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From: chiaravi@silver.bacs.indiana.edu (Lucius Chiaraviglio)
Newsgroups: sci.space,sci.bio
Subject: Re: Babies born in space.
Summary: Mammals may not have same problems as birds and lower vertebrates
Message-ID: <3438@silver.bacs.indiana.edu>
Date: 13 Mar 89 00:42:58 GMT
References: <8Y42Wly00XokQ3qUUv@andrew.cmu.edu> <3870009@hpscdc.HP.COM> <466@jarthur.Claremont.EDU>
Reply-To: chiaravi@silver.UUCP (Lucius Chiaraviglio)
Distribution: world
Organization: Department of Molecular, Cellular, and Developmental Biology at Indiana University, Bloomington
Lines: 64

_

	This is actually in response to several of the recent sci.space
articles about embryonic development in zero gravity.

In article <466@jarthur.Claremont.EDU> jokim@jarthur.UUCP (John H. Kim) writes:
>I vaguely recall (sorry, don't remember where) a study that found out
>that fetal development in bird eggs involved the cells at the bottom of
>the undifferentiated cell mass *always* differentiated into the head (or
>some other specific body part--I don't remember).  The obvious
>conclusion would be that gravity provides a sort of compass for the same
>types of cells (neural, muscular, etc) to aggregate in the same place.
>I think the source went on to say something about babies conceived and
>developed without gravity possibly ending up as just a mass of cells.

	The problem with generalizing observations of birds, reptiles, and
amphibians to all vertebrates is that these vertebrates (and at least most if
not all fish) have very large yolky eggs whose cell division is distorted by
the yolk, which is itself influenced by gravity (it is heavier, so non-yolky
parts of the egg float over it.  Thus it makes sense that embryonic
development in these animals might evolve to take cues from gravity.  Effects
of gravity on Xenopus development have already been demonstrated, but of
course the effect of zero gravity has not been tested.  Thus, even this is
not necessarily an example of Xenopus (or its ancestors) having evolved
specifically to take a cue from gravity, but may just be an example of
development proceeding abnormally as a result of gravity in the wrong
direction.  Reason for suspecting the latter comes from the same experiments
showing the effects of gravity in the wrong direction, in which some of the
eggs are immune to these effects -- the immunity has been shown to be directly
correlated with rigidity of the cytoplasm of the eggs, which tends to prevent
the cytoplasm from being sheared out of alignment with the cortex (which is
what usually happens to eggs held at the wrong orientation).  I got this
information in personal communication with the principal investigators
performing these experiments and discovering the effect of cytoplasmic
rigidity:  Tony Neff and George Malacinski at Indiana University.  (This
information is also published, but I can't remember which journal it was
published in.)

	Even if the above-mentioned vertebrates do take actual developmental
cues from gravity, it is unlikely that mammals do so.  First of all, mammalian
eggs are very small (microscopic) and do not have much yolk, so forces of 1
gravity are unlikely to effect them unless they have a specific gravity
detection capability.  Even more important, it would be very detrimental for
mammalian embryos (other than those of monotremes such as platypuses) to
depend on gravity in order to develop properly, because they are carried
within their mother, which provides them with many advantages but also means
that it is impossible to guarantee a constant direction of gravity in any but
the largest mammals (and even these roll over occasionally) and impossible to
guarantee even a predominant direction of gravity in highly active mammals
such as tree-climbers and burrowing mammals.  Therefore it seems highly likely
that early mammalian embryonic development will be much affected by zero
gravity unless the physiological state of the mother is altered too much.

	The upshot of all this is that if you want to eat pork or beef in
space you need only have enough room to grow the animals (-: and some
appropriate device to alleviate the obvious problems that will develop in a
barn in zero-gravity :-), but if you want chicken or frog legs you are going
to have to import these items or bring a centrifuge.  8-)

-- 
|  Lucius Chiaraviglio   |  ARPA:  chiaravi@silver.bacs.indiana.edu
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