Giant mitochondria in the myocardium of aging and endurance-trained mice.

During the course of an ultrastructural study on the apex region of the left ventricular myocardium of experimental female C57BL/6J mice, aged between 6 and 27 months, and that had been subjected to long-term daily enforced endurance running schedules, we encountered some instances of unusual giant mitochondria. These mitochondria showed degenerative changes including the disruption, disorganization and loss of cristae and the development of a very electron-lucent matrix. In many instances, these giant mitochondria showed dense inclusions within the matrix. The giant mitochondria appear to have developed as a result of fusion between adjacent hypertrophic mitochondria followed by a sequence of progressive degenerative changes. These giant mitochondria were most common in the myocardium of mice that had followed endurance training schedules for 10 or 15 months and were not encountered in age-matched nonrunning control mice.