DEVELOPMENT OF THE SUBEPICARDIAL MESENCHYME AND THE EARLY CARDIAC VESSELS IN THE DOGFISH (SCYLIORHINUS-CANICULA)

A study was designed to assess the development of the subepicardial me senchyme and the early cardiac vessels in the elasmobranch dogfish (Sc yliorhinus canicula). The findings showed that the subepicardial mesen chymal cells originated, at least in part, from the primitive epicardi um. This process seemed to be more intense where the subepicardium was the widest, namely, at the atrioventricular and conoventricular groov es as well as at the lateral margins of the ventricle. At these sites, the epicardial cells displayed morphological features usually related to epithelial-mesenchymal transitions, i.e., cell hypertrophy, motili ty-like basal appendages, cell overlapping, intercellular gaps, and ac quisition of a secretory phenotype. The epicardial cells which covered other parts of the heart were flattened and showed smaller nuclei; th eir basal surface was fibronectin-immunoreactive, unlike that of the h ypertrophied epicardial cells of the atrioventricular groove. Fibronec tin immunoreactivity also developed in the subepicardial space as the mesenchymal cell population increased. In the dogfish, a subepicardial network of capillaries developed subsequent to the epicardial coverin g of the heart. Before this network was established, numerous capillar y-like structures were present in the subepicardial space. These capil lary-like structures appeared as single cells with a large vacuole or as connections of cytoplasmic processes of one or several cells by jun ctional complexes. The cells that formed the capillary-like structures probably originated from the subepicardial mesenchymal cells. The mai n ultrastructural difference between the mesenchymal cells and the cap illary-like structures was the presence, in the latter, of membrane-bo und, electron-dense cytoplasmic inclusions 0.2-1.0 mu m in diameter. M orphological evidence suggested that both the subepicardial capillary plexus and the endothelial precursors of the adult cardiac veins resul ted from the coalescence of capillary-like structures. (C) 1996 Wiley- Liss, Inc.