Intercellular gaps in the early development of chick mural endocardium. A TEM study.

Embryonic chick hearts stages 15 to 21 according to the Hamburger-Hamilton series (1951) were subjected to TEM and light-microscopical examination to investigate ventricular mural endocardium during early trabeculation. Serial sectioning of consecutively-aged embryos indicated an invagination of the endocardial layer which lines the intertrabecular and intermuscular spaces. During the proceeding development, frequently occurring approaches and contacts between mural endocardium and myocardium could be observed. At all stages examined, the cardiac jelly appeared as an acellular, at particular sites very attenuated matrix. The intercellular gaps of the endocardium could not be correlated to the invagination and the endothelialization of the intertrabecular spaces.