Postnatal differentiation of ventricular myocardial cells of the opossum (Didelphis virginiana Kerr) and T-tubule formation.

Cardiac muscle cells in the right ventricle of the postnatal opposum were studied ultrastructurally, with particular attention to the developmental stage of T-tubule formation. Animals ranging from 5.5 cm (day 29 of postnatal life) to 41 cm (young adult) in body length were used. Typical T-tubules were first recognized in a few myocardial cells of the 7.5 cm (day 43) opossum. T-tubules increased in number as cardiac muscle cells continued to differentiate until 22 cm in body length (about 105 days after birth). At this stage of development most of the T-tubules were completely established. The general mode of differentiation and development of cardiac muscle cells appeared essentially the same as in other mammals.