Re: R.B.C. has a biconcave shape, WHY?

Red blood cells, or erythrocytes, are biconcave for two reasons.  First, 
erythrocytes have no nucleus or internal membranes. Second, erythrocytes 
have an unusual cytoskeleton.  All cells have a cytoskeleton, which is made 
out of long protein polymers, and gives cells their shape.  The erythrocyte 
cytoskeleton is a dense fibrous structure lining the whole cell and making 
many contact points.  The primary component of this lining is spectrin, a 
long fibrous protein.  The spectrin is attached to the membrane at many 
points.  Spectrin chains help to pull the red blood cell into its 
characteristic shape, and allows them to squeeze through tiny capillaries. 
Mice that have mutant spectrin have spherical red blood cells, which 
illustrates the importance of spectrin in holding the appropriate shape of 
the erythrocytes. 

For more information, check out a cell bio textbook like:
Molecular and Cell Biology
Harvy Lodish et al.

Or see the spectin web page:
 http://chazin.scripps.edu/cabp_database/other_res/
spectrin_sites.html

Erin