In the apical region of
MRCs is a collection of tubules and vesicles
(tubulovesicular system) that is distinct from the
tubular system (Fig. 9). The apical membrane can
be quite variable in appearance ranging from
concave to convex, sometimes forming deep crypts.
In addition, the surface topography has been
shown to be smooth, or having microvilli of
varying densities and lengths as well as highly
branched, to give a sponge-like appearance. The
use of morphological features has led to the
classification of different types of MRCs.