![]() The streak eventually extends 60–75% of the length of the area pellucida. The elongation of the primitive streak appears to be coextensive with the anterior migration of these secondary hypoblast cells. At the same time, the secondary hypoblast cells continue to migrate anteriorly from the posterior margin of the blastoderm. As these cells enter the primitive streak, the streak elongates toward the future head region. This thickening is caused by the ingression of endodermal precursors from the epiblast into the blastocoel and by the migration of cells from the lateral region of the posterior epiblast toward the center ( Figure 11.10B Vakaet 1984 Bellairs 1986 Eyal-Giladi et al. ![]() This streak is first visible as a thickening of the epiblast at the posterior region of the embryo, just anterior to Koller's sickle ( Figure 11.10A). The major structural characteristic of avian, reptilian, and mammalian gastrulation is the primitive streak. Fate maps of the chick epiblast are shown in Figures 11.10 and 1.8 ( Schoenwolf 1991). All three germ layers of the embryo proper (plus a considerable amount of extraembryonic membrane) are formed from the epiblastic cells. Rather, the hypoblast cells form portions of the external membranes, especially the yolk sac and the stalk that links the yolk mass to the endodermal digestive tube. The hypoblast does not contribute any cells to the developing embryo ( Rosenquist 1966, 1972). The avian embryo comes entirely from the epiblast. Thus, although the shape and formation of the avian blastodisc differ from those of the amphibian, fish, or echinoderm blastula, the overall spatial relationships are retained. The two-layered blastoderm (epiblast and hypoblast) is joined together at the margin of the area opaca, and the space between the layers forms a blastocoel. Shortly thereafter, a sheet of cells from the posterior margin of the blastoderm (distinguished from the other regions of the margin by Koller's sickle, a local thickening) migrates anteriorly to join the polyinvagination islands, thereby forming the secondary hypoblast ( Eyal-Giladi et al. At this time, most of the cells of the area pellucida remain at the surface, forming the epiblast, while other area pellucida cells have delaminated and migrated individually into the subgerminal cavity to form the polyinvagination islands ( primary hypoblast), an archipelago of disconnected clusters containing 5–20 cells each ( Figure 11.9B). * Some of the marginal zone cells become very important in determining cell fate during early chick development.īy the time a hen has laid an egg, the blastoderm contains some 20,000 cells. Between the area pellucida and the area opaca is a thin layer of cells called the marginal zone (or marginal belt) ( Eyal-Giladi 1997 Arendt and Nübler-Jung 1999). The peripheral ring of blastoderm cells that have not shed their deep cells constitutes the area opaca. This part of the blastoderm forms most of the actual embryo. At this stage, the deep cells in the center of the blastoderm are shed and die, leaving behind a one-cell-thick area pellucida. This space is created when the blastoderm cells absorb fluid from the albumin (“egg white”) and secrete it between themselves and the yolk ( New 1956). Between the blastoderm and the yolk is a space called the subgerminal cavity ( Figure 11.9A). These cells become linked together by tight junctions ( Bellairs et al. Thereafter, equatorial and vertical cleavages divide the blastoderm into a tissue five to six cell layers thick. As in the fish embryo, these cleavages do not extend into the yolky cytoplasm, so the early-cleavage cells are continuous with each other and with the yolk at their bases ( Figure 11.8E). The first cleavage furrow appears centrally in the blastodisc, and other cleavages follow to create a single-layered blastoderm ( Figure 11.8). Cleavage occurs only in the blastodisc, a small disc of cytoplasm 2–3 mm in diameter at the animal pole of the egg cell. Like fish eggs, the yolky eggs of birds undergo discoidal meroblastic cleavage. The egg is telolecithal (like that of the fish), with a small disc of cytoplasm sitting atop a large yolk. The chick embryo can be surgically manipulated and, since it forms most of its organs in ways very similarly to those of mammals, it has often served as a surrogate for human embryos.įertilization of the chick egg occurs in the oviduct, before the albumen and the shell are secreted upon it. Thus, large numbers of embryos can be obtained at the same stage. Moreover, at any particular temperature, its developmental stage can be accurately predicted. It is accessible all year and is easily raised. Ever since Aristotle first followed its 3-week development, the domestic chicken has been a favorite organism for embryological studies.
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