Nuclear and cellular size of myocytes in different segments of the developing rat heart.

Background: In the embryonic heart, the individual cardiac segments show different growth rates. For the analysis of changing form in relation with changing function, data on number and shape of cardiomyocytes are necessary. Such data will give insight into the process of hypertrophy and/or hyperplasia as they may take place in the myocardium in the embryonic period.

Methods: We have measured the volumes of the nuclei and myocytes as well as the surface areas of the nuclear envelope and cellular membrane using stereological tools in rat embryos from 11 days postcoitum to 17 days postcoitum. From the data of the cellular volume of the myocytes and the myocardial volume of the individual segments, we have calculated the total number of myocytes during the developmental period.

Results: It is shown that the sinus venosus, sinu-atrial junction, and atrium increase their cellular volume during development, whereas the other cardiac segments show no difference in cellular volume. Similarly, the surface area of the cell membrane of the sinus venosus and sinu-atrial junction had increased during development. The nuclear volume and the surface area of the nuclear envelope did not differ during the period studied. The total number of myocytes showed a conspicuously smaller increase in the atrio-ventricular canal and distal outlet segment than in the other segments.

Conclusions: The increase of the cellular volume in the segments sinus venosus and sinu-atrial junction seems to be due to a late differentiation process. In general, however, the increase of the myocardial volume in the individual cardiac segments is caused by hyperplasia of the cardiomyocytes in these segments and not by hypertrophy. The surface area of cells has a fixed relationship with cell volume, indicating that no important changes take place in the developmental period studied.