Direct and indirect innervation of smooth muscle cells of rat stomach, with special reference to the interstitial cells of Cajal

Interstitial cells of Cajal in the circular (ICCCM) and longitudinal (ICC-LM) muscle layer of the rat gastric antrum and their innervation were studied ultrastructurally. Both ICC-CM and ICC-LM are characterized by many mitochondria, rough and smooth endoplasmic reticulum, caveolae, and formation of gap junctions with each other and with muscle cells, though ICC-LM tend to show more variable cytoplasmic features depending on section profiles. Close contacts between nerve terminals and both ICC-CM and ICC-LM are observed. These possible synaptic structures are characterized by: (1) accumulation of synaptic vesicles in nerve varicosities, (2) a narrow gap (about 20 nm) between pre- and postjunctional membranes, (3) lack of a basal lamina between pre- and postjunctional membranes, and (4) the presence of an electron-dense lining on the inner aspect of prejunctional membranes. Almost the same characteristics are observed between the nerve terminals and the muscle cells of both circular and longitudinal muscle layers of the same specimens. Therefore, we conclude that the smooth muscle cells of both circular and longitudinal layers of the rat antrum are directly and indirectly innervated via ICC. Their functional significance is discussed.