Meiosis

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yaracuy

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Human female cells at what point in time stop going trough meiosis ?

Due all mammals females produce only a certain number of ova ?
 
yaracuy said:
Human female cells at what point in time stop going trough meiosis ?

Due all mammals females produce only a certain number of ova ?
Click to expand...
It is believed that females are born with all the "eggs" they'll ever have.

Oogenesis


Egg formation takes place in the ovaries.

In contrast to males, the initial steps in egg production occur prior to birth. Diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes. By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed. By the time she is born, 1–2 million of these remain. Each has begun the first steps of the first meiotic division stopping at the diplotene stage of meiosis I.

No further development occurs until years later when the girl becomes sexually mature. Then the primary oocytes recommence their development, usually one at a time and once a month.

The primary oocyte grows much larger and completes meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.

In humans (and most vertebrates), the first polar body does not go on to meiosis II, but the secondary oocyte does proceed as far as metaphase of meiosis II and then stops.

Only if fertilization occurs will meiosis II ever be completed. Entry of the sperm restarts the cell cycle

breaking down MPF (M-phase promoting factor) and
turning on the anaphase-promoting complex (APC).
Completion of meiosis II converts the secondary oocyte into a fertilized egg or zygote (and also a second polar body).
Click to expand...

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Sexual_Reproduction.html
 
The prevailing paradigm in the last several decades for female primate gametogenesis is that germ cells undergo meiosis during embryonic development to form oogonia that are stalled at the final stage of meiosis. The total number of stalled oogonia that are present at the end of ovary development represent the total ova that the female is theoretically capable of producing (but will never actually produce) during her reproductive lifetime.

During each estrous cycle, one of the oogonia in one of the ovaries will complete the final meiotic division to produce a haploid mature oocyte (ovum) and is released from the ovary (*).

However, in the past several years there have been some prominent published experiments that cast doubt on this prevailing paradigm. (I don’t have the details at hand; I can try to locate the papers if you are really keen.) These experiments suggest that mammalian ovary stem cells are capable of limited mitosis to replenish the stock of germ cells that can subsequently undergo meiosis to produce oocytes. I left this field several years back so I haven’t kept up with this interesting topic. My understanding is that this has not yet been widely accepted enough to say that the prevailing paradigm has been overthrown.


(*) Yes yes I know, sometimes there is two from one or one from each. :p
 
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