What happens to all the female germ cells?
Here’s a little semi-interactive drawing I put together that shows what happens to the germ cells over the course of a female’s life, from fetal life until menopause.
The directions for how to get the most out of this diagram are listed under the left hand column below, entitled “A picture is worth a thousand words.” Basically, just start at the top, and scroll down to see how the number of germ cells changes at different ages. And then hover over ages to see if there’s a red oval, which means there’s a little more explanation about what happens at that particular age.
Then, SCROLL WAY, WAY DOWN to the bottom of the page to see the TAKEAWAY POINTS!
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Here are the main takeaway points.
1. By the time baby is born, all her oogonia are gone!
They’ve all transformed into primary oocytes (which are stuck in prophase I).
2. Almost all of these primary oocytes will remain halted at prophase forever.
Only a few (around 50) are selected for maturation at the beginning of each menstrual cycle.
3. Most of the 50 primary oocytes selected each month end up dying – only one makes it to ovulation.
All 50 primary oocytes in the chosen group start the process of follicular maturation – but along the way, most of them die. Only one actually makes it to the Graafian follicle stage. When LH surges, that lonely primary oocyte (in its Graafian follicle) completes meiosis I, and becomes a secondary oocyte, which is then ovulated.
4. Every year, starting at birth, the population of primary oocytes dwindles.
At birth, there are around 2 million primary oocytes – but by puberty, there are only around 400,000 left. At that point, a measly 50 primary oocytes are given a shot at maturation each month. The rest just sit there, wondering if they’ll be chosen next time. Meanwhile, they keep dying off – and by menopause, there are no primary oocytes left.