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 regarded as not being significantly affected by disease and formed the basis for study of the normal thymus. Most other koalas had chronic illness and, consequently, disease affected (involuted) thymuses. Histologically, thymuses showed obvious corticomedullary differentiation with small Hassall's corpuscles visible in koalas more than 8 mo of age. Most cortical and medullary lymphocytes stained for CD3 and CD5 (T lymphocyte markers) while some cells (predominantly medullary) stained for CD79b (B lymphocytes and plasma cells), IgG (plasma cells) or MHC class II (reticular epithelium, macrophages and possibly lymphocytes). Adults of up to 5-6 y of age which had died through trauma had little evidence of involution and had prominent Hassall's corpuscles and medullary epithelial thymocytes. Thymic eosinopoiesis was an inconsistent finding. In traumatised animals over this age, involution was obvious with 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, centrally located in lobules, were still present in the oldest koala examined. In these involuted thymuses, remaining lymphocytes stained for CD3 and lesser numbers of CD5 and CD79b. Plasma cells were common and often stained both for IgG and MHC class II. Thymuses of chronically diseased koalas showed accelerated involution when age matched with thymuses from traumatised koalas. Chronically ill koalas as young as 18-24 mo showed advanced involution, but the morphological and immunohistological characteristics of involuted thymus from diseased koalas could not be distinguished from those of involuted thymuses derived from traumatised koalas. It was concluded that development of the koala thymus is completed at 8 mo of age and that for normal koalas involution is a gradual process which starts not at but after sexual maturity. Immunohistological characterisation of the thymus was comparable to that reported for a variety of eutherian mammals.