[Development of the spatial organization of the myocardium]

The spatial organization of cardiac muscle was studied in the developmental stages of the rat. In the 10-day embryo the heart loop has formed and the different cardiac segments can be distinguished by the presence of intersegmental constrictions of the myocardial wall. The intersegmental myocardium looks already well organized, whereas the free segmental walls still lack a lengthwise orientation of the myocytes. On the 15th embryonic day the intersegmental zones are still the best organized, the free ventricular walls still showing areas of unorganized myocytes. Early differentiation thus seems to be concentrated in the intersegmental myocardium. This differentiation could serve contraction, as well as impulse conduction. Development of the endomysial collagen skeleton was seen in later stages. On the 13th embryonic day, collagen fibres were only seen in the septum transversum and around the venous sinus. Collagen development appeared to progress subepicardially from the venous towards the arterial pole. On the 16th day collagen fibres were found within the ventricular myocardium, but mainly in the intersegmental zones. The free ventricular walls were invested with collagen only on the 17th embryonic day, but the subendocardial myocardium, which is trabeculated and traversed by endocardial sinusoids, was relatively poor in collagen. Thus, a comparison can be made between the development of collagen and that of the coronary vascular system, which arises from epicardial as well as endocardial contributions. Further morphometric studies showed that the collagen content of ventricular myocardium gradually rises until the end of the 4th postnatal week.