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From: eddy@boulder.UUCP
Newsgroups: sci.bio
Subject: Re: Human Asymmetry
Message-ID: <898@sigi.Colorado.EDU>
Date: Thu, 9-Apr-87 20:44:33 EST
Article-I.D.: sigi.898
Posted: Thu Apr  9 20:44:33 1987
Date-Received: Sat, 11-Apr-87 14:46:39 EST
References: <633@batcomputer.tn.cornell.edu>
Sender: news@sigi.Colorado.EDU
Reply-To: eddy@beagle.Colorado.EDU (Sean Eddy)
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Organization: University of Colorado, Boulder
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In article <633@batcomputer.tn.cornell.edu> plimpton@batcomputer.UUCP (Steve Plimpton) writes:
>does this mean that one could take a human egg and by examining its
>cilia/microtubule structure or motion (assuming you could look at it in
>sufficient microscopic detail), one could assign a unique top/bottom,
>front/back, left/right (TBFBLR) to it?  And that *all* human eggs are the same
>in that respect?  So that in essence every egg is a human in miniature with the
>same TBFBLR orientation as each other and adults? 
>
>	If this isn't so, then I'm still having trouble visualizing, when the
>embryo consists of a few cells (20 or 20,000 - I don't know) and there is a
>definable top/bottom and front/back, but not yet a left and right, how a cell
>on the left is in any different a chemical/biological environment than one on
>the right and hence senses it's the one that should become a heart.  It doesn't
>(to me) seem enough to say (for example) all the cilia are beating towards the
>left at that point therefore that's where the heart will end up, because if the
>top or front had formed the opposite way (which it seems they could have if
>there wasn't a TBFBLR to start with), the cilia would be beating to the right. 
>Any help here? 

OK, I'll try. First of all, remember that you asked a comparatively
simple question to start. The question Craig and I answered was "why
does the human heart almost always end up on the left side?" The answer
we gave is a typical biology answer. A syndrome is known in humans in
which the heart's position is randomized with respect to left and right.
When you look for the specific genetic defect associated with this
syndrome, the defect lies in the ciliary system.

Therefore, says the biologist, the ciliary system determines the asymmetry
of the heart with respect to left and right position in the body. One
may also assume that the fact that cilia beat in one direction has some
significance. But, to my knowledge, that's all we know and/or safely
assume. So I can't say more than this.

From that simple question, you naturally are asking a broader question,
that of asymmetry in general. That's a large part of what developmental
biologists are trying to answer, and far beyond the scope of my knowledge,
at least in human development. (Side note of clarification: the
asymmetry of the egg itself does not bear directly on the heart question.
Cilia are expressed later in development, I believe.)

So what I'm saying is, I can't explain human developmental biology.
Neither can anyone else, for now. All that's known is bits and pieces,
and some of the underlying themes.


- Sean Eddy
- Dept. of Molecular, Cellular, Developmental Biology
- Univ. of Colorado, Boulder; Boulder, CO 80309
- 
- "Ph.D.'s are for suckers."  -- from 'Ask Mr. Science'