0.1 Module 2 anatomy and physiology of the female reproductive  (Page 4/61)

When only the one dominant follicle remains in the ovary, it begins to greatly increase estrogen production. It produces so much estrogen that the normal negative feedback doesn't occur. Instead, these extremely high concentrations of estrogen in the blood triggers the anterior pituitary to begin secreting large amounts of LH and FSH into the bloodstream (see [link] ).

It is this large burst of LH that leads to ovulation of the dominant follicle. The LH surge induces many changes in the dominant follicle, including stimulating the return to cell division. As noted earlier, the polar body that results from unequal cell division simply degrades. The dominant follicle bulges out from the surface of the ovary. This combined with pressure from the large, fluid-filled follicle, results in the expulsion of the oocyte into the peritoneal cavity. This release is ovulation    .

The surge of LH also stimulates a change in the follicle cells that remain in the follicle after the oocyte has been ovulated. This change is called luteinization (recall that the full name of LH is luteinizing hormone), and it transforms the collapsed follicle into a new endocrine structure called the corpus luteum    , a term meaning “yellowish body” (see [link] ). Instead of estrogen, the corpus luteum begins to produce large amounts of the sex steroid hormone progesterone , a hormone that is critical for the establishment and maintenance of pregnancy . The increase in progesterone signals the brain to keep GnRH, LH, and FSH secretions low, so no new dominant follicles develop at this time.

If pregnancy does not occur within 10 to 12 days, the corpus luteum will stop secreting progesterone and degrade into the corpus albicans    , a nonfunctional “whitish body” that will disintegrate over a period of several months. During this time progesterone production is greatly reduced and FSH and LH are once again stimulated producing new growino grow and secrete estrogen.

The uterine tubes

The uterine tubes    (also called fallopian tubes or oviducts) serve as the pathway of the oocyte from the ovary to the uterus ( [link] ). Each of the two uterine tubes is close to, but not directly connected to, the ovary and divided into sections. The isthmus    is the narrow medial end of each uterine tube that is connected to the uterus. The wide distal infundibulum    flares out with slender, finger-like projections called fimbriae    . The middle region of the tube, called the ampulla    , is where fertilization often occurs. The uterine tubes also have three layers: an outer serosa, a middle smooth muscle layer, and an inner mucosal layer. In addition to its mucus-secreting cells, the inner mucosa contains ciliated cells that beat in the direction of the uterus, producing a current that will be critical to move the oocyte.

Following ovulation, the uterine tubes receive the oocyte. Unlike sperm, oocytes lack flagella, and therefore cannot move on their own. So how do they travel into the uterine tube and toward the uterus? Around the time of ovulation, contractions occur in the smooth muscle along the length of the uterine tube. These contractions result in a coordinated movement that sweeps the egg toward the opening of the uterine tube. Current flowing toward the uterus is generated by coordinated beating of the cilia that line the outside and the length of the uterine tube. Once the egg is inside, the muscular contractions and beating cilia move the oocyte slowly toward the uterus. When fertilization does occur, sperm typically meet the egg while it is still moving through the uterine tube.