The cytoplasm of the mesenchymal cells contained mitochondria, endoplasmic reticulum, some tissue inclusions and debris, but no myofibrils or glycogen. The condensation of the mesenchyme increased gradually between E12 and E14. They were located in the muscular edge of the projection of the primary atrioventricular valve as well. Large mesenchymal cells with denser cytoplasm were also seen near the myocardial border. The endocardial cushion has a critical role in heart development because it contributes to the valves and septa.
Differentiation includes proliferation and apoptosis.
The occurrence of HSP25 in the developing cardiomyocytes but not in the endocardial cushion indicates that an association of HSP25 and apoptosis in the myocytes and in the endocardial cushion might occur via internal or external stimuli in response to the differentiation of the mesenchyme. Moreover, HSP25 coincided with relatively less apoptosis in the myocardium than in the mesenchyme on days 11 through 14, suggesting that the role of HSP25 is most likely to be protective. The mesenchymal cells multiply, increasing the population of the cushion cells. Some mesenchymal cells differentiate to myocytes and fibroblasts. The presence of HSP25 in these developing cells indicates that it is apparently involved in the differentiation of mesenchymal cells to myocytes.