questions about mitochondria

Iuval Clejan clejan at mindspring.com
Sun Sep 10 11:27:29 EST 2000

Jim Cummins wrote:

> > Does this mean that the oocyte divides into an egg with a sperm and a polar
> > body, after the sperm enters it? Metaphase is before actual division, so this
> > seems to be the case. But it doesn't sound right.
> No.  Sperm entry triggers oocyte activation: calcium oscillations and
> release of cortical granule proteases that harden the zona pellucida
> against further sperm entry, plus completion of meiosis and release of the
> oocyte's second polar body.  The haploid sperm nucleus decondenses. In
> most species (except Murid rodents as far as we know) the sperm centrosome
> in the neck region acts as a mirotubule organising centre: MTs radiate
> throughout the egg cytoplasm and eventually locate and reel in the female
> haploid pronucleus. The two pronulei are brought together, nuclear
> membranes break down and the two haploid sets of chromosomes pair up on
> the first mitotic spndle.
> >

Why "No"? The only difference between what I said and what you said is the level of
detail.. Doesn't "completion of meiosis" mean second division into polar body and
the egg (which now has sperm DNA in it)? Please explain.

> > Refined hypothesis A: During telophase I and/or II of meiosis, "good"
> > mitochondria stay in the oocyte and "bad" mitochondria end up in the polar
> > body. This mechanism is responsible for resetting of the ageing clock in most
> > vertebrates. Any ideas on what it would take to test this hypothesis?
> There's no evidence to support or refute this.

I'm asking how to test the hypothesis by generating evidence which will either
support or refute it, not whether evidence already exists.

> > Hypothesis B: "Bad" mitochondria only occur at cellular senescence, or in non
> > dividing cells. Since eggs don't experience cellular senescence, there is no
> > need to reset the clock. How to test this one?
> The vast majority of eggs are never released - in humans numbers peak at 7
> million in mid-gestation  but of these only a few hundred are ever
> released. Eggs DO undergo senescence - there's a vast literature on the
> problems of the aged oocyte both in vivo and in vitro.

Thanks for this. I will look up litterature on egg senescence. But I meant
replicative senescence. The germ line doesn't have replicative senescence since
there is tellomerase present in the mitotic stage of the cells, and when they
differentiate into oocytes, they are post mitotic. Maybe the hypothesis needs to be
refined if old eggs experience more damaged mitochondria.


> --
> Jim Cummins
> Murdoch University
> <cummins at central.murdoch.edu.au>

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