Continued from page 1Oviduct
The left oviduct is attached to the dorsal body wall by the mesovarian ligament. Glandular development within the oviduct results in a thickening of its walls which differentiate it into five functional regions associated with egg formation. These portions are termed the infundibulum, magnum, isthmus, uterus and vagina.
Infundibulum
The infundibulum is the funnel shaped structure at the proximal end of the oviduct along with a very small portion of the tubular shaped proximal oviduct. The funnel shaped portion of the infundibulum forms an elongated slit which faces the ovary. The ovulated secondary oocyte is literally swallowed or "caught" by the funnel portion of the infundibulum. This "catching" process is facilitated by the adjacent air sac which tightly encloses the ovary and forms the "ovarian pocket", leaving only the direction of the infundibulum for easy movement of the secondary oocyte. Not all ovulated secondary oocytes are successfully captured or caught by the infundibulum. "Internal laying" refers to some secondary oocytes that are lost into the body cavity where they are reabsorbed or potentially become involved in "egg yolk peritonitis". Penetration by spermatozoa occurs in the funnel of the infundibulum. Formation of the yolk membrane's outer layers probably begins in the tubular portion of the infundibulum. These outer layers of yolk membrane are termed the chalaziferous layer of albumen and the chalazae.
Magnum
The magnum is the longest and most coiled portion of the oviduct. It is distinguished by its greater external diameter and markedly thicker wall caused by the presence of numerous secretory glands which account for the prominent mucosal folds of the magnum. These glands secrete the thick albumen protein around the ovum. The stimulus to secrete albumen may be mechanical, arising from passage of the ovum along the magnum. Smooth muscle contraction peristaltically moves the ovum along the oviduct. In a sexually active bird, the magnum undergoes tremendous enlargement as compared in size to that of an inactive hen. Added length of the enlarged magnum causes a folding of the oviduct upon itself.
Isthmus
This region of the oviduct is short and has less prominent mucosal folds than the magnum. The division between the magnum and the isthmus is marked by a thin translucent line which can be seen on the mucosal surface with the unaided eye in domestic fowl. The isthmus produces two shell membranes which are loosely secreted around the ovum and albumen.
Uterus
The uterus is initially a similar diameter as the isthmus, but rapidly expands to form a pouch which retains the egg during the entire portion of egg shell formation. There is a normal "rocking" motion of the egg in the uterus as the shell is formed, and the egg spins along its linear axis during its development.
Vagina
The short terminal portion of the oviduct immediately proximal to its junction with the urodeum is the vagina. Strong muscle of the vaginal wall and a well-developed muscular sphincter at the uterovaginal junction serve to expel the egg during oviposition. The lining of the vagina has tubular crypts that act as "sperm nests" for storing sperm. The capacity for fertilization is retained for several weeks after a single insemination in domestic fowl as well as many other domestic bird species. Within minutes after insemination, sperm reaches the infundibulum, but they disappear within 24 hours, only to reappear within the lumen in smaller numbers at the time of each subsequent ovulation. How sperm survive in the vagina or what causes them to be released is unknown.
Formation of the Egg
In domestic fowl, the time required for the egg to traverse the entire length of the oviduct is approximately 25 hours. Most yolk precursors are synthesized in the liver and transported to the ovarian cortex hematogenously. The primary oocyte increased in size as the yolk precursors are assembled in their cytoplasm. This dramatic increase in size of the primary oocyte is referred to as the stage of rapid development. Approximately 15 minutes prior to ovulation, the primary oocyte goes through a maturation division to form the secondary oocyte and first polar body. Following ovulation the ovum passes through the infundibulum in approximately 15 minutes. Fertilization most likely occurs in the infundibulum. During the passage through the infundibulum, the chalaziferous layer of albumen and chalazae are formed and directly surround the ovum. Both the chalazae and the chalaziferous layers are composed of thick or dense albumen. The chalazae connect to the chalaziferous layer and function to suspend the yolk between the two ends of the egg.
The egg takes about three hours to traverse the magnum, during which time it acquires albumen. Albumen is much less viscous than yolk, the solid component being composed almost entirely of protein. Albumen contributes to the aqueous environment of the embryo, has antibacterial properties and is a source of nutrition for the embryo. The egg moves through the isthmus in approximately one and one half hours. The inner and outer shell membranes are formed during transit through the isthmus. The egg spends most of its total transit time in the uterus (approximately 20 hours). During the initial eight hours in the uterus, "plumping" occurs. This refers to the addition of watery solutions from selective uterine glands into the egg. The weight of the egg is doubled during this process and the albumen becomes multi-layered.
Following "plumping", calcification of the egg shell occurs relatively quickly, taking approximately 15 hours. The shell consists of the shell membranes, the testa and the cuticle. The air cell of the egg is formed by the separation of the two shell membranes at the apical end of the egg as the egg cools after laying. The bulk of the shell is the testa. The testa consists of an organized matrix of fine fibers and a far more bulky 98% solid inorganic component consisting mainly of calcite (calcium carbonate). Egg pores open on the surface of the shell and extend down to the shell membrane. Overlying the testa and pores is an extremely thin organic layer called the cuticle. The cuticle functions as a water repellent, reduces water loss from the egg and acts as a bacterial barrier. Egg color is due to the secretion of porphyrins by the uterine epithelium. The egg travels through the vagina in a matter of seconds during oviposition.
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