The fine structure of desmosomes and intercalated discs in the toad heart is discussed. A definite relationship between the dense components of these structures and the dense region of the Z band is demonstrated. The dense region of the Z band characteristically widens at its approach to the plasma membrane, and often terminates beneath it in a distinct discoidal plaque. Cardiac desmosomes appear to be structures which result from the intimate apposition of plaques of Z band material. These desmosomes retain the Z band function as sites of attachment for myofilaments. The suggestion is made that rotation of a desmosome through 90° and splitting of filaments from the adjacent sarcomere could result in the formation of a simple step-like intercalated disc. Intermediate stages in this process are illustrated. Complex discs present in the toad probably represent the alignment of groups of simple discs produced by contractile forces. Possible physiologic functions of the disc and desmosome are discussed. Other morphologic features of toad cardiac cells include a distinct amorphous outer coat to the sarcolemma, a prominent N band, and a granular sarcoplasm with poorly developed reticulum.

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