Atresia in Ovarian Follicles

Atresia

• Involves programmed cell death of follicles that don't reach maturity during the menstrual cycle.
• Stages of atresia:
  • Granulosa cells detach
  • Oocyte undergoes self-digestion
  • Zona pellucida collapses
  • Macrophages invade and phagocytose apoptotic material and debris

Ovulation

• Process where an oocyte is released from the ovary.
• Pre-ovulation:
  • Dominant follicle grows and bulges at the outer layer of the ovary (tunica albuginea).
  • A translucent area called the Stigma appears on the follicle due to tissue compaction and blocked blood flow.
• Just before ovulation:
  • Oocyte completes its first meiotic division, which it had started and paused during fetal development.
  • Division gives rise to two cells: a secondary oocyte and a first polar body.
• Secondary oocyte begins the second division but stops at metaphase, only completing it if fertilized.

FSH, GnRH, and Estrogen

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

LH Surge

• Causes meiosis I to be completed, resulting in primary oocyte and first polar body.
• Stimulates granulosa cells to produce more follicular fluid.
• Weakens the ovarian wall.
• Initiates smooth muscle contractions in the theca externa.

Oocyte Release and Fertilization

• Oocyte, along with its surrounding corona radiata and follicular fluid, is expelled and adheres loosely to the ovary surface.
• Oocyte must be fertilized within about 24 hours or it begins to degenerate.

Hormonal Function and Fate of the Corpus Luteum

• Non-Pregnancy Scenario:
  • Corpus luteum secretes progesterone for 10-12 days after ovulation due to the LH surge.
  • Without pregnancy and further LH stimulation, the corpus luteum cells stop producing steroids and undergo apoptosis.
  • 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.
  • Regressing corpus luteum is phagocytosed by macrophages and replaced by a scar of dense connective tissue, the corpus albicans.
• Pregnancy Scenario:
  • If pregnancy occurs, the trophoblast cells of the embryo produce human chorionic gonadotropin (hCG), which maintains the corpus luteum.
  • hCG promotes the corpus luteum to continue producing progesterone, essential for maintaining the uterine mucosa and preventing menstruation.
  • Corpus luteum grows larger and is maintained by hCG for 4-5 months until the placenta takes over progesterone production.
  • By 4-5 months, the placenta produces enough progesterone (and estrogens) to maintain the uterine mucosa, after which the corpus luteum degenerates into a large corpus albicans.

Atresia

• Involves programmed cell death of follicles that don't reach maturity during the menstrual cycle.
• Stages of atresia:
  • Granulosa cells detach
  • Oocyte undergoes self-digestion
  • Zona pellucida collapses
  • Macrophages invade and phagocytose apoptotic material and debris

Ovulation

• Process where an oocyte is released from the ovary.
• Pre-ovulation:
  • Dominant follicle grows and bulges at the outer layer of the ovary (tunica albuginea).
  • A translucent area called the Stigma appears on the follicle due to tissue compaction and blocked blood flow.
• Just before ovulation:
  • Oocyte completes its first meiotic division, which it had started and paused during fetal development.
  • Division gives rise to two cells: a secondary oocyte and a first polar body.
• Secondary oocyte begins the second division but stops at metaphase, only completing it if fertilized.

FSH, GnRH, and Estrogen

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

LH Surge

• Causes meiosis I to be completed, resulting in primary oocyte and first polar body.
• Stimulates granulosa cells to produce more follicular fluid.
• Weakens the ovarian wall.
• Initiates smooth muscle contractions in the theca externa.

Oocyte Release and Fertilization

• Oocyte, along with its surrounding corona radiata and follicular fluid, is expelled and adheres loosely to the ovary surface.
• Oocyte must be fertilized within about 24 hours or it begins to degenerate.

Hormonal Function and Fate of the Corpus Luteum

• Non-Pregnancy Scenario:
  • Corpus luteum secretes progesterone for 10-12 days after ovulation due to the LH surge.
  • Without pregnancy and further LH stimulation, the corpus luteum cells stop producing steroids and undergo apoptosis.
  • 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.
  • Regressing corpus luteum is phagocytosed by macrophages and replaced by a scar of dense connective tissue, the corpus albicans.
• Pregnancy Scenario:
  • If pregnancy occurs, the trophoblast cells of the embryo produce human chorionic gonadotropin (hCG), which maintains the corpus luteum.
  • hCG promotes the corpus luteum to continue producing progesterone, essential for maintaining the uterine mucosa and preventing menstruation.
  • Corpus luteum grows larger and is maintained by hCG for 4-5 months until the placenta takes over progesterone production.
  • By 4-5 months, the placenta produces enough progesterone (and estrogens) to maintain the uterine mucosa, after which the corpus luteum degenerates into a large corpus albicans.