Experimental study on the formation of the epicardium in chick embryos.

In chick embryos, the formation of the epicardium proceeds from the attachment of a secondary sinuventricular mesocardium. This mesocardium is formed by the adhesion of pericardial villi with the dorsal surface of the heart. It was the aim of this study to clarify the role of the pericardial villi in the formation of the epicardium. For this purpose, the contact between the pericardial villi and the heart was prevented by placement of a piece of the shell membrane between them. After re-incubation, the hearts of the experimental embryos could be assigned to one of two different groups: hearts completely lacking a secondary mesocardium (Group A), and hearts without the sinu-ventricular but with a dystopic secondary mesocardium (Group B). In Group A, the formation of the epicardium and subepicardial mesenchyme was found to be severely disturbed. In Group B, the formation of the epicardium proceeded from the point of attachment of a dystopic secondary mesocardium; defective development of the subepicardial mesenchyme was not encountered. These results support the view that the epicardium is derived from the pericardial epithelium.