Problem
Trace the complete life history of an oocyte from its formation in fetal life to the production of a mature ovum, specifying the stage, ploidy, and arrest conditions at each step.
Solution
Step 1 — Fetal life (~weeks 10–20 of gestation)
- Primordial germ cells migrate to the gonad → become oogonia (2n)
- Oogonia divide by mitosis → millions of oogonia formed
- Oogonia enter meiosis I → become primary oocytes (2n)
- PRIMARY OOCYTES ARREST AT PROPHASE I
- Surrounded by granulosa cells → primordial follicles formed
- ~2 million primary oocytes at birth
Step 2 — From birth to puberty
- Primary oocytes remain arrested at prophase I
- Atresia reduces pool to ~60,000–80,000 by puberty
Step 3 — Each menstrual cycle (from puberty)
- FSH from anterior pituitary stimulates a cohort of follicles
- One dominant follicle grows to Graafian follicle
- LH surge → primary oocyte resumes meiosis I
- Meiosis I completes → secondary oocyte (n) + polar body I
- Secondary oocyte ARRESTS AT METAPHASE II
Step 4 — Ovulation (~day 14)
- Graafian follicle ruptures → secondary oocyte (n) released
- Fimbriae collect it → it enters the fallopian tube
Step 5 — If fertilized (in the ampulla)
- Sperm penetrates zona pellucida (acrosomal reaction)
- Cortical reaction occurs (polyspermy block)
- wave → metaphase II arrest breaks
- Meiosis II completes → mature ovum (n) + polar body II
- Ovum nucleus (female pronucleus) + sperm nucleus (male pronucleus) → fuse → zygote (2n)
Answer Summary
Oogonium (2n) → Primary oocyte (2n, arrested Prophase I) → Secondary oocyte (n, arrested Metaphase II) → Mature ovum (n, only after sperm entry)