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Gametogenesis is the formation of haploid gametes (eggs and sperm) from diploid germ cells through meiosis. Oogenesis and spermatogenesis differ dramatically in timing, output, and the type of errors they generate, differences that explain the maternal age effect on aneuploidy and why most trisomies are maternal in origin.
- Begins in fetal life: primordial germ cells migrate to the ovary and enter meiosis I by 8–13 weeks' gestation.
- Arrests in prophase I (dictyate stage) before birth. All of a female's oocytes (about 1–2 million at birth, about 400 ovulated in a lifetime) are arrested at this point.
- Completion is triggered each cycle: the LH surge completes meiosis I at ovulation; meiosis II completes only upon fertilization.
- Asymmetric division: each meiotic division produces one large secondary oocyte/ovum and a small polar body. Net yield: one ovum per primary oocyte over 10–50 years.
- Begins at puberty and continues throughout adult life.
- Spermatogonia (stem cells) divide mitotically; some commit to meiosis and become primary spermatocytes.
- Meiosis I and II proceed continuously, no arrest. Full cycle takes about 74 days.
- Symmetric division: each primary spermatocyte yields 4 spermatids → 4 spermatozoa after maturation.
- High cell-division count means spermatogenesis accumulates point mutations with age (paternal age effect).
| Feature | Oogenesis | Spermatogenesis |
|---|---|---|
| Start | Fetal life | Puberty |
| Arrest | Prophase I (decades) | None |
| Gametes per precursor | 1 | 4 |
| Size | Large, nutrient-rich | Small, motile |
| Dominant error type | Nondisjunction (aneuploidy) | Point mutations (new AD mutations) |
| Age effect | Aneuploidy risk rises after 35 | New dominant mutations rise after 45 |
- Maternal age effect: prolonged meiotic arrest degrades cohesin proteins holding chromatids together, increasing nondisjunction. Trisomy 21 incidence rises from about 1 in 1,200 at age 25 to about 1 in 100 at age 40.
- Paternal age effect: continued cell division raises the rate of new point mutations. Associated with achondroplasia (FGFR3), Apert syndrome (FGFR2), neurofibromatosis type 1, and a subset of autism-spectrum disorders.
- Most trisomies are maternal in origin (>90% of trisomy 21). An exception is 47,XYY, which is always paternal (meiosis II error).
- Female gametes are "old eggs" (aneuploidy risk); male gametes are "new errors" (point mutation risk).
- The second polar body extrudes at fertilization, not before.
- MII arrest (metaphase II) is held by cytostatic factor until fertilization releases it.