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.

Learn about the process of atresia in ovarian follicles, including the stages of cell death and the role of macrophages. Understand how it relates to the menstrual cycle and oocyte development.