A mechanical momentum in ultrastructural development of the heart.

The question of whether mechanical moments participate in ultrastructural development of the heart cannot be decided by examining heart tissue samples during embryonic life, for during maturation processes in vivo mechanical factors are always present. The significance of mechanical moments can only be investigated in a system which allows a distinction to be made between pulsations of heart myocytes under conditions of contraction against and under conditions of contraction without mechanical stress. This double possibility is provided by cell culture. An example in which mechanical influences are absent are the so-called "minihearts"; these are cell agglomerations which are detached from the culture substrate. Their cell contractions do not act against any mechanical resistance. In the electron microscope they show abundant filaments, Z-line material and membrane specializations. The highest degree of development, however, does not exceed that characterizing the cells with which the cultures were started. Even some dedifferentiation and degeneration are apparent in the minihearts. No intercalated discs can be found even after 12 weeks, provided that the culture had been started with really isolated cells. The representatives in culture of a myocyte's connection with the heart's mechanical action are the cords, along which attached points alternate with free strands. The pulsation of these strands is restrained during each contraction by their ends, which are fixed on the culture substrate. Thus, the myocytes contract against a resistance, and in this respect their pulsations resemble those of the heart, in which the myocytes' contraction acts against the pressure of the blood. The myocyte culture under these mechanical condition, after a culture time of 10 weeks, produces electron micrographs of mature cells with reduced cytoplasm, aligned mitochondria and fibrils and intercalated discs.