Lymphatic drainage from the distal small intestine in sheep

Lymphatic drainage from the wall of the distal small intestine, important especially in young sheep as a major site of gut-associated lymphoid tissue, begins with a series of longitudinally oriented subserosal vessels. These vessels convey lymph to the mesenteric border of the intestinal wall and unite to form larger vessels which course through the mesentery to the mesenteric lymph nodes. These nodes lie towards the periphery of a broad, fan-shaped mesentery, adjacent to major arteries and veins. Mesenteric vessels convey lymph from the jejunum and part of the proximal ileum to the jejunal nodes. Lymph from most of the ileum is conveyed to caecal nodes. The larger mesenteric lymphatics have well formed smooth muscle and connective tissue layers surrounding the endothelium. They are often adjacent to, but rarely within, the connective tissue band anchoring the major arteries and veins to one or both lamellae of mesentery. Few anastomoses occur between vessels from opposing sides of the gut wall or the mesentery. Afferent lymphatics enter the subcapsular and trabecular sinuses of the nodes over most surfaces apart from the hilar region. Lymph flows through cortical tissue to the medulla, which occupies most of the node. In the medulla, sinuses occur within medullary cords as well as between them. Initial efferent lymphatics occur throughout medullary tissue. Efferent vessels emerge at a hilus then coalesce and drain into the jejunal or ileal trunk. The hilus of the node varies from a flat, poorly defined area on the lesser curvature, to a depression or groove. The latter commonly occurs in elongated jejunal nodes.(ABSTRACT TRUNCATED AT 250 WORDS)