Development of the subepicardial mesenchyme and the early cardiac vessels in the dogfish (Scyliorhinus canicula)

Abstract

A study was designed to assess the development of the subepicardial mesenchyme and the early cardiac vessels in the elasmobranch dogfish (Scyliorhinus canicula). The findings showed that the subepicardial mesenchymal cells originated, at least in part, from the primitive epicardium. This process seemed to be more intense where the subepicardium was the widest, namely, at the atrioventricular and conoventricular grooves 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, motility-like basal appendages, cell overlapping, intercellular gaps, and acquisition of a secretory phenotype. The epicardial cells which covered other parts of the heart were flattened and showed smaller nuclei; their basal surface was fibronectin-immunoreactive, unlike that of the hypertrophied epicardial cells of the atrioventricular groove. Fibronectin 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 covering of the heart. Before this network was established, numerous capillary-like structures were present in the subepicardial space. These capillary-like structures appeared as single cells with a large vacuole or as connections of cytoplasmic processes of one or several cells by junctional complexes. The cells that formed the capillary-like structures probably originated from the subepicardial mesenchymal cells. The main ultrastructural difference between the mesenchymal cells and the capillary-like structures was the presence, in the latter, of membrane-bound, electron-dense cytoplasmic inclusions 0.2–1.0 μm in diameter. Morphological evidence suggested that both the subepicardial capillary plexus and the endothelial precursors of the adult cardiac veins resulted from the coalescence of capillary-like structures. © 1996 Wiley-Liss, Inc.