Ontogenetic Changes in the Reptilian Heart: An Investigation of Cardiomyocyte Proliferation and Hyperplasia in the Leopard Gecko ( Eublepharis macularius )

Abstract

Relative rates of cardiomyocyte proliferation vary among and within species. Whereas embryonic and fetal mammals demonstrate robust rates of cardiomyocyte proliferation, this capacity sharply declines within days following birth. Continued growth of the heart in mammals primarily involves cardiomyocyte hypertrophy. In contrast, cardiomyocytes in some teleost fish and salamanders retain a relative high proliferative capacity, even as adults. In these species, continued growth involves increasing the relative population of cardiomyocytes (cardiomyocyte hyperplasia). Previously, we have demonstrated that subadult leopard geckos (Eublepharis macularius, hereafter ‘geckos’) demonstrate relatively high rates of cardiomyocyte proliferation. As subadult geckos are actively undergoing somatic growth, we sought to determine if the rates of cardiomyocyte proliferation changed in adult geckos. We hypothesized that the elevated rate of cardiomyocyte proliferation observed in subadults is a function of ontogeny, and contributes to overall heart growth through cardiomyocyte hyperplasia. To document cardiomyocyte proliferation, we performed double immunofluorescence on heart ventricles from subadult and adult geckos. To identify proliferating cardiomyocytes we immunostained for the motor protein marker myosin heavy chain with either the DNA synthesis (S) phase marker proliferating cell nuclear antigen or the mitotic (M) phase marker phosphorylated histone H3. We found that there were significantly fewer cardiomyocytes in S phase (~0.1%) and M phase (~0%) in adult geckos as compared to subadult geckos (~11% and ~0.5%, respectively). This decline in proliferation likely reflects a shift from tissue growth to maintenance and remodeling. We also determined that the ventricle of adult geckos had more than twice as many cardiac cells when compared to subadult geckos. These data indicate that, similar to zebrafish (and unlike mammals), ontogenetic growth of the gecko heart involves continued cardiomyocyte proliferation and hyperplasia.