Ovulation and Corpus Luteum Formation

Corpus Luteum Formation

• After ovulation, granulosa cells and theca interna form a temporary endocrine gland called corpus luteum.
• Corpus luteum is influenced by LH, which causes granulosa cells to increase in size and become granulosa lutein cells, while theca interna cells become smaller theca lutein cells.

Hormone Production

• Under LH influence, granulosa lutein cells and theca lutein cells produce estrogen and progesterone for 10-12 days.
• If pregnancy does not occur, hormone production stops, and LH levels decrease, leading to apoptosis of the cells.

Menstrual Cycle

• The decrease in hormone production leads to menstruation.
• FSH secretion increases, and the growth of a new group of follicles begins, starting the next menstrual cycle.
• The corpus luteum that lasts only one menstrual cycle is called the corpus luteum of menstruation.

Corpus Luteum Regression

• After the corpus luteum degenerates, remnants are phagocytosed by macrophages, and fibroblasts invade the area, producing a scar called corpus albicans.

Pregnancy

• If pregnancy occurs, the hormone hCG is produced by the trophoblast cells of the implanted embryo, maintaining progesterone levels.
• The corpus luteum of pregnancy grows larger over time and is maintained by hCG for 4-5 months.
• Eventually, the placenta takes over progesterone production, and the corpus luteum of pregnancy is replaced by corpus albicans.

Follicular Development and Hormonal Regulation

• Follicular development depends on follicle-stimulating hormone (FSH) from the pituitary gland, which is stimulated by gonadotropin-releasing hormone (GnRH) from the hypothalamus.
• The dominant follicle secretes estrogen, which increases the release of GnRH and leads to a surge in luteinizing hormone (LH) from the pituitary gland.

Hormonal Mechanisms

• Inhibin, a hormone produced by granulosa and luteal cells, reinforces the negative feedback of estrogen and progesterone on the hypothalamus and anterior pituitary.
• Estrogen and progesterone have a negative feedback mechanism on the hypothalamus and anterior pituitary.

Ovulation and Oocyte Release

• The LH surge causes meiosis I to be completed, resulting in a primary oocyte and first polar body.
• Granulosa cells are stimulated to produce more follicular fluid, and the ovarian wall weakens.
• Smooth muscle contractions begin in the theca externa, leading to increased pressure and wall weakening, causing the follicle to rupture at the stigma.
• The oocyte, along with its surrounding corona radiata and follicular fluid, is expelled and adheres loosely to the ovary surface.
• The oocyte must be fertilized within about 24 hours or it begins to degenerate.

Hormonal Function and Fate of the Corpus Luteum

• In the absence of pregnancy, the corpus luteum secretes progesterone for 10-12 days after ovulation due to the LH surge.
• Without further LH stimulation, the corpus luteum cells stop producing steroids and undergo apoptosis.
• The decrease in progesterone leads to menstruation, the shedding of the uterine mucosa.
• Lowered estrogen levels lead to increased FSH secretion, stimulating new follicle growth and starting the next menstrual cycle.
• The regressing corpus luteum is phagocytosed by macrophages and replaced by a scar of dense connective tissue, the corpus albicans.