Ultrastructure of capillaries in the conduction system of the heart in various mammals.

The capillaries in the atrioventricular node and atriventricular bundle of the heart (mammals) were investigated by thin sections and freeze-etch replicas. Fenestrated endothelial cells were found to occur in all mammals examined in this study. These fenestrations (500 +/- 50 A diameter) are bridged by a diaphragm and occur in relatively small sieve areas. In contrast to the AV-node and AV-bundle, the non-specialized myocardium is supplied by continuous capillaries. The presence of the fenestrated capillary in the conduction system of the heart may serve to increase both permeability and passive transport capacity.