Indevelopmental biology,animal embryonic development, also known asanimal embryogenesis, is the developmental stage of ananimalembryo. Embryonic development starts with thefertilization of anegg cell (ovum) by asperm cell (spermatozoon).[1] Once fertilized, the ovum becomes a singlediploid cell known as azygote. The zygote undergoesmitoticdivisions with no significant growth (a process known ascleavage) andcellular differentiation, leading to development of a multicellular embryo[2] after passing through an organizational checkpoint during mid-embryogenesis.[3] Inmammals, the term refers chiefly to the early stages ofprenatal development, whereas the termsfetus andfetal development describe later stages.[2][4]
The main stages of animal embryonic development are as follows:
The embryo then transforms into the next stage of development, the nature of which varies among different animal species (examples of possible next stages include afetus and alarva).
The egg cell is generally asymmetric, having ananimal pole (futureectoderm).It is covered with protective envelopes, with different layers. The first envelope – the one in contact with themembrane of the egg – is made ofglycoproteins and is known as thevitelline membrane (zona pellucida inmammals). Differenttaxa show different cellular and acellular envelopes englobing the vitelline membrane.[2][5]
Fertilization is the fusion ofgametes to produce a new organism. In animals, the process involves asperm fusing with anovum, which eventually leads to the development of anembryo. Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside in the case of external fertilization. The fertilized egg cell is known as thezygote.[2][5]
To prevent more than one sperm fertilizing the egg (polyspermy), fast block and slow block to polyspermy are used. Fast block, the membrane potential rapidly depolarizing and then returning to normal, happens immediately after an egg is fertilized by a single sperm. Slow block begins in the first few seconds after fertilization and is when the release of calcium causes thecortical reaction, in which various enzymes are released from cortical granules in the eggs plasma membrane, causing the expansion and hardening of the outside membrane, preventing more sperm from entering.[6][5]
Cell division with no significant growth, producing a cluster of cells that is the same size as the original zygote, is calledcleavage. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called themorula. In the early mouse embryo, the sister cells of each division remain connected duringinterphase bymicrotubule bridges.[7] The different cells derived from cleavage, up to theblastula stage, are calledblastomeres. Depending mostly on the amount ofyolk in the egg, thecleavage can beholoblastic (total) ormeroblastic (partial).[8][9]
Holoblastic cleavage occurs in animals with little yolk in their eggs,[10] such as humans and other mammals who receive nourishment as embryos from the mother, via theplacenta ormilk, such as might be secreted from amarsupium. Meroblastic cleavage occurs in animals whose eggs have more yolk (i.e. birds and reptiles). Because cleavage is impeded in thevegetal pole, there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote.[8][9]
In holoblastic eggs, the first cleavage always occurs along the vegetal-animal axis of the egg, and 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:
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The end of cleavage is known asmidblastula transition and coincides with the onset of zygotictranscription.
In amniotes, the cells of themorula are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, thetrophoblast, which does not contribute to the formation of the embryo proper, and aninner cell mass, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into avesicle, called the blastodermic vesicle. The inner cell mass remains in contact, however, with the trophoblast at one pole of the ovum; this is named the embryonic pole, since it indicates the location where the future embryo will develop.[18][9]
After the seventh cleavage has produced 128cells, the morula becomes ablastula.[8] The blastula is usually a spherical layer of cells (theblastoderm) surrounding a fluid-filled or yolk-filled cavity theblastocoel.[citation needed]
Mammals at this stage form a structure called theblastocyst,[1] characterized by aninner cell mass that is distinct from the surrounding blastula.[19][20][21] The blastocyst is similar in structure to the blastula but their cells have different fates. In the mouse, primordialgerm cells arise from the inner cell mass (theepiblast) as a result of extensivegenome-wide reprogramming.[22] Reprogramming involves globalDNA demethylation facilitated by the DNAbase excision repair pathway as well aschromatin reorganization, and results in cellulartotipotency.[23][20]
Beforegastrulation, the cells of the trophoblast become differentiated into two layers: The outer layer forms asyncytium (i.e., a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells), termed thesyncytiotrophoblast, while the inner layer, thecytotrophoblast, consists of well-defined cells. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of thechorion and play an important part in the development of theplacenta. On the deep surface of the inner cell mass, a layer of flattened cells, called theendoderm, is differentiated and quickly assumes the form of a small sac, called theyolk sac. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called theamniotic cavity is gradually developed. The floor of this cavity is formed by theembryonic disk, which is composed of a layer of prismatic cells – the embryonic ectoderm, derived from the inner cell mass and lying in apposition with the endoderm.[18][20]
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Theembryonic disc becomes oval and then pear-shaped, the wider end being directed forward. Towards the narrow, posterior end, an opaqueprimitive streak, is formed and extends along the middle of the disc for about half of its length; at the anterior end of the streak there is a knob-like thickening termed theprimitive node or knot, (known asHensen's knot in birds). A shallow groove, theprimitive groove, appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, theblastopore, with theyolk sac. The primitive streak is produced by a thickening of the axial part of the ectoderm, the cells of which multiply, grow downward, and blend with those of the subjacent endoderm. From the sides of the primitive streak a third layer of cells, themesoderm, extends laterally between the ectoderm and endoderm; thecaudal end of the primitive streak forms thecloacal membrane. The blastoderm now consists of three layers, an outer ectoderm, a middle mesoderm, and an inner endoderm; each has distinctive characteristics and gives rise to certain tissues of the body. For many mammals, it is sometime during formation of the germ layers thatimplantation of the embryo in theuterus of the mother occurs.[18][20]
During gastrulation cells migrate to the interior of the blastula, subsequently forming two (indiploblastic animals) or three (triploblastic)germ layers. The embryo during this process is called agastrula. The germ layers are referred to as the ectoderm, mesoderm and endoderm. In diploblastic animals only the ectoderm and the endoderm are present.[8]* Among different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
In most animals, a blastopore is formed at the point where cells are migrating inward. Two major groups of animals can be distinguishedaccording to the blastopore's fate. Indeuterostomes the anus forms from the blastopore, while inprotostomes it develops into the mouth.[9]
In front of the primitive streak, two longitudinal ridges, caused by a folding up of the ectoderm, make their appearance, one on either side of the middle line formed by the streak. These are named theneural folds; they commence some little distance behind theanterior end of theembryonic disk, where they are continuous with each other, and from there gradually extend backward, one on either side of the anterior end of the primitive streak. Between these folds is a shallowmedian groove, theneural groove. The groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, theneural tube or canal, the ectodermal wall of which forms the rudiment of the nervous system. After the coalescence of the neural folds over the anterior end of the primitive streak, the blastopore no longer opens on the surface but into the closed canal of the neural tube, and thus a transitory communication, theneurenteric canal, is established between the neural tube and the primitivedigestive tube. The coalescence of the neural folds occurs first in the region of thehind brain, and from there extends forward and backward; toward the end of the third week, the front opening (anterior neuropore) of the tube finally closes at the anterior end of the futurebrain, and forms a recess that is in contact, for a time, with the overlying ectoderm; the hinder part of the neural groove presents for a time arhomboidal shape, and to this expanded portion the termsinus rhomboidalis has been applied. Before the neural groove is closed, a ridge of ectodermal cells appears along the prominent margin of each neural fold; this is termed theneural crest or ganglion ridge, and from it thespinal andcranial nerve ganglia and the ganglia of thesympathetic nervous system are developed.[citation needed] By the upward growth of the mesoderm, the neural tube is ultimately separated from the overlying ectoderm.[24][9]
Thecephalic end of the neural groove exhibits several dilatations that, when the tube is closed, assume the form of the threeprimary brain vesicles, and correspond, respectively, to the futureforebrain (prosencephalon),midbrain (mesencephalon), andhindbrain (rhombencephalon) (Fig. 18). The walls of the vesicles are developed into the nervous tissue and neuroglia of the brain, and their cavities are modified to form its ventricles. The remainder of the tube forms thespinal cord (medulla spinalis); from its ectodermal wall the nervous and neuroglial elements of the spinal cord are developed, while the cavity persists as thecentral canal.[24][9]
The extension of the mesoderm takes place throughout the whole of the embryonic and extra-embryonic areas of the ovum, except in certain regions. One of these is seen immediately in front of the neural tube. Here the mesoderm extends forward in the form of two crescentic masses, which meet in the middle line so as to enclose behind them an area that is devoid of mesoderm. Over this area, the ectoderm and endoderm come into direct contact with each other and constitute a thin membrane, thebuccopharyngeal membrane, which forms a septum between the primitive mouth andpharynx.[18][9]
In front of the buccopharyngeal area, where the lateral crescents of mesoderm fuse in the middle line, thepericardium is afterward developed, and this region is therefore designated the pericardial area. A second region where the mesoderm is absent, at least for a time, is that immediately in front of the pericardial area. This is termed the proamniotic area, and is the region where the proamnion is developed; in humans, however, it appears that a proamnion is never formed. A third region is at the hind end of the embryo, where the ectoderm and endoderm come into apposition and form the cloacal membrane.[18][9]
Somitogenesis is the process by whichsomites (primitive segments) are produced. These segmented tissue blocks differentiate into skeletal muscle, vertebrae, and dermis of all vertebrates.[25]
Somitogenesis begins with the formation ofsomitomeres (whorls of concentric mesoderm) marking the future somites in the presomitic mesoderm (unsegmented paraxial). The presomitic mesoderm gives rise to successive pairs of somites, identical in appearance that differentiate into the same cell types but the structures formed by the cells vary depending upon the anteroposterior (e.g., thethoracic vertebrae have ribs, thelumbar vertebrae do not). Somites have unique positional values along this axis and it is thought that these are specified by theHoxhomeotic genes.[25]
Toward the end of the second week after fertilization,transverse segmentation of theparaxial mesoderm begins, and it is converted into a series of well-defined, more or less cubical masses, also known as the somites, which occupy the entire length of the trunk on either side of the middle line from theoccipital region of the head. Each segment contains a central cavity (known as a [myocoel), which, however, is soon filled with angular and spindle-shape cells. The somites lie immediately under the ectoderm on the lateral aspect of the neural tube andnotochord, and are connected to thelateral mesoderm by theintermediate cell mass. Those of the trunk may be arranged in the following groups, viz.:cervical 8,thoracic 12,lumbar 5,sacral 5, andcoccygeal from 5 to 8. Those of the occipital region of the head are usually described as being four in number. In mammals, somites of the head can be recognized only in the occipital region, but a study of the lower vertebrates leads to the belief that they are present also in the anterior part of the head and that, altogether, nine segments are represented in the cephalic region.[26][25]
At some point after the different germ layers are defined,organogenesis begins. The first stage invertebrates is calledneurulation, where theneural plate folds forming the neural tube (see above).[8] Other common organs or structures that arise at this time include theheart and somites (also above), but from now on embryogenesis follows no common pattern among the different taxa of theanimalia.[2]
In most animals organogenesis, along withmorphogenesis, results in alarva. The hatching of the larva, which must then undergometamorphosis, marks the end of embryonic development.[2]
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