oocyte - Meiosis I

A mature oocyte (egg cell) does not complete meiosis right after ovulation. Here’s the timeline in simple steps:

1. Before birth – Primary oocytes begin meiosis I but stop in prophase I (they stay paused for years).

2. At puberty – Each menstrual cycle, under FSH and LH signals, some primary oocytes continue meiosis I.

   • One usually completes meiosis I just before ovulation → producing a secondary oocyte + a small first polar body.

3. At ovulation – The secondary oocyte starts meiosis II but stops again in metaphase II.

4. Completion of meiosis II – This only happens after fertilization by a sperm. Then, the oocyte quickly finishes meiosis II, producing a mature ovum and a second polar body.

👉 So to answer clearly:

• A mature oocyte completes meiosis I just before ovulation, not after.

• It only completes meiosis II after fertilization.