Pathways of blood flow to and through superficial lymph nodes in the dog.

Wide variations occur in the arrangement of the blood vessels of the superficial lymph nodes in dogs. These vessels were studied using Microfil and Mercox casts, scanning and transmission electron microscopy, and the dye Alcian blue. Most nodes receive some arteries at a hilus, but this varies from a single indentation to one or more grooves and in some cases is a smooth and flattened area. Some nodes have no identifiable hilus and arteries enter at many different points. Most nodes that receive arteries at a hilus also receive additional vessels, some of which are derived from a network at the capsule; some of these enter by crossing the subcapsular sinus. In both cortex and medulla, some capillaries are within lymph sinuses, or adjacent to them. Within the cortex, networks of arterioles, capillaries and venules occur near subcapsular and trabecular sinuses, and around nodules, but the nodules themselves are relatively avascular. Some medullary capillaries are fenestrated, but all other capillaries have a complete endothelial lining, with basement membrane and pericytes. All venules, except those in the deep cortex, have a low endothelial lining. Although the distribution of vessels within the nodes follows a general pattern in dogs, the variation in the pattern of supply is such that it is not possible to ascribe any particular pattern to superficial nodes in general, or even to the nodes of a particular lymph centre. This contrasts with the commonly accepted description which implies that all blood vessels to all lymph nodes pass through a well defined hilus.