Growth of the myocardial volumes of the individual cardiac segments in the rat embryo.

Abstract

Although the growth of the developing heart in relation to an increase of ventricular systolic pressure and the growth of the entire embryo during development has been described, no data are available on the growth of the individual segments and intersegmental junctions. Because these different portions are known to function differently, the need for data on their individual development is obvious. We have measured the volumes of these different compartments by Cavalieri's point counting method in rat embryos from 11 to 17 days. It is shown that sinus venosus and sinu-atrial junction as well as the main compartments atrium, inlet, and proximal outlet segment grow roughly proportional to the total myocardial volume. Atrio-ventricular canal and distal outlet segment show a restricted growth and their proportional volumes decrease in time. The inlet segment is the most important part of the ventricular mass at 11 days of gestation, when it is still larger than the proximal outlet segment and, thus, takes the greater part in systolic action of the ventricular mass. The growth of the primary fold increases from day 13 onwards and can be considered as part of the wall of the inlet segment which gives rise to the main part of the ventricular septum. The timing of the septal volume increase fits with qualitative descriptions of ventricular septation. The atrio-ventricular canal and distal outlet segment have an important constrictive function in early stages, when valves are not yet present. Slow conduction and contraction patterns have been reported to be a characteristic feature of these portions of the embryonic heart. With development of valves these segments are loosing their mechanical function and, thus, their proportional volume declines.