A picture is worth a thousand words.
Female germ cell development can be confusing.
Part of the problem is that words can't quite capture the dramatic and complicated changes that happen to the germ cells over the course of a woman's life.
So I thought I'd try to make a more visual representation of what happens to the germ cells over time.
Here's how to use this diagram:
1. Take a look at the timeline and scroll down to see how the population of germ cells dwindles down to nothing over time.
2. Then, click on "fetal life," "birth," and "puberty" to get a little more information on what's going on at those three stages. Scroll up to read the text!
Let me know what you think!
What happens in utero?
During fetal development, oogonia proliferate, reaching a maximum number of about 7 million.
They then undergo mitosis, becoming primary oocytes. A single layer of follicular cells surrounds each primary oocyte, forming a primordial follicle.
Primary oocytes begin meiosis 1, but they stop at prophase. Most will remain frozen in prophase forever!
How many follicles are present in a newborn baby?
Most of the primordial follicles formed during fetal life undergo atresia (meaning, they just disappear).
By the time the baby is born, fewer than half of the original number of germ cells remain! Although nobody seems to agree on an exact number, a good rough estimate of the number of follicles remaining at birth is around one million per ovary.
Follicular atresia continues throughout life. By the time menopause rolls around, virtually no follicles remain.
What happens at puberty?
At puberty, menstruation begins. Every month, at the beginning of the menstrual cycle, a "team" of about 50 primordial follicles (containing primary oocytes still frozen in prophase of meiosis I) is chosen for maturation.
Although the primary oocytes themselves remain frozen in prophase I, their follicles undergo maturation, changing from primordial to primary to secondary to Graafian (mature) follicles.
Over time, most of the team members drop out (or undergo atresia). By the Graafian follicle stage, only one or two follicles remain.
And only one Graafian follicle responds to LH and undergoes ovulation.
Just before ovulation, the primary oocyte in the Graafian follicle completes meiosis I, turning into a secondary oocyte. It then begins meiosis II, stopping at metaphase, where it will remain unless fertilization occurs (at which point it will complete meiosis II and become an ovum).