Background— Whether adult cardiomyocytes have the capacity to regenerate in response to injury and, if so, to what extent are still issues of intense debate. In human heart failure, cardiomyocytes harbor a polyploid genome. A unique opportunity to study the mechanism of polyploidization is provided through the setting of hemodynamic support by left ventricular assist devices. Hence, the cardiomyocyte DNA content, nuclear morphology, and number of nuclei per cell were assessed before and after left ventricular assist device support.

Methods and Results— In 23 paired myocardial samples, cardiomyocyte ploidy was investigated by DNA image cytometry, flow cytometry, and in situ hybridization. Nuclear cross-sectional area and perimeters were measured morphometrically, and the binucleated cardiomyocytes were counted. The median of the cardiomyocyte DNA content and the number of polyploid cardiomyocytes both declined significantly from 6.79 c to 4.7 c and 40.2% to 23%, whereas a significant increase in diploid cardiomyocytes from 33.4% to 50.3% and in binucleated cardiomyocytes from 4.5% to 10% after unloading was observed.

Conclusions— The decrease in polyploidy and increase in diploidy after left ventricular assist device suggest a numeric increase in diploid cardiomyocytes (eg, through cell cycle progression with completion of mitosis or by increased stem cells). The cardiac regeneration that follows may serve as a morphological correlate of the recovery observed in some patients after unloading.