Ultrastructural changes in the left ventricular rat myocardial cells with age

Abstract

The ultrastructural changes in the left ventricular myocardial cells of rats that ranged in age from 6 to 33 months is described. There were marked age-associated changes which begin to occur at about 18 months and continue through 33 months. These changes were: (1) increases in the number of residual bodies among mitochondria in the nuclear pole zones and in the mitochondrial rows separating the myofilament masses; these bodies were localized in only these two regions; (2) increases in the number of primary lysosomes in close association with the mitochondria in the nuclear pole zones and in the mitochondrial rows separating the myofilament masses; (3) the lysosomal degradation of mitochondria which occurred by two methods: (a) the direct coalescence of mitochondria with primary lysosomes without their prior segregation by cytomembranes from other components in the sarcoplasmic matrix; this mechanism of mitochondrial degradation has thus far not been reported although it was the most common type observed in myocardial cells in this study; and (b) the segregation and digestion in autophagic vacuoles, commonly observed in other cell types, but rarely encountered in myocardial cells; large numbers of glycogen particles were similarly degraded with mitochondria in this fashion; (4) increases with age in the number of lipid droplets, changes in their electron density and solubility, and their partial or complete segregation by smooth membranous elements of the sarcoplasmic reticulum, for their presumed degradation in autophagic vacuoles. The present studies suggest that lysosomes function in aging rat myocardial cells in the degradation of mitochondria, and cellular inclusions, glycogen, and lipid, in droplet form. The extent of this degradation may decrease the capacity of the heart to function with age, as shown by others.