Oogenesis in mammals

Published on June 30, 2019   55 min

Other Talks in the Series: The Female Reproductive System: from Basic Science to Fertility Treatments

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Hello, my name is Hugh Clarke. I'm a professor at McGill University, and I'm going to talk to you today about "Oogenesis in Mammals". The process of oogenesis in mammals actually begins before birth. I'm going to talk to you about the process of oogenesis afterbirth. Before I do so I'll just say a little bit about what happens before birth.
Before birth, the female germ cells which are known as oogonia proliferate to generate a large starting population of oogonia cells. Then, again before birth these oogonia, enter into the mitotic phase of the cell cycle. At this stage we call them oocytes. So these oocytes, which are now in meiosis begin the process of recombination, and they progress as far as diplotene of the meiotic cell cycle. At this point, they become arrested. So this is before the birth of the female, and they'll remain arrested at diplotene until the time of ovulation, which could be many months or in the case of women many years later. By the time of birth or shortly after birth, all of the oocytes have been assembled into what we call primordial follicles. Each primordial follicle consists of one oocyte surrounded by a small number of somatic follicular cells that will term granulosa cells. So this means that at the time of birth or shortly after birth, the ovary of the female is populated by thousands or hundreds of thousands of primordial follicles. Before I explain to you the process of oocyte development after birth, I just want to say a few words about what happens to oocytes before birth or female germ cells before birth.
During embryonic development, the female generates an enormous number of oogonia and eventually, oocytes. So you can see that midway through fetal development, a human female has approximately seven million oocytes. By the time of birth, however, the number of oocytes has dropped to around two million, and by the time of puberty, there is in the neighborhood of a quarter of a million oocytes remaining. Well, that's plenty because during her lifetime, a human female will only ovulate about 500, so a quarter of a million is plenty. But it remains a mystery why they're such a drastic decline in the number of oocytes from around seven million to around a quarter of a million before puberty. That mystery, why so many oocytes are lost even before puberty, remains unsolved. Now, I'm going to move to talk to you about the development of oocytes after birth.