Gap junctions between electrotonically coupled cells in tissue culture and in brown fat

Although circumstantial evidence has suggested that gap junctions mediate intercellular electrotonic coupling, it has not been possible in most tissues to exclude the involvement of other, coexisting cell junctions. We have made an electron microscopic study of replicas of frozen-fractured BHK21 cells (from tissue culture) and of brown fat cells of newborn mice. Both of these cell types are known to exhibit intercellular electrical coupling. In each case, the only junctions found between the cells are small macular gap junctions (less than 1 mum in diameter) characterized by clusters of 6-nm (60 A) particles or depressions on membrane cleavage faces. Several replicas confirm the association of these particles and depressions with regions of narrowing of the intercellular space, i.e., with the sites of cell junctions. We have also determined the frequency of occurrence of gap junctions on the membrane cleavage faces of both cell types. Gap junctions occupy about 1-2% of the surface area of brown fat cells, but only 0.05% of the surface area of BHK21 cells. These observations indicate that gap junctions, when they are the only intercellular junctions present, are sufficient to account for electronic coupling between cells.