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

An epithelial-mesenchymal transition is involved in two main morphogenetic events of cardiac morphogenesis, namely the differentiation of the valvulos eptal tissue from the endocardial endothelium, and the formation of subepic ardial mesenchyme from the epicardial mesothelium. We have proposed that th e dogfish (Scyliorhinus canicula) is a suitable model for the study of basi c processes of cardiac morphogenesis in vertebrates, since the heart of thi s primitive fish probably outlines the original bauplan of the vertebrate h eart. In order to study in this model the endocardial and epicardial epithe lial-mesenchymal transition under scanning electron microscopy, we have use d a technique of paraffin-embedding, partial sectioning, dewaxing and criti cal-point drying. Our results showed: 1) A centrifugal pattern of epicardia l development from the atrioventricular groove to the sinus venosus and con us arteriosus; 2) A close spatial and temporal relationship between the end ocardial and epicardial epithelial-mesenchymal transition, although the tra nsformation of the endocardium starts earlier and ends later the epicardial transformation; 3) A complex arrangement of the fibrous extracellular matr ix which is established prior to the migration of the mesenchymal cells. Su bepicardial, but not subendothelial mesenchymal cells, coalesce in unicellu lar or pluricellular ring-like structures that probably are related to the origin of the cardiac vessels.