Structure and distribution of lymphatic capillaries and fenestrated blood capillaries in the conduction system of the rabbit heart.

The distribution and structure of lymphatic and blood capillaries in the rabbit heart conduction system were investigated by transmission electron microscopy. The sinuatrial node, atrioventricular node, and atrioventricular bundle possessed a rich network of lymphatic capillaries, which were situated not only at the periphery but also in the interior of the conduction system. The fine structure of these lymphatic capillaries was essentially similar to those within the atria and the ventricles. Although blood capillaries within working myocardium were nonfenestrated, the heart conduction system was often supplied by fenestrated blood capillaries. In the atrioventricular node and bundle especially, fenestrated blood capillaries and lymphatic capillaries were topographically associated, forming an extensive microcirculatory system. The presence of fenestrated capillaries suggests that a fast transcapillary passage of metabolites occurs in these regions, while the lymphatic capillaries may play an important role in the removal of macromolecules and excess intercellular fluid.