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. The regional correlation of HSP70 with abundant apoptotic cells on day 14 suggests that it is involved in the death of some cells in the cushion mesenchyme. The involvement of HSP70 in this apoptosis might be either via a permissive or triggering role, as HSP70 was also detected in some normal cells. HSP70 appears to have a triggering role in the differentiation of the mesenchyme to myocytes, because it appears during clear maturation of the cushion to myocytes. Our results thus suggest that maintenance of the development of the endocardial cushion is mediated by cell division and differentiation factors.