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From: werner@aecom.UUCP
Newsgroups: sci.bio
Subject: Re: Human asymmetry
Message-ID: <1003@aecom.UUCP>
Date: Fri, 3-Apr-87 19:30:44 EST
Article-I.D.: aecom.1003
Posted: Fri Apr  3 19:30:44 1987
Date-Received: Sun, 5-Apr-87 10:19:52 EST
References: <586@batcomputer.tn.cornell.edu>
Distribution: sci
Organization: Albert Einstein Coll. of Med., NY
Lines: 47
Summary: Dynein arms

In article <586@batcomputer.tn.cornell.edu>, plimpton@batcomputer.tn.cornell.edu (Steve Plimpton) writes:
> 
> 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?  

	Everything biological is assymetrical.  Why do we metabolize only
D-sugars, not L-sugars, or use only D-amino acids, and not their L
enantiomers.  Chemists cannot tell prochiral centers apart, but enzymes
can.
	It turns out that the size of macromolecules ensures that they
will always be assymetrical, and an assymetric molecule can react with
a symmetrical one assymetrically.

	The molecule in question here is Dynein, the component of
microtubules that provides the molecular motor.  It interacts with
polymerized tubulin to form the skeleton of cilia.  Since tubulin
has a direction, dynein binds only one way, and hence all cilia
have a forward beat and a reverse beat.

	It turns out that this ciliary movement ensures that the heart will
always be on the left side, and the liver on the right.  The large
intestine also undergoes a rotation to ensure that the rectum is on
the left, and the ileocecal valve (and the appendix) on the right.

	Certain people are born without functional cilia.  In addition
to being sterile, and have chronic respiratory disorders, half of them
have their internal organs reversed (i.e., without cilia to give
direction, rotation IS random).  The syndrome of immotile cilia due
to defective Dynein arms is called Kartegner's Syndrome, and the
reversed organs is referred to as 'Dextrocardia with Situs Invertus',
and has been coined by the anatomist Francis Baker-Cohen as
'Vice versa viscera'.

	One of my medical puzzles dealt with left sided abdominal pain 
distant heart sounds (why?) and pulmonary congestion. It was a
left sided appendicitis, and the other signs were hints.



-- 
			      Craig Werner (MD/PhD '91)
				!philabs!aecom!werner
              (1935-14E Eastchester Rd., Bronx NY 10461, 212-931-2517)
Everything's different. Nothing's changed. Well, only maybe slightly rearranged.