Lymphatic and blood vessels of the popliteal node in sheep.

Our aim was to describe the lymphatic and blood vascular pathways to and from the popliteal lymph node in sheep. The blood vessels and lymphatics were filled with Microfil, and were cleared in methyl salicylate. Afferent lymphatics divide and anastomose as they pass dorsally along the lateral saphenous vein, and 6-12 lymphatics reach the node. Each branches extensively on the surface of the node giving rise to 20-50 terminal afferents which enter the node over a roughly circular area. Most enter the subcapsular sinus, but some penetrate deeply into the node. Lymph leaves the node through numerous initial efferent lymphatics, many of which contain valves. These join forming progressively larger vessels, and 2-4 efferent trunks emerge from the hilus. The hilus varies considerably in shape, depth and location, and it is filled with fat. Either a single artery, or up to 10-12 arteries derived from an anastomotic network or circle, enter the node from the hilar fat pad. Arteries may also enter at other sites. The arteries originate from the caudal femoral, or the medial circumflex femoral artery; a single node may receive blood from both arteries. This arrangement may help to maintain blood flow especially during an immune response, and despite external pressures applied to the arteries and node during movements of the animal.