HISTOLOGICAL AND IMMUNOHISTOLOGICAL STUDY OF THE DEVELOPING AND INVOLUTING SUPERFICIAL CERVICAL THYMUS IN THE KOALA (PHASCOLARCTOS-CINEREUS)

The thymuses of 44 koalas, ranging from less than 30 d to more than 14 y of age, were examined histologically and immunohistologically. The thymuses from 17 of these koalas dying acutely through trauma were reg arded as not being significantly affected by disease and formed the ba sis for study of the normal thymus. Most other koalas had chronic illn ess and, consequently, disease affected (involuted) thymuses. Histolog ically, thymuses showed obvious corticomedullary differentiation with small Hassall's corpuscles visible in koalas more than 8 mo of age. Mo st cortical and medullary lymphocytes stained for CD3 and CD5 (T lymph ocyte markers) while some cells (predominantly medullary) stained for CD79b (B lymphocytes and plasma cells), IgG (plasma cells) or MHC clas s II (reticular epithelium, macrophages and possibly lymphocytes). Adu lts of up to 5-6 y of age which had died through trauma had little evi dence of involution and had prominent Hassall's corpuscles and medulla ry epithelial thymocytes. Thymic eosinopoiesis was an inconsistent fin ding. In traumatised animals over this age, involution was obvious wit h fibrous replacement of lobules, loss of Hassall's corpuscles and the development of dilated ducts lined by nonciliated epithelium. However , loss of lymphocytes was gradual and pockets of lymphocytes, centrall y located in lobules, were still present in the oldest koala examined. In these involuted thymuses, remaining lymphocytes stained for CD3 an d lesser numbers of CD5 and CD79b. Plasma cells were common and often stained both for IgG and MHC class II. Thymuses of chronically disease d koalas showed accelerated involution when age matched with thymuses from traumatised koalas. Chronically ill koalas as young as 18-24 mo s howed advanced involution, but the morphological and immunohistologica l characteristics of involuted thymus from diseased koalas could not b e distinguished from those of involuted thymuses derived from traumati sed koalas. It was concluded that development of the koala thymus is c ompleted at 8 mo of age and that for normal koalas involution is a gra dual process which starts not at but after sexual maturity. Immunohist ological characterisation of the thymus was comparable to that reporte d for a variety of eutherian mammals.