INTERSTITIAL-CELLS OF CAJAL - MEDIATORS OF COMMUNICATION BETWEEN CIRCULAR AND LONGITUDINAL MUSCLE LAYERS OF CANINE COLON

The network of interstitial cells of Cajal associated with Auerbach's (myenteric) plexus in the canine colon was investigated to determine i ts role in facilitating communication between circular and longitudina l muscle layers. Electrical coupling between the muscle layers was dem onstrated by propagating extracellularly evoked electrotonic pulses fr om circular muscle cells to nearby longitudinal muscle cells. The like lihood of cytoplasmic continuity across Auerbach's plexus was further demonstrated by the ability of neurobiotin to spread between the inter stitial cells and the circular and longitudinal muscle cells. Importan tly, direct neurobiotin spread between circular and longitudinal muscl e cells was not observed even when they were in close proximity as det ermined by confocal microscopy. When neurobiotin did spread across the two muscle layers, the intervening interstitial cells were always neu robiotin-positive. In regions where circular and longitudinal muscle c ells approach each other closely, electron microscopy revealed the pre sence of close appositions between interstitial cells and smooth muscl e cells. Gap junctions between interstitial cells and smooth muscle ce lls of both layers, as judged by electron microscopy, were extremely r are. Neither gap junctions nor close appositions were observed between longitudinal and circular muscle cells. The special arrangement for e lectrotonic coupling across Auerbach's plexus through interstitial cel ls of Cajal suggests controlled coupling between the two muscle layers , explaining the preservation of their distinct electrical activities.