Ultrastructural observations of fibroblast-like cells forming gap junctions in the W/W(nu) mouse small intestine.

The ultrastructure of the wild-type (+/+) mice small intestine was compared with c-kit mutant (W/W(nu)) mice which only have few interstitial cells of Cajal (ICC) associated with Auerbach's plexus, in order to elucidate whether the specialized membrane contacts are general features of so-called fibroblast-like cells that are widely distributed in the tunica muscularis of the alimentary tract. Fibroblast-like cells in the Auerbach region were found in approximately equal number in W/W(nu) mice as in +/+ mice, while ICC associated with Auerbach's plexus (ICC-AP) could not be demonstrated in W/W(nu) mice in the present investigation. Fibroblast-like cells were characterized by cytoplasm of moderate to high electron density, well developed rough endoplasmic reticulum and nuclei with thick peripheral accumulations of heterochromatin. There were no basal lamina and caveolae along the cell membrane. It was observed that single fibroblast-like cells formed probable small gap junctions with muscle cells of both circular and longitudinal layers. Fibroblast-like cells with the same features were also observed in the region of the deep muscular plexus in both +/+ and W/W(nu) mice. The present observation, together with our previous studies on rats and guinea-pigs, suggest the common presence of gap junctions or gap junction-like structures on fibroblast-like cells in the gastrointestinal musculature and their involvement in the regulatory system of gastrointestinal motility by passing electrical or molecular signals to influence the state of muscle tonus.