Xref: utzoo sci.bio:1248 sci.med:5785
Path: utzoo!attcan!uunet!husc6!mailrus!iuvax!silver!chiaravi
From: chiaravi@silver.bacs.indiana.edu (Lucius Chiaraviglio)
Newsgroups: sci.bio,sci.med
Subject: Re: Mortality
Summary: Females are born with many oocytes, but many of these die before puberty
Keywords: aging, DNA damage, fertility, evolution of external testicles
Message-ID: <1901@silver.bacs.indiana.edu>
Date: 14 Jun 88 07:06:25 GMT
References: <36@feedme.UUCP> <1894@silver.bacs.indiana.edu> <1898@silver.bacs.indiana.edu> <4810@blia.BLI.COM>
Reply-To: chiaravi@silver.UUCP (Lucius Chiaraviglio)
Organization: Department of Molecular, Cellular, and Developmental Biology at Indiana University, Bloomington
Lines: 73

In article <4810@blia.BLI.COM> heather@blia.BLI.COM (Heather Mackinnon) writes:
>In article <1898@silver.bacs.indiana.edu>, chiaravi@silver.bacs.indiana.edu
>(Lucius Chiaraviglio) writes:
>> Animals become less fertile or infertile with age as the gametes quit
>> functioning due to somatic aging, and also in females because of running out
>> of eggs (no more eggs are made after the early part of life (before birth in
>> humans)).
>
>Human females are some of the only animals who outlive their time of
>sexual fertility.  I can't believe that running out of eggs is a cause
>of menopause.  Human females are born with some 100,000 eggs.  If you
>assume that 25 eggs are used per year (2 per menstrual cycle, a very
>high estimate), a human female still has enough eggs for about 4,000
>years of fertility.

	Problem is that not all of the oocytes have a chance to be used --
before puberty occurs, the great majority of them die.  I forget the
percentage that survives, but it is something horrendously small.  In rats,
this has been shown to be preventable (or at least reducible) by giving very
young rats (some time fairly close to birth) extra amounts of some hormone
(I forget the name of it -- this is from a Scientific American article of
*many* years ago) which is ordinarily limiting for oocyte survival; the
problem with this is that then too many eggs are released at each ovulation
(conceivably this problem might be less severe in humans, which have stricter
control over the release of eggs so that normally only one is released at a
time).  Nevertheless, untreated rats do (as you say below) die of old age
before menopause.

>                     Also, if running out of eggs was a major cause
>of menopause, we would expect that women with a large number of children
>(each pregnancy causes a cessation of ovulation for roughly 2 years) to
>enter menopause significantly later than women who have fewer children.
>In fact, women tend to enter menopause at roughly the same age that
>their mothers did, regardless of the number of children borne.

	Good point.  I don't know the answer to this one; however, if my
memory serves me right, the supply of eggs is running low by the time
menopause occurs.  However, I shouldn't have said that it is a major cause.

>Hormone levels stop dropping in human females around the age of 25.  They
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                         ^^^^
>continue to drop until the hormone levels are insufficient to cause follicle
 ^^^^^^^^^^^^^^^^
>ripening, ovulation and menstruation.  The reasons for this are not well
>understood.

	Huh?  Which one do you mean?

>The above is fact, as far as I understand.  Conjecture follows.
>
>It may be that menopause is programmed to occur at a certain point in a
>woman's life, much as menarche occurs.  Late childbearing is a risk to
>the mother and puts her other living children at risk also.  Since human
>children are dependent upon their parents for such a long time, menopause
>may be a way of helping to insure that the parents stick around long enough
>for the children to achieve independence.

	This might be a case for menopause in humans, but some animals (such
as chickens) that normally do not survive long enough to attain menopause, and
whose offspring grow up fast enough for menopause not to be a factor, do
attain menopause if maintained under conditions suitable for unusually long
survival.  Back in very primitive times, this would have been the case for
humans as well to a fair extent, since the life expectancy was something well
under 30; my memory is fuzzy on the situation with respect to menopause for
other primates, but I think they have a lifespan marginally long enough to
reach menopause under conditions unusually favorable to survival (read
carefully-maintained captivity).

	-- Lucius Chiaraviglio
	   chiaravi@silver.bacs.indiana.edu
	   lucius@tardis.harvard.edu	(in case the first one doesn't work)
Better to open your mouth and prove yourself a fool than to leave people
hanging in suspense.