Final answer:
The tertiary follicle stage is vital for ovulation, where the dominant follicle releases a secondary oocyte. Most tertiary follicles undergo atresia, while the surviving one becomes a corpus luteum after release of the oocyte. Hormones like GnRH, LH, and FSH regulate this process, and subsequent fertilization can lead to pregnancy; otherwise, menstruation occurs.
Step-by-step explanation:
Once the stage of tertiary follicle has been reached, a thin acellular membrane called the zona pellucida is secreted by the primary oocyte, which will play a critical role in fertilization. A follicle becomes a tertiary follicle when the antrum, a large pool of follicular fluid, has formed fully. While several follicles may reach this stage together, most will undergo atresia, a process where the follicle degenerates. The dominant follicle that survives will grow until ovulation occurs, releasing a secondary oocyte from the ovary.
During this time, hormonal changes regulated by gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) guide the process. The surge in LH triggered by elevated estrogen levels from the dominant follicle leads to ovulation, which signifies the end of the follicular phase of the menstrual cycle. Post-ovulation, the ruptured follicle forms the corpus luteum, which produces progesterone, supporting the endometrial lining and inhibiting the release of further FSH and LH.
If fertilization occurs, the secondary oocyte completes meiosis II, forming a zygote that may implant in the uterus. If the secondary oocyte is not fertilized, it disintegrates, leading to the eventual shedding of the endometrial lining during menstruation.