Subject: Re: [NABOKV-L] Victor Fet on symmetry in biology
From: "Nicholas Grundy" <[email-masked]>
To: "Vladimir Nabokov Forum" <[email-masked]>
Date: Wed, 20 Sep 2006 13:32:16 -0400

(This is basically a note about developmental biology as it applies to 
left-right symmetry breaking - so it may not be relevant to the list, 
although arguably chirality is... I leave the decision over whether to 
run it entirely to you!)

Victor Fet is spot-on about external symmetry and internal asymmetry, 
and discoveries in the last few years have revealed just how left and 
right are defined in the human and other creatures. In short, the 
question isn't "why is symmetry broken?" - there are several theories 
about why asymmetric hearts and viscera are more efficient (they involve

fluid dynamics and organ packing in the abdomen) - the question is "how 
is it broken in the same way every time?". People have their hearts on 
the left hand side almost without fail, and the mirror-image picture 
(/situs inversus/) is vanishingly rare - but defining left as distinct 
from right is extremely difficult.

The development of the other two body axes, the superior/inferior (or 
rostral/caudal), and the front/back (or anterior/posterior) , is 
understood - but even with those in place, how can you get left 
reliably? You get an idea of it if you try, using *only* instructions 
featuring 'front', 'back', 'up', and 'down', to define your left hand. 
Until recently the existence of a chiral F-molecule was postulated, 
which the developing organism could align with its superior-inferior and

anterior-posterior axes and have left defined by the third bar (make a 
fist, point your index finger forward, your thumb up, and your middle 
finger towards your other hand, and you get the idea!).

What actually happens is that there are little hair-like structures 
(cilia) which rotate clockwise (and, because of the intrinsic chirality 
of proteins, only clockwise) in a pit on the surface of the organism. In

and of itself, of course, that doesn't create a flow in either direction

- but then the organism simply angles the cilia 45 degrees back, so that

the rotation from 9 through 12 to 3 (as on a clockface) hits the floor 
of the pit and creates no flow, and the rotation from 3 through 6 to 9 
creates a leftward flow.

Not strictly relevant - so I half apologise - but it's got a certain 
magic to it, IMHO.

Nick.

PS: Chris McManus' 'Left hand, Right hand' is very good on this and on 
all sorts of other left-right distinctions in culture, society, and 
development.

-- 
"Forgotten is forgiven." F.Scott Fitzgerald
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