Anatomy and Physiology of the Ovary

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🌸 Roles

• Endocrine function: Production of steroid hormones - oestrogen, progesterone, and androgens.
• Reproductive function: Gametogenesis (development and release of oocytes).

🔬 Gross Anatomy

• Paired almond-shaped organs located on either side of the uterus in the lateral pelvic wall.
• Average size in reproductive years: 4 × 3 × 2 cm; weight approximately 8–10 g.
• Lie close to the fimbrial end of the fallopian tubes, facilitating oocyte capture at ovulation.
• Attached to the posterior layer of the broad ligament by the mesovarium, a peritoneal fold carrying vessels, lymphatics, and nerves.
• Suspended laterally by the suspensory ligament of the ovary (infundibulopelvic ligament), which contains the ovarian vessels.
• Medially attached to the uterus via the ovarian ligament.

🩸 Blood Supply and Drainage

• Arterial supply: Ovarian arteries arise directly from the abdominal aorta just below the renal arteries.
• Additional contribution from the uterine artery (anastomotic supply).
• Venous drainage: Ovarian veins form the pampiniform plexus; right drains into the IVC, left drains into the left renal vein.
• Lymphatic drainage: Para-aortic (lumbar) lymph nodes.

🏗️ Microscopic Structure

• Outer cortex: Contains follicles at various stages of development embedded in a dense stromal matrix.
• Inner medulla: Looser connective tissue containing blood vessels, lymphatics, and nerves.
• Surface covered by a single layer of cuboidal epithelium (historically termed “germinal epithelium”).
• Underlying dense fibrous capsule called the tunica albuginea.
• Primordial oocytes supported by stromal connective tissue and follicular cells.

🔄 Physiology: Follicular Development and Ovulation

• At menarche, approximately 400,000–500,000 primordial follicles remain in the ovarian cortex.
• Primordial follicles (~0.1 mm) consist of an oocyte surrounded by a single layer of granulosa cells.
• Granulosa cells produce oestradiol (via aromatisation of theca-derived androgens).
• Stromal cells differentiate into:
  • Theca interna: Androgen-producing layer (LH-responsive).
  • Theca externa: Structural layer with no endocrine function.
• During each menstrual cycle, multiple follicles are recruited under FSH influence, but usually one dominant follicle develops into a mature Graafian follicle.
• The mature follicle reaches ~20 mm in diameter before ovulation.
• The LH surge triggers completion of meiosis I:
  • Primary oocyte (46 chromosomes) divides.
  • Produces a secondary oocyte and first polar body (each haploid - 23 chromosomes).
• Ovulation releases the secondary oocyte into the peritoneal cavity, usually captured by the fimbriae of the fallopian tube.

🌕 Corpus Luteum Formation

• After ovulation, the ruptured follicle collapses.
• Granulosa cells become granulosa lutein cells.
• Theca interna becomes theca lutein cells.
• Forms the corpus luteum, visible as a yellow, crinkled structure.
• Secretes progesterone and oestrogen to support endometrial preparation.
• If no fertilisation:
  • Degenerates after ~10–14 days.
  • Forms a fibrotic scar - the corpus albicans.
• If pregnancy occurs:
  • Corpus luteum enlarges (up to 3 cm).
  • Persists for 8–12 weeks under hCG stimulation before placental takeover.

🥚 Oocyte Numbers Across the Lifespan

• ~7 million primordial germ cells at 15 weeks’ gestation.
• ~2 million at birth.
• ~400,000–500,000 at puberty.
• Approximately 400–500 ovulated during reproductive life.
• Remaining follicles undergo atresia.

🧠 Key Physiological Concepts

• Follicular development is regulated by the hypothalamic–pituitary–ovarian axis (GnRH → FSH/LH → ovarian hormones).
• Ovarian steroid production operates via the two-cell, two-gonadotropin model (LH stimulates theca → androgens; FSH stimulates granulosa → aromatisation to oestrogen).
• Menopause results from depletion of functional follicles, leading to reduced oestrogen and loss of negative feedback on FSH/LH.