Development and proliferative capacity of cardiac muscle cells.

Abstract

In this article the cytological features of cardiac enlargement are reviewed. During the embryonic development of the myocardium, both undifferentiated cells and cells containing myofibrillar proteins divide. As the heart grows and approaches maturity, its muscle cells progressively lose their mitotic activity, and hypertrophy of myocytes becomes the principal process of cardiac enlargement. Cytological features of cardiac enlargement secondary to work overloading depend on the stage of heart development. In the early postnatal heart increased work load results in enlargement characterized by an increase in both the number and size of myocardial cells. The adult heart enlarges only by hypertrophy of its myocytes. Morphometric studies of diseased human hearts enlarged in excess of 500 g indicate an increased number of muscle cells. Some of the control mechanisms regulating mitotic activity in myocytes are discussed: (1) Critical mitosis theory: it has been suggested that synthesis of DNA and of cell specific proteins are mutually exclusive processes. A block of DNA synthesis then occurs during differentiation of myoblasts as a conseqence of a specific mitosis leading to a new phenotype cell. (2) ''Critical mass'' theory: it is postulated that the gradually accumulating cell specific structures present a hindrance to mitotic division ofmore » nucleus. After a critical mass has been reached, no more mitoses are possible. (3) Repression of DNA synthesis: synthesis of DNA in developing myocytes declines with maturation. Nevertheless, under rare conditions DNA synthesis can be reactivated, suggesting a repression rather than an irreversible block of DNA synthesis.« less