IMMUNOHISTOCHEMICAL STUDY OF THE ORIGIN OF THE SUBEPICARDIAL MESENCHYMNE IN THE DOGFISH (SCYLIORHINUS-CANICULA)

A mesenchyme appears at the subepicardium of the vertebrate embryo clo sely following the epicardial ensheathing of the heart. We have sugges ted earlier that a localized epithelial-mesenchymal transition (= tran sdifferentiation) of the epicardial mesothelium might explain the orig in of the subepicardial mesenchymal cells (SEMC) in a primitive verteb rate, the dogfish Scyliorhinus canicula. We have tested our hypothesis through the immunolocalization, in the embryonic dogfish heart, of th ree proteins involved in the epithelial-mesenchymal transitions. Fibro nectin immunoreactivity (FN-IR) was present between epicardium and myo cardium from the earliest stages of the epicardial development. Howeve r, in embryos ranging between 20 and 35 mm TL, FN-IR disappeared from the basal surface of the epicardial cells at the atrioventricular and conoventricular grooves and increased in association with the SEMC. Pr oliferating-cell nuclear antigen immunoreactivity was intense in most epicardial cells and SEMC in these areas and developmental stages, but it declined in later embryos. The JB3 antigen, a fibrillin-related pr otein, was detected in the cytoplasm and basal surface of the epicardi al cells, as well as on the SEMC surface. These immunohistochemical pa tterns were remarkably similar at the subepicardium and at the endocar dial cushions, which are populated by mesenchymal cells derived from t he transdifferentiation of the endocardium. (C) 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. Ail rights re served.