Development of the thymus, spleen, lymph nodes and liver in the marsupial,Isoodon macrourus (Northern brown bandicoot, Peramelidae)

We report for the first time the development and morphological characterist ics of the spleen thymus, lymph nodes and liver of the northern brown bandi coot, Isoodon macrourus. To date few marsupial species have been studied. T he development and morphological characteristics of the organs shared the t ypical features of those few other marsupials studied as well as those of e utherians. These suggest comparative functional properties with the eutheri an immune system. The thymus differentiated within the first week of pouch life and showed evidence of immunolymphopoiesis up to juvenile stages. The spleen, though seeded by lymphocytes within the first week of pouch life, w as slower to mature, but differentiated and showed signs of immunocompetenc y by the time young left the pouch. The mature spleen displayed the same an atomical blood filtering and immunosurveillance properties as that of the e utherian spleen, with evidence of erythrocyte destruction, thrombopoietic a ctivity, activation and differentiation of immunocompetent lymphocytes. How ever, the absence of sheathed capillary structures in the spleen may indica te differences in the humoral response to circulating antigens. Similarly, lymph nodes also mature by this stage with anterior nodes appearing before posterior nodes. The mature lymph nodes displayed structural features of se condary immuno-lymphoid organs consistent with production of immune respons es. Finally, the liver displayed haemopoietic activity for the first four w eeks of pouch life. The pattern of development in the bandicoot appears to parallel the pattern reported for other marsupials, yet the thymus matures considerably earlier than previously reported and may be of significance in the development and onset of cell-mediated immunity. Current studies to ch aracterise cellular components, such as T/B lymphocytes and accessory cells of these organs will help to define the mechanisms of immune recognition, activation and hence outline the asis of the marsupial humoral and cellular immunity.