N Animals The Normal Development Of An Embryo Is Dependent On
This article is about embryonic developmt in all types of animals, including humans. For information specific to human embryonic developmt, see Human embryonic developmt. For information specific to plants, see Plant embryonic developmt.
In developmtal biology, animal embryonic developmt, also known as animal embryogesis, is the developmtal stage of an animal embryo. Embryonic developmt starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon).
Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differtiation, leading to developmt of a multicellular embryo
Embryonic Stem Cell
In mammals, the term refers chiefly to the early stages of pratal developmt, whereas the terms fetus and fetal developmt describe later stages.
The embryo th transforms into the next stage of developmt, the nature of which varies betwe differt animal species (examples of possible next stages include a fetus and a larva).
The egg cell is gerally asymmetric, having an animal pole (future ectoderm). It is covered with protective velopes, with differt layers. The first velope – the one in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals). Differt taxa show differt cellular and acellular velopes globing the vitelline membrane.
Embryonic Origins Of Adult Pluripotent Stem Cells
Fertilization is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which evtually leads to the developmt of an embryo. Depding 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 the zygote.
To prevt more than one sperm fertilizing the egg (polyspermy), fast block and slow block to polyspermy are used. Fast block, the membrane pottial rapidly depolarizing and th returning to normal, happs immediately after an egg is fertilized by a single sperm. Slow block begins in the first few seconds after fertilization and is wh the release of calcium causes the cortical reaction, in which various zymes are released from cortical granules in the eggs plasma membrane, causing the expansion and harding of the outside membrane, prevting more sperm from tering.
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 dse ball of at least sixte cells called the morula. In the early mouse embryo, the sister cells of each division remain connected during interphase by microtubule bridges.
Fetal Outcome At Gestational Day 20 In N And Md W And L Rats. Bars...
The differt cells derived from cleavage, up to the blastula stage, are called blastomeres. Depding mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial).
Such as humans and other mammals who receive nourishmt as embryos from the mother, via the placta or milk, such as might be secreted from a marsupium. Meroblastic cleavage occurs in animals whose eggs have more yolk (i.e. birds and reptiles). Because cleavage is impeded in the vegetal pole, there is an unev 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, and the second cleavage is perpdicular to the first. From here the spatial arrangemt of blastomeres can follow various patterns, due to differt planes of cleavage, in various organisms:
See A Rare Baby Dinosaur Curled Up In Its Fossilized Egg
In amniotes, the cells of the morula are at first closely 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, and an inner cell mass, from which the embryo is developed. Fluid collects betwe the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a vesicle, 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.
The blastocyst is similar in structure to the blastula but their cells have differt fates. In the mouse, primordial germ cells arise from the inner cell mass (the epiblast) as a result of extsive gome-wide reprogramming.
Reprogramming involves global DNA demethylation facilitated by the DNA base excision repair pathway as well as chromatin reorganization, and results in cellular totipotcy.
Animal Development Ii: Gastrulation & Organogenesis
Before gastrulation, the cells of the trophoblast become differtiated into two layers: The outer layer forms a syncytium (i.e., a layer of protoplasm studded with nuclei, but showing no evidce of subdivision into cells), termed the syncytiotrophoblast, while the inner layer, the cytotrophoblast, 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 the chorion and play an important part in the developmt of the placta. On the deep surface of the inner cell mass, a layer of flatted cells, called the doderm, is differtiated and quickly assumes the form of a small sac, called the yolk sac. Spaces appear betwe the remaining cells of the mass and, by the largemt and coalescce of these spaces, a cavity called the amniotic cavity is gradually developed. The floor of this cavity is formed by the embryonic 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 doderm.
The embryonic disc becomes oval and th pear-shaped, the wider d being directed forward. Towards the narrow, posterior d, an opaque primitive streak, is formed and extds along the middle of the disc for about half of its lgth; at the anterior d of the streak there is a knob-like thicking termed the primitive node or knot, (known as Hs's knot in birds). A shallow groove, the primitive groove, appears on the surface of the streak, and the anterior d of this groove communicates by means of an aperture, the blastopore, with the yolk sac. The primitive streak is produced by a thicking of the axial part of the ectoderm, the cells of which multiply, grow downward, and bld with those of the subjact doderm. From the sides of the primitive streak a third layer of cells, the mesoderm, extds laterally betwe the ectoderm and doderm; the caudal d of the primitive streak forms the cloacal membrane. The blastoderm now consists of three layers, an outer ectoderm, a middle mesoderm, and an inner doderm; 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 that implantation of the embryo in the uterus of the mother occurs.
During gastrulation cells migrate to the interior of the blastula, subsequtly forming two (in diploblastic animals) or three (triploblastic) germ layers. The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm, mesoderm and doderm. In diploblastic animals only the ectoderm and the doderm are prest.
Asynchronous Division At 4–8 Cell Stage Of Preimplantation Embryos Affects Live Birth Through Icm/te Differentiation
In most animals, a blastopore is formed at the point where cells are migrating inward. Two major groups of animals can be distinguished according to the blastopore's fate. In deuterostomes the anus forms from the blastopore, while in protostomes it develops into the mouth.
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 the neural folds; they commce some little distance behind the anterior d of the embryonic disk, where they are continuous with each other, and from there gradually extd backward, one on either side of the anterior d of the primitive streak. Betwe these folds is a shallow median groove, the neural groove. The groove gradually deeps 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, the neural tube or canal, the ectodermal wall of which forms the rudimt of the nervous system. After the coalescce of the neural folds over the anterior d of the primitive streak, the blastopore no longer ops on the surface but into the closed canal of the neural tube, and thus a transitory communication, the neurteric canal, is established betwe the neural tube and the primitive digestive tube. The coalescce of the neural folds occurs first in the region of the hind brain, and from there extds forward and backward; toward the d of the third week, the front oping (anterior neuropore) of the tube finally closes at the anterior d of the future brain, and forms a recess that is in contact, for a time, with the overlying ectoderm; the hinder part of the neural groove prests for a time a rhomboidal shape, and to this expanded portion the term sinus rhomboidalis has be applied. Before the neural groove is closed, a ridge
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