Epithelial‐mesenchymal transitions in the developing heart of the dogfish (Scyliorhinus canicula). A scanning electron microscopic study

Abstract

An epithelial‐mesenchymal transition is involved in two main morphogenetic events of cardiac morphogenesis, namely the differentiation of the valvuloseptal tissue from the endocardial endothelium, and the formation of subepicardial mesenchyme from the epicardial mesothelium. We have proposed that the dogfish (Scyliorhinus canicula) is a suitable model for the study of basic processes of cardiac morphogenesis in vertebrates, since the heart of this primitive fish probably outlines the original bauplan of the vertebrate heart. In order to study in this model the endocardial and epicardial epithelial‐mesenchymal transition under scanning electron microscopy, we have used a technique of paraffin‐embedding, partial sectioning, dewaxing and critical‐point drying. Our results showed: 1) A centrifugal pattern of epicardial development from the atrioventricular groove to the sinus venosus and conus arteriosus; 2) A close spatial and temporal relationship between the endocardial and epicardial epithelial‐mesenchymal transition, although the transformation of the endocardium starts earlier and ends later the epicardial transformation; 3) A complex arrangement of the fibrous extracellular matrix which is established prior to the migration of the mesenchymal cells. Subepicardial, but not subendothelial mesenchymal cells, coalesce in unicellular or pluricellular ring‐like structures that probably are related to the origin of the cardiac vessels.