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From: plimpton@batcomputer.UUCP
Newsgroups: sci.bio
Subject: Human asymmetry
Message-ID: <586@batcomputer.tn.cornell.edu>
Date: Wed, 1-Apr-87 15:18:38 EST
Article-I.D.: batcompu.586
Posted: Wed Apr  1 15:18:38 1987
Date-Received: Sat, 4-Apr-87 09:51:15 EST
Reply-To: plimpton@batcomputer.UUCP (Steve Plimpton)
Distribution: sci
Organization: Theory Center, Cornell University, Ithaca NY
Lines: 47

This is a question from a physics-type to bio-types so apologies if this is
basic MicroBio 101 stuff. 

Humans (all animals?) are asymmetric right vs. left.  Namely, my heart is on
my left side, my appendix on the right, liver someplace, etc., etc.  And a
nurse told me virtually everyone (99%+) is the same.  My question is, given
that we all start from a one-cell symmetric egg, how do we all develop the
same asymmetry?  It doesn't puzzle me (as much) that we are asymmetric at
all, but that we are all the same!  That is, why aren't 50% of the people
around mirror images of me with their heart on the right side?  In physics
lingo it seems like a violation of the law of parity.  I guess what I'm
really asking is, that as the fetus cells are busily dividing along, how do
ones on the left know not just that it's time to become heart cells, but
that they're not on the right? 

Wild guesses (in descending likelihood):

1.   I know there's left and right-handed proteins and humans are all made
     of only one kind (which?).  So maybe this low-level asymmetry works its
     way up thru the development to a heart on the left side?  If so this is
     pretty amazing, and you still need a mechanism to explain how it
     happens ...
2.   Maybe the egg isn't symmetric and as it sits in a gravity field,
     different stuff is always on the right vs. the left and splitting into
     more cells just accentuates the difference ... 
3.   The fetus develops in an asymmetric environment (Mom), so maybe the
     fetus "hears" Mom's heart above and to the left and does the same ...
4.   Coriolis effect (my favorite):  Only people born in the northern
     hemisphere have hearts on the left; southern hemisphere people on the
     right ...

Related questions: 

1.   How soon is there left/right asymmetry in the growing fetus?  
2.   I assume there's a gene for right handed/brainedness vs. left and that 
     it controls the development of turning the left side of the brain into
     something different than the right.  So how do the chemicals or
     proteins or whatever this gene makes know how to travel to the right
     brain vs. the left, either before birth or after? 
3.   Since a small % of people do have their heart on the right side, how did
     they get turned around?  Are they complete mirror images of "normal"
     people?  Is this genetic (Republican parents) or just random chance?

Thanks in advance ...

Steve Plimpton			ArpaNet:  plimpton@cheme.tn.cornell.edu
UseNet:  {cmcl2,shasta,uw-beaver,rochester}!cornell!batcomputer!plimpton