Quantitative morphology of the embryonic heart: an approach to development of the atrioventricular valves

In 32 human embryos ranging from 4.0 to 42 mm CR-length, the volumes of the atrioventricular endocardial cushions and of the ventricular myocardium were estimated by the point counting method. The surface densities of the left and right ventricular apical trabeculations were estimated by the point and intersection counting method. It is concluded that the cushions do not grow after the 25 mm stage, by that time having reached the maximal value of 0.074 mm3. This supports the concept that the cushions do not materially contribute to the definitive atrioventricular valves. In young embryos, the left ventricular trabeculations are thicker (as concluded from their higher surface density) than the right ventricular trabeculations. Only around the 25 mm stage, the ratio becomes 1. After this stage, right ventricles have thicker trabeculations than left ventricles have. This supports the concept that the trabecular pattern is changed during the period of valve formation, which process is characterized by delamination of the inner myocardial layers.