The marsupial and monotreme thymus, revisited

The anatomy and histology of the thymus in monotremes and marsupials were r evisited and several new observations made. Tissues of the mediastinum and/ or ventral neck of 134 animals altogether (12 Australian marsupial and mono treme species) were examined and the locations of thymic tissues documented . Descriptions not previously recorded include the gross anatomy of the thy mus in monotremes, the bandicoot (Isoodon macrourus, Isoodon obesulus), the wombat (Lasiorhinus latifrons), and koala (Phascolaretos cinereus), as wel l as light and electron microscopic features in all species examined. In mo notremes, thymic tissue was diffusely spread in the mediastinum and extende d over the ventral surface of the pericardium. In the bandicoot, a small, d eep cervical thymus was persistently present as well as the expected thorac ic thymus and contrary to previous observations, a thoracic thymus was foun d in the wombat but not the koala. Both species also have a cervical thymus . In the marsupial species examined, additional thymic tissue was often pre sent in the vicinity of the carotid bifurcations, in the deep Ventral regio ns of the neck. In many animals, thymic tissue of deep cervical origin as w ell as thoracic thymus was mingled with parathyroid tissue, In several spec imens, the two tissue types lacked intervening connective tissue barriers a nd at the electron microscopic level, attenuated processes of epithelial re ticular cells were the only tissue components separating parathyroid and th ymic tissues. The role of the cervical thymus in diprotodont marsupials rem ains to be elucidated. The presence of aberrant thymic tissue in marsupials is important in the interpretation of experiments involving thymectomy and immunological development because complete removal of thymic tissue may no t occur. One aspect that has received little attention is the function of t he thymic epithelial reticular cells in marsupial development and a suggest ion is made that perhaps the extra thymic tissue found in marsupials provid es extra epithelial reticular cells necessary for the production of cytokin es and other secretions to sustain the development of immunological compete nce.