Immunostained sections and whole-mount preparations of the layers of the guinea pig jejunum were investigated by an improved peroxidase-antiperoxidase method using an antiserum to S-100 protein. A delicate latticework of S-100 protein immunopositive glial cells was demonstrated extending in the longitudinal muscle layer, myenteric or Auerbach's plexus, circular muscle layer including the deep muscular plexus, submucous layer including the submucous or Meissner's plexus, lamina muscularis mucosae and lamina propria mucosae. The whole enteric nerve plexuses consisted of two subsystems; nerve plexuses of the muscular coat and those of the submucous and mucous coats. These two subsystems were joined to each other by thick, connecting branches perforating the inner circular muscle layer. Extrinsic nerves entering the myenteric plexus formed a specialized junctional structure containing S-100 protein immunopositive glial cells, whereas those entering the submucous plexus ran along the submucous arteries. We proposed the term enteroglial cells to designate the S-100 protein immunopositive cells which ensheathed the somata and processes of the enteric neurons. The frameworks of all structures in the enteric nerve plexuses from the largest ganglia to the thinnest nerve fasciculi were constructed of these enteroglial cells.
A spectrum of the enteroglial cells was presented. Those in the myenteric and submucous ganglia were found similar to the astroglia of the central nervous system and to the satellite cells in the peripheral ganglia. Those in the primary and secondary fasciculi of the myenteric plexus formed a kind of neuropil together with the neuronal processes. Those in the tertiary fasciculi of the muscular coat formed the framework of the autonomic ground plexus. We tentatively concluded that the interstitial cells of Cajal contain an immunoreactivity for S-100 protein, and thus are glial in nature. The occurrence of specialized enteroglial cells with a neuron-like function was discussed in the autonomic ground plexus of the muscular coat. In the lamina propria mucosae, there was a fine latticework of the S-100 protein immunopositive enteroglial cells. This latticework corresponded to that of the interstitial cells of Cajal in the villous and periglandular plexuses.