During organogenesis, the heart is one the first organs to develop and the earliest organ to function. The early appearance of cardiac activity in the tubular hearts of chick and rat embryos was noted many years ago [1, 2]. It arises from two plates of the splanchnic mesoderm which fuse to form a single tubular structure composed of endocardial and myocardial cells and, between them, the extracellular cardiac matrix. There is considerable variation in the formation of the extracellular matrix in the various regions of the heart during development. The endocardial lining cells of the vertebrate embryos show a regional specificity that remains an unexplained phenomenon in cardiac morphogenesis. The great majority of the endocardial lining cells remain epithlial. However, a restricted population of endothelial cells, lining the atrioventricular (AV) canal and the reputed proximal outflow tract (OT), transforms into mesenchyme; the latter being the reputed progenitor of the valves and membranous septa.

The purpose of this study was to investigate the extracellular cardiac matrix of the human fetal heart in different regions and in various stages of development, and also the heterogeneity of the endocardial cell lining, in connection with the endothelial cells of other cardiac vessels. Identification of the mesenchymal cells/extracellular matrix was confirmed by immunohistochemical techniques using the fol­lowing monoclonal antibodies: actin, desmin, vimentin, collagen IV and fibronectin. The present results provide evidence that the extracellular matrix of the heart is of mesodermal origin but at the level of the valves the mesenchyme is derived from the endothelial lining cells rather than the primitive mesenchyme.