Aug 2, 2014
This was a difficult concept for me to understand, and it's pretty shaky. Can someone kindly confirm if this is correct:

So in utero, the primordial germ cells (endoderm) lining the yolk sac go through the umbilical cord into the urogenital ridge (mesoderm) stimulating the gonadal formation. Lack of SRY gene on short arm of Y chromosome means no Testes Determining Factor, so the central part of the urogenital ridge becomes the ovary (outer part forms the kidneys & ureters). Also in utero, the primordial germ cells are induced to become Type A Oogonia which are the stem cells (?) and can undergo mitosis to produce more of themselves (Type A Oogonia) or to produce Type B Oogonia (46 chromosomes, 2n). Type B Oogonia undergo mitosis to double their chromatids and become primary oocytes (46 chromosomes, 4n). Primary oocytes begin mieosis 1 but become arrested in Prophase 1. So by 5th month of gestation, the female fetus has produced all of the primary oocytes that she will ever have in her lifetime, but they are all arrested at Prophase 1.

After birth until puberty - she still has primary oocytes arrested in prophase 1. At menarche, once a month, one of her primary oocytes will become un-arrested and continues meisosis 1 and forms a secondary oocyte (and a polar body). The secondary oocyte starts meiosis 2 then stops at Metaphase 2. If there is no fertilization, the secondary oocyte dies. If there is fertilization, then the secondary oocyte continues to complete meiosis 2 and forms an ovum.

My major issue is with the follicles.
The primordial follicle contains granulosa cells surrounding the primary oocyte arrested at Prophase 1. According to what I read, at puberty, in response to FSH, 20 of the primary oocytes arrested in prophase 1 of the primordial follicles will complete meiosis 1 and become arrested in Metaphase 2 as secondary oocytes, becoming the Primary Follicle. I've also read that after puberty, one of your primary oocytes gets arrested, but then why are so many primordial follicles becoming primary follicles (I get that only one of those follicles will eventually become the mature follicle and undergo ovulation while the rest become atretic, but doesn't that mean that atleast 20 other primordial follicles mature every month, so why does Kaplan say it's only 1 primary oocyte that gets unarrested every month)?

My main question is when (at what stage of follicle development) does a primary oocyte become a secondary oocyte? And which is ovulated out of the ovary - a primary or a secondary oocyte?
Last edited:
Mar 5, 2014
Medical Student
i skimmed this but your second question is secondary oocyte(you answered your own question) secondary oocyte is whats ovulated bc only 1 primary oocyte completes meisois 1 to form a secondary oocyte to be released
Aug 2, 2014
Ok that makes sense so that means that a primary oocyte becomes a secondary oocyte when the graffian follicle is formed because all the other follicles will become atretic. So that way you only have one oocyte completing meiosis 1 and becoming a secondary oocyte which is ovulated.