including those of the liver and pancreas. The endoderm forms: the pharynx, the esophagus, the stomach, the small intestine, the colon, the liver, the pancreas, the bladder, the epithelial parts of the trachea and bronchi, the lungs, 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; the formation of a mesoderm leads to the development of a coelom. Organs formed inside a coelom can move and develop independently of the body wall while fluid cushions and protects them from shocks; the mesoderm has several components which develop into tissues: intermediate mesoderm, paraxial mesoderm, lateral plate mesoderm, chorda-mesoderm. The chorda-mesoderm develops into the notochord; the intermediate mesoderm develops into gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, dermis; the lateral plate mesoderm develops into the circulatory system, the wall of the gut, wall of the human body. Through cell signaling cascades and interactions with the ectodermal and endodermal cells, the mesodermal cells begin the process of differentiation; the mesoderm forms: muscle, cartilage, connective tissue, adipose tissue, circulatory system, lymphatic system, genitourinary system, serous membranes, notochord.
The ectoderm generates the outer layer of the embryo, it forms from the embryo's epiblast. The ectoderm develops into the surface ectoderm, neural crest, the neural tube; the surface ectoderm develops into: epidermis, nails, lens of the eye, sebaceous glands, tooth enamel, the epithelium of the mouth and nose. The neural crest of the ectoderm develops into: peripheral nervous system, adrenal medulla, facial cartilage, dentin of teeth; the neural tube of the ectoderm develops into: brain, spinal cord, posterior pituitary, motor neurons, retina. Note: The anterior pituitary develops from the ectodermal tissue of Rathke's pouch; because of its great importance, the neural crest is sometimes considered a fourth germ layer. It is, derived from the ectoderm. Histogenesis
An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell; the zygote possesses half the DNA from each of its two parents. In plants and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism; the result of this process is an embryo. In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh-week gestational age. After this time the embryo is referred to as a fetus. First attested in English in the mid-14c; the word embryon itself from Greek ἔμβρυον, lit. "young one", the neuter of ἔμβρυος, lit. "growing in", from ἐν, "in" and βρύω, "swell, be full". In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula and organogenesis; the blastula stage features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells called blastomeres.
In a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus. An embryo is called a fetus at a more advanced stage of development and up until hatching. In humans, this is from the eleventh week of gestation. However, animals which develop in eggs outside the mother's body, are referred to as embryos throughout development. During gastrulation the cells of the blastula undergo coordinated processes of cell division, and/or migration to form two or three tissue layers. In triploblastic organisms, the three germ layers are called endoderm and mesoderm; the position and arrangement of the germ layers are species-specific, depending on the type of embryo produced. In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer", is thought to have been an important novelty in the evolution of head structures. During organogenesis and cellular interactions between germ layers, combined with the cells' developmental potential, or competence to respond, prompt the further differentiation of organ-specific cell types.
For example, in neurogenesis, a subpopulation of ectoderm cells is set aside to become the brain, spinal cord, peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis, myogenesis and many others. In botany, a seed plant embryo is part of a seed, consisting of precursor tissues for the leaves and root, as well as one or more cotyledons. Once the embryo begins to germinate—grow out from the seed—it is called a seedling. Bryophytes and ferns produce an embryo, but do not produce seeds. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated; the inner wall of the archegonium lies in close contact with the "foot" of the developing embryo. The structure and development of the rest of the embryo varies by group of plants; as the embryo has expanded beyond the enclosing archegonium, it is no longer termed an embryo.
Embryos are used in various fields of research and in techniques of assisted reproductive technology. An egg may be fertilized in vitro and the resulting embryo may be frozen for use; the potential of embryonic stem cell research, reproductive cloning, germline engineering are being explored. Prenatal diagnosis or preimplantation diagnosis enables testing embryos for conditions. Cryoconservation of animal genetic resources is a practice in which animal germplasms, such as embryos are collected and stored at low temperatures with the intent of conserving the genetic material; the embryos of Arabidopsis thaliana have been used as a model to understand gene activation and organogenesis of seed plants. In regards to research using human embryos, the ethics and legalities of this application continue to be debated. Researchers from MERLN Institute and the Hubrecht Institute in the Netherlands managed to grow samples of synthetic rodent embryos, combining certain types of stem cells; this method will help scientists to more study the first moments of the process of the birth of a new life, which, in turn, can lead to the emergence of new effective methods to combat infertility and other genetic diseases.
Fossilized animal embryos are known from the Precambrian, are found in great numbers during the Cambrian period. Fossilized dinosaur embryos have been discovered; some embryos do not survive to the next stage of development. When this happens it is called spontaneous abortion or miscarriage. There are many reasons; the most common natural cause of miscarriage is chromosomal abnormality in animals or genetic load in plants. In species which produce multiple embryos at the same time, miscarriage or abortion of some embryos can provide the remaining embryos with a greater share of maternal resources; this can disturb the pregnancy, causing harm to the second embryo. Genetic strains which miscarry their embryos are the source of commercial seedl
Embryonic development embryogenesis is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe stages. Embryonic development starts with the fertilization of the egg cell by a sperm cell. Once fertilized, the ovum is referred to a single diploid cell; the zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo. Although embryogenesis occurs in both animal and plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals; the egg cell is asymmetric, having an "animal pole" and a "vegetal pole". It is covered 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, which leads to the development of an embryo. 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. To prevent more than one sperm fertilizing the egg, fast block and slow block to polyspermy are used. Fast block, the membrane potential depolarizing and returning to normal, happens after an egg is fertilized by a single sperm. Slow block begins the first few seconds after fertilization and is when the release of calcium causes the cortical reaction, various enzymes releasing from cortical granules in the eggs plasma membrane, to expand and harden the outside membrane, preventing more sperm from entering. Cell division with no significant growth, producing a cluster of cells, the same size as the original zygote, is called cleavage.
At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. The different cells derived from cleavage, up to the blastula stage, are called blastomeres. Depending on the amount of yolk in the egg, the cleavage can be holoblastic or meroblastic. Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via the placenta or milk, such as might be secreted from a marsupium. On the other hand, meroblastic cleavage occurs in animals; because cleavage is impeded in the vegetal pole, there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms: The end of cleavage is known as midblastula transition and coincides with the onset of zygotic transcription.
In amniotes, the cells of the morula are at first aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast, which does not contribute to the formation of the embryo proper, an inner cell mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, thus the morula is converted into a vesicle, called the blastodermic vesicle; the inner cell mass remains in contact, with the trophoblast at one pole of the ovum. After the 7th cleavage has produced 128 cells, the embryo is called a blastula; the blastula is a spherical layer of cells surrounding a fluid-filled or yolk-filled cavity Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass, distinct from the surrounding blastula. The blastocyst must not be confused with the blastula. In the mouse, primordial germ cells arise from a layer of cells in the inner cell mass of the blastocyst as a result of extensive genome-wide reprogramming.
Reprogramming involves global DNA demethylation facilitated by the DNA base excision repair pathway as well as chromatin reorganization, results in cellular totipotency. Before gastrulation, the cells of the trophoblast become differentiated into two strata: The outer stratum forms a syncytium, termed the syncytiotrophoblast, while the inner layer, the cytotrophoblast or "Layer of Langhans", consists of well-defined cells; as stated, the cells of the trophoblast do not contribute to the formation of the embryo proper. On the deep surface of the inner cell mass, a layer of flattened cells, called the endoderm, is differentiated and assumes the form of a small sac, called the yolk sac. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic c