Freeze-fracture studies of gap junctions in the developing and adult amphibian cardiac muscle.

The freeze-fracture technique has been used to characterize the junctional devices involved in the electrical coupling of Ambystoma cardiac tissue. These cells are connected by junctions formed by either linear or circular arrays of particles. Such structures can be interpreted as a special type of gap junction. Gap junctions have also been investigated during the growth and differentiation of two amphibians, Rana and Xenopus. In both genera the earliest stage of junctional assembly is characterized by linear rows of particles. Later, a gradual transformation of these linear rows into circles was found. Finally, in the fully formed gap junctions, these circles appeared to join together into clusters. In summary, in the adult amphibian myocardial cells, three different types of gap junctions can be described. The first type, which has been observed in all embryonic stages and in adults in all three genera, consists of linear or circular arrays of particles: this is the only type of gap junction seen at any age in Xenopus. The second type, consisting of a variable number of anastomosing circles forming regular networks, is never observed in embryonic cells. It is typical of the adult frog heart and may also be seen in Ambystoma. The third type is characteristic only of adult Ambystoma heart and consists of geometrically packed particles identifiable with classic communicating macula. The fact that only the first class of structure is observed in Xenopus heart strongly supports the conclusion that such linear arrays of intramembranous particles really represent true functional electrical junctions.