**2.1. Overview**

The goal of AI is to produce a succession of fertilized eggs between successive inseminations. To accomplish this, weekly inseminations must replenish the sperm population in the uterovaginal junction (UVJ) sperm storage tubules (SSTs). Birds do not have an estrous cycle that synchronizes copulation with ovulation. Alternatively, about 7-10 days before their first ovulation, hens mate, sperm ascend the vagina and then enter the SSTs. At the onset of egg production, individual sperm are slowly released from the SSTs, transported to the anterior end of the oviduct, and interact with the surface of the ovum (see [9-10] for recent reviews). Whether fertilized or not, over the next 24-26 hr the ovum is transported though the oviduct, accruing the outer perivitelline layer (PL) in the infundibulum, the albumen in the magnum, the shell membrane in the isthmus, and the hard shell in the uterus (also referred to as the shell gland) before oviposition. If fertilized, the blastoderm in the first laid egg consists of 40,000-60,000 cells in the turkey and 80,000-100,000 cells in the chicken.

Ovary: In the hen only the left ovary and oviduct become functional organs. About 2-3 wk before the onset of lay, small (less than 1 mm in diameter) white-yolk follicles begin to accumulate yellow yolk with some being recruited into a hierarchy of maturing yellow-yolk follicles (Figure 1). At the time of ovulation, the largest follicle, designated as F1, is ovulated. About 17 days were necessary for the 1 mm diameter white yolk follicle to mature to a preovulatory 40 mm diameter yellow yolk follicle [11]. After the F1 follicle is ovulated, the next largest follicle, formerly designated F2, becomes the F1 follicle and will ovulate at the beginning of the next daily "ovulatory cycle" in 24-26 hr.

The follicular sheath surrounding the maturing oocyte consists of histologically distinct con‐ centric layers of cells: the outer serosa (germinal epithelium); the theca externa, which forms the greatest portion of the follicle wall, provides structural support to the follicle and has steriodo‐ genic cells; the theca interna, a highly vascularized layer, which like the theca externa has ste‐ roid-producing cells (both thecal layers synthesize androgens and estrogens); and, the granulosa cell layer, enveloping the oocyte, which is responsible for progesterone secretion and the synthesis of the inner PL. The inner PL is homologous to the mammalian zona pellucida and is a fibrous reticulum about 2 μm thick. At ovulation, only the inner PL envelops the ovum. While there is no corpus luteum formation in birds, the thecal layers and the granulosa of the post-ovulatory follicle (POF) produce prostaglandins [12] and progesterone, respectively [13-16] then regress over the next 72 hr. The POF has a pocket like appearance after ovulation (Figure 1). On the surface of the inner PL overlying the germinal disc (GD), which is a 3.5 mm di‐ ameter disc of white yolk containing the haploid pronucleus and associated organelles, are sperm receptors. Sperm bind to the receptors overlying the GD, hydrolyze a path through the inner PL, and are incorporated into the ovum. Polyspermy is normal in birds but only one sperm in apposition to the female pronucleus undergoes nuclear decondensation and initiates synga‐ my, the reconstitution of the diploid number of chromosomes.

**Figure 1.** The ovary and oviduct of a turkey hen in egg production occupy much of the abdominal cavity. The ovarian follicular hierarchy consisting of ovarian follicles at various stages of develop (7 maturing follicles visible in this photo‐ graph) is observed. The largest follicle, F1 follicle is the next to ovulate. The ovum ovulated about 10 hr earlier has accrued albumen in the magnum (m), a shell membrane in the isthmus, and is observed in the uterus (ut) undergoing shell formation. Its post-ovulatory follicular sheath (POF) appears as an open pocket. The vagina (distal to the uterus and not visible) is embedded in connective tissue and enveloped by the abdominal fat pad.
