Final answer:
At the beginning of each estrous cycle, a hormone surge triggers primary oocytes to resume meiosis, transforming into secondary oocytes. Only one of these oocytes is typically released during ovulation, while the rest degenerate.
Step-by-step explanation:
Near the beginning of each estrous cycle, multiple oocytes undergo a process that initiates their development and potential release during ovulation. Just prior to ovulation, a surge in luteinizing hormone (LH) stimulates the resumption of meiosis in a primary oocyte, leading to the transition from primary to secondary oocyte. This division is unequal, resulting in a large secondary oocyte and a smaller first polar body. The secondary oocyte may be released from the ovary during ovulation, while the smaller polar body typically disintegrates.
As the cycle continues, the level of estrogen influences the rise and fall of follicle-stimulating hormone (FSH) and LH, with the LH surge triggering the rupture of the most mature follicle, which releases the egg. The other follicles that do not rupture degenerate, and their oocytes are lost. In the uterus, the endometrium thickens in preparation for a potential embryo, but if the secondary oocyte is not fertilized, it does not complete meiosis II and disintegrates.