Re: RC: Re: Breeding theories (was *Muscat)

[Peggy Rinehart <rubyjwl@cts.com>]

Dear Claudia and Lif,

In humans, the ideal maternal age for reproduction appears to be from 18-30
years of age.  The liklihood of severe chromosomal abnormality in the embryo
increases significantly after the age of 35 years.  Female mammals (humans,
horses, whatever) are born with millions of primary oocytes already present
in their ovaries.   By puberty only about 30-40 thousand remain.  Of these
only a small fraction will mature to secondary oocytes and be expelled at
ovulation during the reproductive period.  As the oocytes age, the
chromosomal material can degrade, increasing the risk for conceiving an
embryo with chromosomal abnormality.

It is also undesirable for the father to be an advsanced age at the time of
conception.  Although it is true that spermatogenesis is an ongoing process,
the possibility for gene mutation increases with paternal age.  The older
the father is, the more likely he is to have accumulated mutations that the
luckless embryo may inherit.

Those who may have elderly mares and stallions who are producing wonderful
foals, please do not take offense.  While it is certainly possible for any
mammal of reproductive age to have normal offspring, the odds of having an
unintended, or undesired outcome increase with advanced maternal and
paternal age.

You mentioned turtles in your post.  Reptilian reproduction is another
matter altogether.  Some reptiles can carry fertilized embryo in a
"suspended animation" state for years.  When environmental conditions are
favorable, they then lay the eggs and embryological development proceeds.
In some species, sex determination of the embryos is a function of
environmental temperature!  Although the reproductive biology of reptiles is
fascinating, I don't believe we can apply conclusions about maternal age in
reptiles to mammalian reproduction

Peggy Rinehart
Ramona, CA

At 12:06 PM 9/10/99 -0600, you wrote:
>Claudia wrote:  "my resources tell me (vets, A&M, Colo. State Vet school
>and tech support) that an older mare has older eggs."
>
>This is true about any female that produces eggs - all the eggs are
>there when the female is a baby, and she just releases them over her
>lifetime (as opposed to males, who manufacture new sperm as they go), so
>as the females age, so do the eggs.  I believe however, that 35+ year
>old eggs aren't considered that old - at least I know they aren't in
>humans - because women can have perfectly normal, healthy babies from 35
>year and older eggs.  I don't know if this translates to horses because
>I do not know if the viability/age of eggs is species specific.  Turtles
>(or is it tortoises?) live to be ancient - well over human child bearing
>years - and they produce viable eggs all along.  I certainly would like
>to know if any of our more educated list members have the answers!  Lif
>
>--
>                                     ____________
>Lif & Paul Strand   STRAND ENTERPRISES   http://www.fasterhorses.com
>           Arabian Horses for Distance Riding
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>                     Quemado, NM  USA
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Ridecamp is a service of Endurance Net, http://www.endurance.net.    
Information, Policy, Disclaimer: http://www.endurance.net/RideCamp   
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