An ultrastructural study and stereological analysis of the colon wall in the cockroach Periplaneta americana.

The colonic epithelium has been examined for ultrastructural evidence of physiological activity. The cells show extensive folding of the apical plasma membrane, associated with mitochondria and an internal coating of particles about 120 A diameter. Anteriorly many apical infoldings are dilated at the tip to form substantial extracellular spaces up to 0.8 micron wide. Narrow intercellular channels are present, opening to the haemolymph side of the epithelium. Pinocytosis is seen frequently at the basal surface. The surface densities of apical plasma membranes were not significantly different in the posterior mid-gut and colon. Similarly the volume densities of mitochondria were equal in the colonic epithelium and rectal pads, but the surface density of outer mitochondrial membranes was greater in the colon. It is suggested that the colon may absorb organic solutes from the gut lumen.