Myocyte mitotic division in the aging mammalian rat heart.

To determine whether myocyte mitotic division occurs in the adult mammalian heart and whether this cellular process is affected by aging, we measured the percentage of myocyte nuclei showing metaphase chromosomes in myocytes isolated from the left and right ventricles of rats at 8-12, 19-24, and 28-32 months after birth. Metaphase chromosomes were found at all ages in both ventricles. However, from 8-12 to 28-32 months, the fraction of nuclei exhibiting metaphase chromosomes increased 6.3-fold and 2.3-fold in the left and right ventricles, respectively. Thus, myocyte cellular hyperplasia is present in the adult and aging myocardium as a compensatory mechanism to regenerate tissue mass and recover function, which are lost with the progression of life and senescence.