Plexus muscularis profundus and associated interstitial cells. II. Ultrastructural studies of mouse small intestine

The ultrastructure of plexus muscularis profundus (PMP) of the mouse small intestine was investigated subsequent to vascular perfusion with ruthenium red-containing and routine aldehyde fixatives. Four types of nerve terminals were revealed. Type I: numerous 500-A agranular vesicles and few 1,000-A granular vesicles. Type II: predominantly large (1,000-1,500 A), granular vesicles and fewer 500-A agranular vesicles. Type III: an abundance of mitochondria and many flattened vesicles (300 A X 700-1,300 A). Type IV was identified by abundant smooth cisternae 200 A in width. Types I-III formed close (200 A), synapse-like contacts to interstitial cells of Cajal (ICC-III). Presynaptic densities were frequent in type I endings. A direct innervation of muscle cells via PMP was only very occasionally suggested. ICC-III possessed a basal lamina and numerous caveolae associated with subsurface SER-cisternae. Mitochondria were very abundant in ICC-III-processes. ICC-III formed multiple, large gap junctions with outer circular-muscle cells and with other ICC-III. Also reflexive gap junctions were observed. Fibroblastlike cells (FLC) were distinguished by their prominent GER, the frequent presence of lipid droplets, and the lack of caveolae and a basal lamina. FLC never participated in synaptic arrangements or gap junctions. Macrophage-like cells were occasionally encountered. It is concluded that possible efferent and afferent nerve terminals in PMP may chiefly, if not exclusively, innervate ICC-III, the ultrastructure of which is compatible with efferent and/or afferent modulatory actions.