Reproductive System

The Female Reproductive Tract

The function of the female chicken reproductive system is to produce eggs.  The avian female reproductive system also has the ability to store spermatozoa temporarily in the infundibulum because of the chicken’s short fertilization period. Sperm can also be stored in tubules in the uterovaginal region. The different structures present in the avian reproductive tract are the ovaries, follicles, the stigma, the post-ovulatory follicles, the infundibulum, the magnum, the isthmus, the shell gland, and the vagina. 

The Ovaries 

Female birds have both left and right ovaries during the embryonic stage; however, during chicken, turkey, and domestic geese development the right organs degenerate and only the left side develops. The ovary is located on the left side of the abdominal cavity and it is cranioventral to the left kidney. The size of the ovary is dependent on the reproductive status of the bird. Before the first laying period, the ovary is small and smooth; however, as puberty advances, the ovary begins to have a granular appearance because of the growing size of the follicles. The growth and development of the follicles occur in order to prepare the bird for ovulation. There are three different stages of follicles that can be identified by their appearance. The large yellow follicles are arranged in size order and the largest follicle is the follicle that will be the next to ovulate. The largest follicle is termed F1 and the next largest follicle is termed F2. The F2 follicle normally ovulates the day after the F1 follicle ovulates. The small yellow follicles are less developed than the large yellow follicles; however, they continue to develop and as the larger follicles ovulate, these smaller follicles replace the larger yellow follicles. The small white follicles are the follicles that replace the small yellow follicles after the small yellow follicles replace the large yellow follicles. 

Fertilization

During the time of ovulation, the follicle ruptures at a nonvascular area called the stigma. After ovulation of the oocyte, the remaining follicular structure is termed the post-ovulatory follicle. Then the mature ovum is released and enters into the infundibulum. The infundibulum is a portion of the oviduct that receives the oocyte and then directs it into the magnum of the oviduct. The infundibulum is the site of fertilization and it is one of the sites where avian species store sperm. Spermatozoa are also stored in specialized tubules in the uterovaginal region. Sperm can be stored at this location for longer periods of time compared to storage regions of the infundibulum. Sperm stored in the uterovaginal region travels to the infundibulum after the expulsion of each egg, which enables fertilization of the following egg. In order for fertilization to occur, spermatozoa must come into contact with the egg before it enters the tubular segment of the infundibulum. If the spermatozoa and egg do not do not come into contact before this period, the egg will not be able to be fertilized because a layer of albumen is secreted and it surrounds the egg. 

The Oviduct

The size of the oviduct varies and expands to its maximum size during the lay period.  The oviduct is composed of four sections, which include the magnum, isthmus, uterus, and vagina. The magnum is thickly walled and contains glands that secrete albumen and other proteins that aid in egg production and provide nutrients for the developing embryo. Fibers from the inner layers of the albumen allow for formation of the chalazae, which are structures that attach the yolk to the egg and are present on opposite sides of yolk. The oocyte remains in the magnum for approximately two hours. After leaving the magnum, the egg enters isthmus and remains within the isthmus for approximately 1-2 hours. The isthmus has a thinner wall compared to the magnum and it is the site of egg membrane formation. 

 

Egg Formation and Oviposition

After remaining in the isthmus for approximately 1-2 hours, the oocyte enters the shell gland. The shell gland is the site in which shell formation and pigmentation occurs, which takes approximately 20 hours. The eggshell consists of four layers, which include the shell membranes, the mammillary cores, a matrix, and the cuticle. The eggshell is primarily made of calcium carbonate, which has been supplied by the mammary core and matrix layers. The cuticle layer of the shell is comprised of lipids and it is the outermost layer of the shell. The cuticle layer functions as a protection against microorganisms, it prevents evaporation, and it repels water. Once the egg is fully developed, arginine vasotocin (AVT) is released from the pituitary and stimulates uterine contractions. AVT is a peptide hormone and it functions in both urine osmolality regulation and regulation of uterine contractions. The entire egg laying process, beginning with ovulation and ending with oviposition, expulsion of the egg, takes approximately 24-26 hours. Ovulation is a continuous cycle and the next egg is ovulated 30-45 minutes after oviposition.

The Male Reproductive Tract

The male reproductive tract functions to create sperm that is then delivered to the female reproductive tract; however, avian species do not need sperm in order to produce eggs. The male reproductive system consists of testes, ductus deferens, and the phallus.

Testes and the Ductus Deferens

In birds, the testes are located cranioventral to the kidneys. During the breeding season, the testes enlarge. They contain seminiferous tubules that are surrounded by interstitium, which is made up of Leydig cells. Furthermore, they are normally located within the body cavity, which allows them to remain at a normal core body temperature. The process of spermatogenesis occurs in the testes and is facilitated by follicle stimulating hormone (FSH) and luteinizing hormone (LH). The ductus deferens is a duct that carries sperm from the testes to the raised papilla. The raised papilla is located on the lateral aspect of the urodeum.

Phallus

The phallus is an organ that is present in roosters and male turkeys. This organ does not protrude from the body and it contains erectile tissue. This tissue allows for the movement of ejaculate from the ductus deferens to the phallus during copulation. During breeding, the transfer of ejaculate is facilitated by the phallus being pressed against the vent of the female.