The early development of the epicardium in Tupaia belangeri.

Development of the epicardium was studied in embryos of Tupaia belangeri from the 13th to 15th day of ontogeny. The greater part of the epithelium of the epicardium does not differentiate locally from the myoepicardium (cardiac splanchnopleure, splanchnic mesoderm), but rather from the coelomic epithelium of the septum transversum. The myoepicardium of the future atria and ventricles differentiates into myocardial cells only. On ontogenetic day 13, bulbar protrusions (the "villi" of Kurkiewicz 1909) are formed on the surface of the septum transversum and extend into the pericardial cavity, primarily between the sinoatrial and the ventricular regions of the embryonic heart. These protrusions are covered by flattened interdigitating cells, and they are filled with intercellular fluid of the mesenchyme of the septum transversum. Many mitoses are found among the cells. From these protrusions free vesicles are formed which are discharged into the pericardial cavity. The vesicles attach to the surface of the myoepicardium, i.e. to the developing myocardial cells. The vesicles open, and their cells spread out onto the surface of the heart to form the primary epicardium. This process begins on the dorsal surface of the heart, close to the protrusions of the septum transversum, there are, however, further isolated patches of primary epicardium in other regions of the surface of the heart. After the epicardial cells have settled onto the myocardium, mitoses become rare among them. On day 15, most of the myocardium is coated by the primary epicardium and the protrusions on the septum transversum disappear. A "bare" myocardium, as found on ontogenetic days 12 and 13 in Tupaia, might be a primitive (plesiomorphic) condition among chordates. In adult Branchiostoma, the coelomic epithelium which coats the contractile blood vessels had been found to differentiate into muscle cells that remain uncoated on the side facing the coelomic cavity (Franz 1933; Joseph 1914, 1928).