Epicardial formation in staged human embryos.

The manner of epicardial formation has been unequivocally clarified by electron microscopy in representative species of the vertebrates. In human embryology, however, information on the epicardial formation is still lacking. To fill the gap of knowledge, we performed the light microscopic observations on serial paraffin sections of human embryos at Carnegie stages 12 to 17. At stage 12, initial formation of the epicardium was seen in 2 of 6 embryos. As was seen in chick embryos, villous protrusions of the mesothelial cells extending from the sinus wall region touched the ventricle on the dorsal side to spread out on it as a simple squamous epithelium. At subsequent stages, the epicardial covering was extended concomitantly with histogenesis and organogenesis of the embryonic heart. Although the final stage was difficult to define at the light microscopic level, the epicardium was considered to cover the entire heart by late stage 16, on the basis of reference to the findings on chick embryonic heart.