THE ARRANGEMENT OF GUT-ASSOCIATED LYMPHOID-TISSUES AND LYMPH PATHWAYSIN THE KOALA (PHASCOLARCTOS-CINEREUS)

Gut-associated lymphoid tissues are poorly developed in koalas. They c omprise 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 lymp hoid parenchyma of the patches consists of a layer of nodules and inte rnodular parenchyma in the submucosa, Apoptosis is common in the nodul es. The mucosa and lymphoid tissue of each patch is continuous over a caecocolic recess, formed by the coalescence of laminae which extend a long the large intestine. Lymph sinuses between and beneath the lympho id 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.