Ultrastructural differentiation characteristics of rat atrial and ventricular cardiac muscle cells in culture.

Changes in differentiation level of cultured atrial (A) and ventricular (V) cardiac muscle cells from 2-week-old rats have been studied to compare the stability of their morpho-functional characteristics. Cultured A and V cells clearly differ in the form, size, spreading pattern and contractile activity. Comparative electron microscopic analysis of A and V myocytes after 7 days in vitro revealed longitudinally arranged and more numerous myofibrils in V myocytes, whereas A cells show less mature contractile apparatus and greater amounts of free components of sarcomeres scattered about the cytoplasm. Both A and V populations were heterogeneous by differentiation level of individual cells: apart from partial dedifferentiation the degree of the contractile apparatus development depends on de novo formation of myofibrils that underlies the redifferentiation process. Leptofibrils commonly connected to Z-lines of myofibrils were observed in V myocytes. A muscle cells actively produce specific atrial granules in the well-developed Golgi complex. Some of V myocytes also possess rare granules similar to the atrial ones. The data obtained indicate preservation of the differences in biology of A and V myocytes under in vitro conditions and are discussed with special reference to their differentiation state in vitro.