Traditionally, the all-asymmetric-division model is seen as
the simpler of the two, and it is certainly the more popular
in textbooks. The suggested mechanism is that stem cells
distribute different cytoplasmic ‘determinants’ to their
apical and basal ends. By constraining cell division planes
to lie perpendicular to the apico-basal axis, determinants
of stemness necessarily pass to just one of the two daughter
cells. The major difficulty with this model is that it works
too well: if stem cell populations are ever to expand (for
example, during embryonic development), there needs to
be some way either to shut off the differential sorting of
determinants, or reorient planes of cell division.