The Reproduction System
High-Yield Summary
- Sex chromosomes (23rd pair): XX = female, XY = male. Y's SRY gene triggers testes development. Males are hemizygous for X-linked genes, so X-linked recessive disorders are more common/severe in males.
- Spermatogenesis: spermatogonium (diploid) → mitosis → primary spermatocyte → Meiosis I → 2 secondary spermatocytes → Meiosis II → 4 spermatids → spermiogenesis → spermatozoa. No arrest points, continuous from puberty, ~64–74 days.
- Oogenesis: oogonia all formed before birth → primary oocytes arrest in Prophase I until puberty → Meiosis I gives secondary oocyte + 1st polar body → arrests again in Metaphase II until fertilization → completes Meiosis II on fertilization → ovum + 2nd polar body. Only 1 functional egg per cycle.
- Hypothalamus (GnRH) → anterior pituitary releases FSH + LH → gonads. Males: FSH→Sertoli cells (spermatogenesis), LH→Leydig cells (testosterone). Females: FSH→follicle development/estradiol, LH surge→ovulation.
- Menstrual cycle (~28 days): follicular (FSH/estradiol rise, endometrium thickens) → ovulation (LH surge, day ~14) → luteal (corpus luteum, progesterone dominant) → menstruation (hormones fall, lining sheds) if no fertilization.
- hCG (from implanting embryo's trophoblast) rescues the corpus luteum to sustain early pregnancy — the hormone pregnancy tests detect.
Key Terms
- SRY gene
- Y-chromosome gene that triggers testes development in the embryo.
- Hemizygous
- Having only one copy of a gene (males, for X-linked genes) — no second allele to mask a recessive or dominant mutation.
- Sertoli cells
- Support developing sperm within seminiferous tubules; targeted by FSH.
- Leydig cells
- Secrete testosterone outside the seminiferous tubules; targeted by LH.
- Corpus luteum
- Structure formed from the ruptured follicle after ovulation; secretes progesterone (+ some estrogen) to maintain the endometrium.
- hCG (human chorionic gonadotropin)
- Hormone from implanting embryo's trophoblast cells; rescues the corpus luteum, detected by pregnancy tests.
- Zona pellucida / corona radiata
- Glycoprotein layer (sperm binding/acrosome reaction) and granulosa-cell layer surrounding a developing oocyte.
Spermatogenesis vs. Oogenesis
| Spermatogenesis | Oogenesis |
|---|---|
| Begins at puberty, continues for life | Set up entirely before birth |
| No meiotic arrest points | Arrests at Prophase I (until puberty) and Metaphase II (until fertilization) |
| 4 functional sperm per cycle | 1 functional egg per cycle (rest become polar bodies) |
Menstrual Cycle Phases
- 1Follicular phase — estrogen/progesterone low, GnRH/FSH/LH rise, follicles mature, dominant follicle raises estradiol, endometrium thickens.
- 2Ovulation (~day 14) — sustained high estradiol flips feedback positive, triggers LH surge, dominant follicle ruptures and releases a secondary oocyte.
- 3Luteal phase (~14 days) — corpus luteum secretes progesterone (dominant), stabilizes endometrium, suppresses GnRH/FSH/LH.
- 4Menstruation — if no fertilization, corpus luteum → corpus albicans, hormones fall, endometrium sheds; cycle restarts.
Common MCAT Trap
- FSH targets Sertoli cells (spermatogenesis) in males but drives follicle development in females — same hormone, different target cell, different downstream effect.
- Oogenesis's two arrest points (Prophase I until puberty, Metaphase II until fertilization) are easy to swap — know which stage pauses for which trigger.
- The LH surge (not FSH) triggers ovulation, caused by sustained HIGH estradiol flipping the hypothalamic/pituitary feedback from negative to positive.
Quick Recall
Why are X-linked recessive disorders more common in males?
What hormone triggers ovulation, and what causes its surge?
At what two points does oogenesis arrest?
What hormone maintains the corpus luteum if fertilization occurs?