Oogenesis - Primary Oocyte formation occurs only prior to birth?

[GomerPyle]

I just want to make clear I understand this correctly...

Oogenesis begins prior to birth when ovarian cells (oogonia stem cells) go through mitosis to create primary oocytes. The formation of primary oocytes continues until the birth, where it halts and never happens again. (This halt occurs at Prophase 1 of Meiosis 1). That person has now a set number of primary oocytes for the rest of her life.

Nothing happens until Puberty, when Meiosis 1 begins and secondary oocytes are formed. The secondary oocytes do not go through meiosis 2 until penetrated by sperm. Once penetrated by sperm, the secondary oocytes form a polar body and an ovum.

I am not sure when secondary oocytes are halted in Meiosis 2 until penetration. I hear that it halts at Metaphase 2, but why? Why not halt at prophase 2? Just makes it more confusing...


Is this correct?

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[MJS4878]

This is correct. The sperm cell penetrating the secondary oocyte stimulates anaphase II.

 

[GomerPyle]

This is correct. The sperm cell penetrating the secondary oocyte stimulates anaphase II.

How does it stimulate anaphase 2? Why is the secondary oocyte halted at metaphase 2 rather than the beginning of meiosis 2 (prophase 2)?

 

[gettheleadout]

How does it stimulate anaphase 2? Why is the secondary oocyte halted at metaphase 2 rather than the beginning of meiosis 2 (prophase 2)?

The secondary oocytes are arrested in metaphase II of meiosis II after ovulation. I have no idea why metaphase II specifically, and I really wouldn't worry about knowing why, just know it. I know that's lame, but I imagine the answer is complicated. It's not explained on this page: http://php.med.unsw.edu.au/embryology/index.php?title=Oocyte_Development

And I haven't seen it explained anywhere else. When the sperm penetrates the oocyte membrane, the presence of the sperm nucleus in the cell triggers the cortical reaction, which seals off the oocyte to other sperm and also resumes meiosis II. To say it "stimulates anaphase II" may be too specific, since we can't say the secondary oocyte was arrested right at the very end of metaphase II. It just resumes where it left off in meiosis II. The sperm pronucleus just sits in the oocyte while meiosis II finishes up and the second polar body is released.

As a note on terminology here, the word "ovum" sucks. It's used differently by every prep book, but all of them fail to mention the fact that it can refer to an oocyte at any point in development (primary, secondary, or post-fertilization).

 

[Laureatebarrel]

Was the primary oocyte halted at Prophase I of meiosis I?

 

[gettheleadout]

Was the primary oocyte halted at Prophase I of meiosis I?

Was it? Yes. Why was it? In general it's because the sexually immature female organism has no use for further maturation of the oocytes until puberty. Specifically why Prophase? No idea.

 

[GomerPyle]

Hmm..well okay, more stuff to "memorize".

Thank you!

 

[sshah92]

Even though this was asked years ago, in case anyone was wondering-

There are a set of cyclin and cyclin-dependent kinases that regulate the transition from phase to phase (G1 to S, or S to G2, and so forth) in both mitosis and meiosis. (Cyclin concentrations fluctuate throughout this cycle and are synthesized and degraded as need be. The kinase concentration stays ~constant throughout the cycle.) One of these complexes, the Master Cyclin-Dependent Kinase (MCDK) will exhibit increased activity during the M phase, and optimal activity will be during metaphase.

The MCDK is what makes everything happen permissive to cell division, specifically all the phosphorylation events necessary. It allows for phosphorylation of the lamins, which cause nuclear envelope breakdown (that triggers prometaphase), for example. But due to its lability, it needs to be stabilized -- and it's stabilized by a cytostatic factor protein called Emi.

During fertilization, when sperm penetrates the ovum via the acrosome reaction, there is an influx of calcium into the ovum which destroys the cytostatic factor. This destabilizes the MCDK (M cyclin/kinase), allowing meiosis to proceed through anaphase and onwards.