Structural Changes in Nuclear Envelopes during Elongation of Heart Muscle Cells

It has recently been shown (1) that myocardial nuclei become longitudinally compressed during systole and extended during diastole . Myocardial nuclear length varies directly with the sarcomere length, and there is no slippage of sarcomere segments past the nuclear envelope . At sarcomere lengths corresponding to diastole the nuclear envelope is straight, but for every incremental decrease in sarcomere length a corresponding degree of envelope redundancy' is observed. Thus the nuclear envelope shows little elasticity at physiologic sarcomere lengths and is pulled taut at diastolic sarcomere lengths. In this communication, ultrastructural differences between the nuclear membranes of extended and contracted heart muscle cells are presented . These ultrastructural features may be the basis for functional differences .