The ovarian cycle | Reproductive system physiology | NCLEX-RN | Khan Academy

The ovarian cycle in females involves the maturation of eggs within the ovaries, leading to the production of a secondary oocyte that can be fertilized. This cycle is also linked to the menstrual cycle. Primary oocytes complete meiosis I just before ovulation and are ejected from the ovary.

Eggs develop in structures called follicles, starting as primordial follicles. A follicle consists of a primary oocyte surrounded by granulosa cells, which become more numerous as the follicle matures and secrete hormones like estrogen, progesterone, and inhibin. The ovarian cycle typically lasts 28 days.

Primordial follicles progress to primary follicles, where a zona pellucida layer forms between the granulosa cells and the oocyte. Granulosa cells nourish the egg through gap junctions and inhibit meiosis I. Next, pre-antral follicles develop with increased granulosa cell numbers and the formation of a theca layer. Theca cells, stimulated by LH, produce androstenedione, which granulosa cells convert to estrogen.

The antrum, a fluid-filled space produced by granulosa cells, forms and expands the follicle. Multiple follicles develop, but only one, called the dominant follicle, reaches maturity and is ovulated. Other follicles undergo atresia, a process of degeneration where both the follicle and the egg die off. A woman loses 15-25 eggs per cycle to atresia.

The dominant follicle enlarges due to the expanding antrum, and granulosa cells form a mound called the cumulus oophorus. The mature follicle balloons out from the ovary surface. On day 14, enzymes break down the common wall between the follicle and the ovary, releasing the egg.

After ovulation, the collapsed follicle transforms into the corpus luteum. Granulosa cells in the corpus luteum enlarge and produce more estrogen, progesterone, and inhibin. Inhibin reduces FSH, which promotes follicle maturation. If the egg is not fertilized, the corpus luteum degenerates around day 25. If fertilized, it persists, continuing to produce progesterone and estrogen to prepare the uterine lining (endometrium) for implantation.

Around age 50-51, females experience menopause, the cessation of ovulation. This occurs because the ovaries lose their ability to respond to gonadotropins (LH and FSH) as most or all follicles and eggs have degenerated through atresia.