The arrangement of gut-associated lymphoid tissues and lymph pathways in the koala (Phascolarctos cinereus).

Gut-associated lymphoid tissues are poorly developed in koalas. They comprise paired caecocolic lymphoid patches, and a few small mesenteric lymph nodes. The patches lie opposite one another in the lateral gut wall at the junction of the ileum, caecum and proximal colon. The lymphoid parenchyma of the patches consists of a layer of nodules and internodular parenchyma in the submucosa. Apoptosis is common in the nodules. The mucosa and lymphoid tissue of each patch is continuous over a caecocolic recess, formed by the coalescence of laminae which extend along the large intestine. Lymph sinuses between and beneath the lymphoid nodules are continuous with efferent lymph vessels in the submucosa. These then enter 2-4 small lymph nodes at the root of the mesentery. The paucity of lymphoid tissue associated with the gut may be related to germicidal activity in the Eucalypt leaves eaten by the koala.