Oogenesis and follicular development

Oogenesis - Egg Genesis Journey

The process of creating an ovum (egg cell) begins in fetal life and halts in specific stages until puberty and fertilization.

• Before Birth: Oogonia (diploid stem cells) multiply via mitosis. They then develop into primary oocytes, which begin Meiosis I but are arrested in Prophase I.

⭐ Exam Favorite: Primary oocytes are arrested in prophase I for years, from birth until just before ovulation. This prolonged suspension increases the risk of meiotic errors with advancing maternal age.

• Puberty to Menopause: Each month, an LH surge prompts one primary oocyte to complete Meiosis I, producing a large secondary oocyte and a small polar body. The secondary oocyte is then arrested in Metaphase II and ovulated.
• Fertilization: Meiosis II is only completed if the oocyte is fertilized, resulting in a mature ovum and a second polar body.

📌 Mnemonic: 'Prophase I until Puberty, Metaphase II until Mating.'

Folliculogenesis - Follicle Formation Fun

📌 Please Pass Some Gravy: Primordial → Primary → Secondary → Graafian

⭐ The Zona Pellucida, a glycoprotein layer secreted by the oocyte and granulosa cells, is essential for sperm binding and preventing polyspermy.

Hormonal Control - Ovary's Orchestra

Based on the Two-Cell, Two-Gonadotropin Theory:

• Theca Cells (LH receptors): Stimulated by LH to produce androgens (androstenedione) from cholesterol.
  • 📌 Mnemonic: 'Luteinizing makes Leydig/Theca Luscious with androgens.'
• Granulosa Cells (FSH receptors): Stimulated by FSH to express aromatase, which converts androgens from theca cells into estrogen.
• Feedback Loop: Estrogen initially provides negative feedback on the HPO axis, inhibiting GnRH, FSH, and LH release.

⭐ The mid-cycle surge in estrogen from the dominant follicle switches from negative to positive feedback on the pituitary, causing the LH surge.

Ovulation - The Great Escape

• LH Surge: The primary trigger, leading to ovulation within 24-36 hours.
• Meiosis I Completion: The primary oocyte completes its first meiotic division, yielding a secondary oocyte and the first polar body.
• Follicular Rupture: The mature follicle wall breaks down under the influence of enzymes (e.g., collagenase) and prostaglandins, releasing the oocyte.
• Corpus Luteum Formation: Post-ovulation, the remaining granulosa and theca cells form the corpus luteum, a temporary endocrine structure.
  • It primarily secretes progesterone and some estrogen, essential for the luteal phase.

⭐ The corpus luteum's primary function is to secrete progesterone, which is essential for preparing the endometrium for implantation and maintaining early pregnancy.

High‑Yield Points - ⚡ Biggest Takeaways

• Primary oocytes are arrested in prophase I of meiosis from fetal life until puberty.
• The LH surge triggers the completion of meiosis I, forming a secondary oocyte.
• This secondary oocyte is then arrested in metaphase II until fertilization.
• Fertilization is required for the completion of meiosis II to form a mature ovum.
• FSH stimulates follicular growth, but only one dominant follicle is typically selected each cycle.
• The corpus luteum, formed post-ovulation, is essential for producing progesterone.