  We study the shapes of human red blood cells using continuum mechanics. In
particular, we model the crenated, echinocytic shapes and show how they may
arise from a competition between the bending energy of the plasma membrane and
the stretching/shear elastic energies of the membrane skeleton. In contrast to
earlier work, we calculate spicule shapes exactly by solving the equations of
continuum mechanics subject to appropriate boundary conditions. A simple
scaling analysis of this competition reveals an elastic length which sets the
length scale for the spicules and is, thus, related to the number of spicules
experimentally observed on the fully developed echinocyte.
