The distribution of organised lymphoid tissue in the alimentary tracts of koalas (Phascolarctos cinereus) and possums (Trichosurus vulpecula and Pseudocheirus peregrinus).

The anatomical arrangement of organised lymphoid tissues of the alimentary tract for 3 Australian marsupials, the koala (Phascolarctos cinereus), the common brushtail possum (Trichosurus vulpecula and the common ringtail possum (Pseudocheirus peregrinus), was determined by gross dissection and acetic acid treatment. Oropharyngeal tonsils were consistently found in the dorsolateral wall of the caudal oropharynx in all 3 species and additionally in the ventral soft palate of the koala. Aggregated lymphoid nodules (Peyer's patches) were present in the small intestine of koalas, ringtail possums and brushtail possums and were of similar appearance for all 3 species. Bilateral large intestinal lymphoid patches were detected in the caecocolic lateral wall adjacent to the termination of the ileum for all 3 species. Caecocolic patches were more complex in koalas and had mucosal folds and a central recess. In addition, solitary and grouped large intestinal lymphoid nodules were variably present in the proximal colon and caecum of the koala. In contrast, possums had solitary and grouped large intestinal lymphoid nodules present in the proximal colon and rectum but not the caecum. Aggregated lymphoid tissue was not detected in the tongue, oesophagus or stomach for all 3 species. In contrast to a previous report, this study did not find a paucity of lymphoid tissue associated with the gut of the koala. The appearance and distribution of gut-associated lymphoid tissue in koalas and possums was found to be similar to that described in other marsupials and eutherian mammals, although some variations in appearance and anatomical location were observed.