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Why are polar bodies produced during oogenesis in human females?

I read that polar bodies are produced during meiosis so that the oogonium (or gamete mother cell) can be converted into a haploid female gamete or ovum by getting rid of the extra genetic material.

So, it is pretty clear that one follicle (containing one gamete mother cell) produces only one ovum.

So what is the point of producing polar bodies? Isn't it just a better mechanism over all for the meiotic devision of the oogonium to result in equal distribution of the cytoplasmic content to both daughter cells? That way, one oogonium can produce four ova, not just one. Th entire process of DNA replication during the synthesis phase (S phase) of the meiotic cell cycle seems like such a waste if 75% of the total genetic material produced is going to be expelled as polar bodies.

There is also the argument that since the ovum is non-motile and always within the female body, it need not be produced in large numbers as it is in a safe and protected environment. But, in that case, would it not be a better system is fewer follicles were produced, but each follicle divided to give four ova? This mechanism would end up producing the same amount of ova, but with lesser labour (such as DNA replication) going waste.

So, taking these arguments into consideration, what exactly is the function of the polar bodies produced?

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  • $\begingroup$ You're saying about multiple eggs. I've a similar feeling too. You should've studied that: initially there are many follicle ( primary follicles) which are selected for oogenesis. Only one of them develops into mature graffian follicle. Can you tell how and why other follicles are kicked out from the race? All of them were in same ovary, close to each other; what caused them to regressed except the one? Should it be better for body to develop all those into mature follicle, each containing a separate egg? If you can answer this, you probably will get your own. $\endgroup$ – علی آفاق Feb 10 '16 at 17:51
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"In other animals and some plants, polar bodies give rise to vital tissues that protect and nourish the embryo " ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3164815/). This could be the same fucntion to humans since we are all eukaryote.

Polar bodies are not waste. They sacrifice themselves after giving their energy to the embryo, completing development and differentiation into a baby.

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Polar bodies are not just mere discarded material in oogenesis as recent experiments have determined its key role in leading to stable embryo development.

Reference:

http://www.ncbi.nlm.nih.gov/pubmed/16102284/

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3164815/

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3663962/

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  • $\begingroup$ I appreciate your quick response. It would be nice if you can actually add what exactly polar body does. You can state and then quote (Ctrl+Q the selected text) from the article. I'll be glad to upvote then. $\endgroup$ – WYSIWYG Feb 10 '16 at 17:34
  • $\begingroup$ Please explain more fully. It would be helpful if you could briefly write the crux of the answer. $\endgroup$ – user21723 Feb 13 '16 at 13:19
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First of all, let me say that you have asked a really nice question!!

Now, if(according to your argument), each oogonium were to produce 4 ova each, then the cytoplasmic content of each ova would remain less. However, the ova has to be rich in cytoplasmic contents in order to compensate for the lesser amount of cytoplasm in the sperm, so that the zygote has enough of cytoplasm. Therefore, there is an unequal division of the oogonium, so that the ovum is larger.

Coming to your second argument, well, the oogonia are produced in large numbers as a preventive measure. What happens is that around 2 million oogonia are produced while the female is still in the fetal stage. No more oogonia can be produced after birth because of the formation of tunica albuginea between germinal epithelium and stroma of ovary. These oogonia develop into primary follicles containing primary oocytes. However, they undergo follicular atresia, resulting in a reduction of the number of follicles. As a result only 60000 to 80000 follicles are found at puberty. There is no guarantee that all of these will develop into graafian follicles, hence the large number.

Also, during ovulation, it is the secondary oocyte which is released into the fallopian tube, and not the ovum. Only when the sperm penetrates the zona pellucida of secondary oocyte, does it complete its meiosis to form an ovum and another polar body. So, if each oogonium were to produce 4 ova, then each secondary oocyte would produce two ova, and that would mean 2 ova present simultaneously in the fallopian tube, both of which would be fertilised. This would mean that humans would always bear 2 children at a time!!

I hope the above counter arguments will have satisfied you...

Cheers!!

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