Ultrastructural studies on the growth of filaments and sarcomeres in mechanically overloaded human hearts.

The ultrastructure of myocardial cells was studied in four patients with left ventricular myocardial hypertrophy due to aortic and/or mitral valvular disease. Signs of cellular lysis and synthesis were present in juxtanuclear, interfibrillar, and subsarcolemmal areas. Subsarcolemmal areas contained a granular matrix with foci of polyribosomes, mitochondria, well preserved membranes, a proliferating sarcoplasmic reticulum, and thick and thin filaments and Z-band formations at different stages of development. Z-band substance and thin filaments appeared in the cytoplasm in close proximity to free polyribosomes and endoplasmic membranes and were seemingly not associated with the sarcolemma. These findings were interpreted as evidence for a proteosynthetic activity leading to the formation of new myofibrils and sarcomeres during the chronic stage of hypertrophy. Comparative studies showed that this process has much in common with the genesis of contractile cardiac units in the mammalian embryo. Anomalous Z-band accumulations were studied in normal myocardial cells of various vertebrates and in cells of the overloaded human heart. No evidence of a proteosynthetic activity was detected in the expanded Z-bands.