Reproductive System

Gametogenesis

• Male: spermatogonium (2n) to primary spermatocyte to secondary spermatocyte (n) to spermatid to spermatozoon — four functional sperm per meiosis, continuous from puberty. Sertoli cells (FSH) nourish developing sperm; Leydig cells (LH) make testosterone.
• Female: the primary oocyte arrests in prophase I before birth, completes meiosis I at ovulation, then arrests in metaphase II; meiosis II finishes only upon fertilization, yielding one ovum plus polar bodies (unequal cytoplasm division).

The Menstrual Cycle

• Follicular phase: FSH grows the follicle, estrogen rises and the endometrium proliferates. Around day 14, sustained high estrogen flips feedback from negative to positive: $\text{estrogen above threshold} \to \text{GnRH/LH surge} \to \text{ovulation}$
• Luteal phase: the corpus luteum secretes progesterone, maintaining a secretory endometrium and restoring negative feedback.

Pregnancy and Contraception

• The implanting trophoblast secretes hCG, an LH mimic that rescues the corpus luteum until the placenta takes over; hCG is what pregnancy tests detect. Without it, progesterone falls and menstruation follows.
• Combined oral contraceptives hold feedback in negative mode, so the LH surge — and ovulation — never occur.

Key Takeaways

• Oogenesis has two arrests: prophase I, then metaphase II until fertilization.
• The LH surge is a rare true positive-feedback loop.
• Progesterone maintains the endometrium; its withdrawal triggers menses.
• hCG rescue of the corpus luteum is the hormonal signature of early pregnancy.