Intercalated discs, nexuses, sarcoplasmic reticulum and transitional cells in the heart of the adult domestic fowl (Gallus gallus domesticus).

Intercalated discs usually consisted of three or four transverse steps of folded cell membrane connected by straight parts running longitudinally. The transverse folded parts contained a variable number of interlocking processes from contiguous cells. Large processes were frequently flanked by smaller ones. Nexuses appeared to be better developed on the straight parts of intercalated discs than on the folded parts. They varied in size, shape and particle-packing density. The largest nexus observed was almost perfectly circular. It covered an area of 0.087 micron2, contained 530 particles and had a particle-packing density of 6078 particles/micron2. The sarcoplasmic reticulum formed a uniform mesh of fine tubes which was distributed in sheets between bundles of myofilaments and longitudinally aligned mitochondria. Neither extended junctional sarcoplasmic reticulum nor fenestrations in the sarcoplasmic reticulum were observed. Peripheral couplings were numerous and widespread. They were not restricted to the sarcolemma in the regions of the Z-line, H- or I-bands. Transitional cells combine the characteristics of contractile and conducting cells in varying proportions. There is some speculation about the mechanism of nexus formation and the interconvertibility of contractile and conducting cells via the intermediary of the transitional cell.