Reconstruction of mammalian heart tissue from embryonic heart cell suspension with reference to the aggregation of adult heart cells.

Aggregations of isolated embryonic and adult heart cells were studied so as to examine in detail the formation of cell contacts, the assembly of cells into multicellular systems, and cell co-operation in forming organized and differentiating tissues. At selected intervals after initiating rotation cultures, aggregates were examined microscopically for evidence of contractility, and subsequently processed for scanning and transmission electron microscopy (TEM, SEM). By continuous accretion of single cells, and the joining of small clusters, the aggregates increased in size. Cells within the aggregates exhibited rhythmic and synchronous contractility by 3--12 h of culture, suggesting the formation of low-resistance inter-cellular junctions between apposed cells. Two populations of cells could be recognized by 9--12 h with SEM. One was spherical in surface view and the other was flattened. Spherical cells possessed myofibrils, and were classified as cardiac myocytes which occupied the core of the aggregate. The flattened cells were devoid of myofibrils and non-muscle in nature. They covered the surface in a multilayered epithelium. At 12h the aggregates were round to oval, covered by flattened cells, but individual round cells could still be recognized. Intercalated discs were frequently observed in 12 h aggregates. The junctional complexes observed in 12--72 h aggregates include desmosomes, fascia adherens and gap junctions. Most of the aggregation was completed by 24 h, and at later time periods, i.e. 48--72 h, the external surface of the aggregates was smoothed out with epithelial investment. In these aggregates myofibrils and intercellular junctions became reconstructed in less than 24 h. Unlike embryonic myocardial cells, adult cells did not form aggregates of numerous cells. Instead, they formed irregular clusters of 2--5 cells during 3--48 h of culture. Intercellular contacts and suggestive desmosomal materials were observed between adherent cardiac muscle cells. When culture continued for 48--72 h, the cells underwent supercontraction and became non-viable, suggesting that the terminally differentiated adult myocardial cells are incapable of regenerating constituents obligatory for histogenetic reconstruction.