What happens to all the female germ cells?

 

Here’s a drawing that shows what happens to the germ cells over the course of a female’s life, from fetal life until menopause. I think it helps to see the numbers of germ cells (and their stages of development) over time.

Scroll down to see the full image – and click on the red highlighted ages to see more information.

Some things to notice:

  • By the time baby is born, all her oogonia are gone! They’ve all transformed into primary oocytes. All of these primary oocytes have begun meiosis I, but have been halted at prophase.
  • Almost all of these primary oocytes will remain halted at prophase forever.
  • Only a few (about 50) primary oocytes are selected at the beginning of each menstrual cycle; and although these 50 primary oocytes all begin the process of follicular maturation (they change their follicles from primordial to primary to secondary etc.), only ONE of them will respond to the LH surge that initiates ovulation! That primary oocyte (now in a Graafian follicle) will complete meiosis I, turning into a secondary oocyte (still within the same Graafian follicle).
  • The unselected primary oocytes just sit there, in prophase of meiosis I, until they degenerate (this is called “atresia”). As you can see from the image, the number of primary oocytes dwindles over time – and by menopause, all primary oocytes are gone.

What happens to all the follicles?
What happens in utero? How many follicles are present in a newborn baby? What happens at puberty?

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! Only one primary oocyte each month is allowed to complete meiosis 1 and turn into a secondary oocyte.

How many follicles are present in a newborn baby?

Many of the primordial follicles formed during fetal life undergo atresia. 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, 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 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).