How might systematic left-right (L/R) asymmetry of the body plan originate in
multicellular animals (and plants)? Somehow, the microscopic handedness of
biological molecules must be brought up to macroscopic scales. Basic symmetry
principles suggest that the usual "biological" mechanisms -- diffusion and gene
regulation -- are insufficient to implement the "right-hand rule" defining a
third body axis from the other two. Instead, on the cellular level, "physical"
mechanisms (forces and collective dynamic states) are needed involving the long
stiff fibers of the cytoskeleton. I discuss some possible scenarios; only in
the case of vertebrate internal organs is the answer currently known (and even
that is in dispute).Comment: 9 pp latex, 6 figures. Proc. Landau 100 Memorial Conf.
(Chernogolovka, June 2008); to appear AIP Conf. series. (v2: added 4 ref's +
revised Sec 2.2.
