During the first reduction division involved in generation of the oocyte, the latter expands from a diameter of about 1-3.5 cm. Most of the content of this extremely large single cell is yolk surrounded by the plasma membrane. The egg nucleus and associated cytoplasm are located peripherally in a region called the germinal vesicle. The "yolk" beneath this structure is clear and less dense than the remaining yellow yolk, causing the germinal vesicle to lie on the uppermost surface of the oocyte. Follicle cells surround the oocyte and pump yolk, synthesized in the maternal liver, from the blood into the oocyte. The follicle ruptures to release the mature oocyte, but the innermost part of the follicle, which is accellular, remains attached to the oocyte and forms the inner layer of the vitelline membrane. When the follicle-derived vitelline membrane is in place, the plasma membrane of the oocyte breaks down.
Fertilization occurs during the time between release of the oocyte and its entry into the end of the oviduct. Sperm penetrate the follicle-derived vitelline membrane to fertilize the oocyte and the second reduction division occurs. The resultant one-cell stage embryo is called a blastodisc. The requirement for rapid fertilization following follicle rupture is met by the female chicken's ability to store sperm in viable form for a number of weeks.
Peristaltic movements carry the egg down the oviduct, a journey that takes about 22 h and, during which the egg is subject to a number of modifications. First, a thickened outer layer is applied to the vitelline membrane, which has two extensions, chalazae, which act as stabilizers for the yolk. Albumen is then applied to the outer surface providing a source of water, protein, and antibiotic agents. Next, the double-layered shell membrane is applied, the layers of which are closely apposed to one another, except at the blunt end of the egg, where the gap between them eventually becomes the air space. Finally, in the shell gland, calcite crystals are deposited in and over the outer shell membrane layer to form the eggshell.
Cleavage of the blastodisc is rapid. The first two cleavage divisions occur within a few hours of fertilization and are not related to future body axes. The third cleavage coincides with application of shell membrane. The chick is telo-
lecithal (yolk is concentrated at one end of the egg), and cleavage is meroblas-tic (incomplete). The first three divisions are radial and incomplete with the cells being opened to the yolk ventrally forming a syncytial blastoderm. The fourth cleavage division is horizontal, producing a bilayered blastodisc and occurs as the shell is being applied. Further divisions increase the thickness of the blastoderm, but the diameter of the embryo remains roughly constant at 3 mm during this period and zygotic transcription is activated.
Subsequently, the blastoderm begins to expand over the yolk and marginal cells of the outer region, known as the area opaca, become specialized to engulf the underlying yolk. The more central region, area pellucida, appears dark owing to the underlying translucent "yolk." The area pellucida comprises an upper layer known as the epiblast from which the embryonic tissues derive and a lower layer of large, yolky cells, the hypoblast, which comprises the extraembryonic endoderm. These two layers are separated by a narrow fissure, which is equivalent to the blastocoel. The hypoblast derives in part from ingression of cells from the overlying blastoderm and in part from the posterior marginal zone. The hypoblast forms a triangle posteriorly, the embryonic shield or posterior marginal zone, and is generated particularly from an adjacent region of epiblast known as Koller's sickle. At this stage, the egg is laid (Hamburger and Hamilton [HH] stage 1) and comprises about 60,000 cells.
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