Developmental morphology of blood and lymphatic capillary networks in mammalian hearts, with special reference to...

The development of blood and lymphatic capillaries in the cardiac ventricles was studied using the rat and monkey hearts from later gestation to adulthood. For scanning electron microscopy, the tissue was treated with ultrasonication followed by HCl in order to remove epicardial mesothelial cell, subepicardial connective tissue and basement membrane. At the end of fetal life and neonate, the blood capillaries ran in parallel between the bundles consisting of several cardiac myocytes with small size. Endothelial extensions were found frequently. Numerous pericytes with short processes were observed on the capillary wall. Blood capillaries in the pubertal myocardium ran between cardiac myocytes to form a dense network. Processes of pericytes became elongated along the capillaries. In the adult, the capillaries showed occasionally Y-shaped branching and H-shaped anastomoses. The lymphatic capillaries were identified by large, variable sizes and absence of pericytes. In early postnatal rats, they appeared in the subepicardial region. In young and adult animals, the lymphatic capillary networks with relatively dense reticular meshes surrounded the bundles of several myocytes. It was noted that numerous nerve fibers were in close to the lymphatic capillary wall.