Failure of 84 cell pairs in 26 different heart-muscle cell cultures to build intercalated discs

Cell contacts formed in cultures between embryonic chick-heart cells were studied. In young cultures, living muscle cells were photographed by phase contrast, each time before and after contact. After fixation the contact sites were observed in the electron microscope. One example where connected cells had beaten together for 3 hours is illustrated here. In the case of these four cell pairs, up to 22 subsequent levels of each contact site were examined. The other 80 cases belonged to 25 different cultures of 2nd, 3rd, 5th, 6th, 8th, 13th, 20th, 47th and 64th days of incubation and were collected within 5 years. On the contact sites of these synchronously-beating muscle cells, membrane specializations, such as intermediate junctions (zonula adhaerens) and gap-junctions (nexus-like), were found, seldom accompanied by desmosomes. Nowhere had these specializations developed an appearance necessary to identify an intercalated disc, i.e., with filament insertion on both sides. These contacts show that common pulsation does not suffice for the formation of an intercalated disc. A developing factor for the maturation of heart myocytes in vivo which is not present in most cultures is to be postulated.