HISTOLOGICAL AND IMMUNOHISTOLOGICAL INVESTIGATION OF ALIMENTARY-TRACTLYMPHOID-TISSUE IN THE KOALA (PHASCOLARCTOS-CINEREUS), BRUSHTAIL POSSUM (TRICHOSURUS-VULPECULA) AND RINGTAIL POSSUM (PSEUDOCHEIRUS-PEREGRINUS)

The histological appearance and distribution of T cells, B cells and p lasma cells were investigated for oropharyngeal tonsils, small intesti nal lymphoid aggregations (Peyer's patches), caecocolic lymphoid patch es and mesenteric lymph nodes of koalas (Phascolarctos cinereus), comm on brushtail possums (Trichosurus vulpecula) and common ringtail possu ms (Pseudocheirus peregrinus). The histological organisation and distr ibution of lymphoid cell subpopulations of these tissues were similar to those described in eutherian mammals, although some differences wer e found in comparison with previous descriptions of American opossum t issues. The main variation among the 3 species was in the structural o rganisations of the oropharyngeal tonsil and the caecocolic lymphoid p atch which were more complex in the koala than in possums. In the koal a the extensive crypts of the oropharyngeal tonsils and folding of the mucosa of the caecocolic lymphoid patch increased their surface area and, in addition, both structures had areas of epithelium heavily infi ltrated with T and B cells. These features could indicate that these s tructures are important in immunological surveillance of orally presen ted antigens in koalas.