Formation of the egg white and the shell in the oviduct 6.6.1
Sequential deposition along the oviduct During ovulation, the yolk released from the largest follicle is captured by the oviduct and undergoes successive deposits in different parts of the tissue according to a predetermined and definite sequence (Fig. 6.2) (Romanoff and Romanoff, 1949; Gilbert, 1979; Sauveur and de Reviers, 1988). The duration between the yolk entrance and the egg expulsion (oviposition) from the oviduct is about 24–26 h.
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The penetration of the yolk into the oviduct is facilitated by the infundibulum, a funnel-shaped structure undergoing muscle contractions during ovulation and capturing the oocyte liberated by the mature follicle. In breeders, fertilization of the oocyte by sperm previously stored in the sperm glands occurs at this site before completion of the vitelline membrane. The outer layer of the vitelline membrane is deposited on the perivitelline layer.
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This thin membrane (10–12 mm thick) limits exchanges between the yolk and the white. Its completion precedes the secretion by the magnum of albumen proteins which are accumulated beforehand in the epithelial and tubular cells. The deposition of these proteins takes about 3.25–3.5 h. Egg white is partially hydrated in the magnum.
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It has a wrinkled aspect when it enters the isthmus, 4 h after ovulation. The perialbumen layer is secreted by the magnum–isthmus junction. The isthmus is the site where the two shell membranes are secreted. In its terminal portion, the isthmus deposits the organic sites for the nucleation of calcium carbonate crystals at the surface of the outer shell membrane (Fig. 6.5b). Then the egg enters the uterus and stays in this compartment for nearly 20 h.
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First, the egg undergoes the second phase of hydration (plumping) which gives the final ovoid shape to the egg (Fig. 6.4) and puts it in close contact with the uterine mucosa. At the same time, the mineralization of the shell starts on the expanding egg (Fig. 6.4). This nucleation phase corresponds to the deposit of the first calcite crystals on the organic nucleation sites (Fig. 6.5b).
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Last Words:
From 10 to 22 h after ovulation, a large quantity of calcium carbonate is quickly deposited, together with a small proportion of proteins composing the organic matrix of the shell. This linear deposit stops about 2 h before egg expulsion (oviposition). During this phase, the shell is covered with the cuticle containing most (two-thirds) of the superficial pigments (porphyrins) of brown shells.Read more about Tamilmv
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