Age-dependent structural changes in the myocardium of rats. A quantitative light- and electron-microscopic study on the right and left chamber wall.

Myocardial fine structure of 6-week-old and 2-year-old female Wistar rats was examined after fixation by perfusion. Qualitatively, lipid droplets, lipofuscin granules and myelin figures were found more often in the older animals. Quantitatively, the volume density of the interstitium of the right ventricular myocardium had significantly increased (52%) in the 2-year-old rats. In these animals a reduction (9%) in the volume fraction of mitochondria in the left ventricular myocytes was seen, while the volume fraction of myofibrils had increased by 10%. The numerical density of mitochondria had significantly increased in the left ventricular myocardium of the older rats by 42%. A decrease in the average size of a mitochondrion by 36% could be shown in the left and by 11% in the right ventricular myocytes of the 2-year-old rats. In these animals the myofibrillar transverse diameter diminished by 15% in the left and by 6% in the right ventricular wall, while the number of myofibrils had increased by 36% and 12%, respectively; these findings signify a close relationship in the number of mitochondria and myofibrils in myocytes. Quantitative alterations in the myocytes of aged rats are more obvious in the left than in the right chamber wall. A reduced mitochondria/myofibril ratio may contribute to the diminished functional adaptability of the aging heart.