Blood vessel architecture in lymph nodes of the dog viewed by scanning electron microscopy.

Scanning electron microscopy was used to investigate the submicroscopic organization of the blood vessels of dog lymph nodes (LN). The LN vessels were casted by systemic perfusion of the animals vasculature with two resins of distinct viscosity. This resulted in the retrieval of two types of LN vascular replicas that depicted either the arterial blood system alone (methacrylate casting) or both the arterial and venous blood systems (Mercox casting). We found that the dog LN showed a significantly higher density of arterial vessels in the cortex than in the medulla. In the cortical domain, a subcapsular layer stood out because of its rich content in arterial capillaries. The use of the high-resolution resin (mercox) resulted in excellent structural detail of the luminal surface of the LN vessels that allowed a clear distinction between arterioles and venules based on the geometrical pattern of the imprints left in the replicas by the nuclei of endothelial cells. At the cortex-medulla frontier, most arterioles showed narrowings of their lumen that suggested the existence of sphincters at this level. Our findings document that the microanatomical arrangement of blood vessels in the LN of the dog is different from that of LN from other mammals studied so far, in particular from rodents where vascular-poor microdomains have been reported in the LN cortex. The arteriolar sphincters that we detected at the innermost zone of the cortex may represent the structural counterpart of the physiological modulation of the blood supply of the cortex exerted by arterial branches coming from the hilus.