Polyploidization of transplanted cardiac myocytes.

Pieces of cardiac ventricular tissue of late embryonic or 1-day postnatal rats, implanted beneath the kidney capsule of adult syngeneic hosts, formed viable, beating transplants. These transplants were investigated over a 40-day postoperative course. In the transplants, cellular binucleation and nuclear polyploidization occurred according to the same schedule as in the heart in situ. The composition of the classes of myocytes was identical both in the hearts in situ and in transplants, but the number of non-diploid myocytes in the intact heart reached 90%, whereas in transplants it varied from 30 to 60%. In contrast to the heart in situ, myocytes in transplants grew feebly after the phase of polyploidization. From these data one can conclude that under conditions of transplantation the temporal sequence of cellular binucleation and nuclear polyploidization follows the normal course, but that a greater number of myocytes remain in a diploid state than is the case in the normal heart. The growth of cardiac myocytes seems to be related to their level of function.