Immunoreactive POMC-derived peptides and cytokines in the chicken thymus and bursa of Fabricius microenvironments: age-related changes.

Changes from hatching to the involutive stage in the thymus and bursa of Fabricius of Gallus domesticus were studied. Pro-opiomelanocortin (POMC)-derived peptides and cytokines were also tested by immunocytochemical procedures. Thymic histological modifications appeared at 3 months and involved an increase in connectival argyrophilic reticular fibres, a proliferation of non-epithelial cells in the reticulum network, the presence of mucous cells and small mucous cysts, and an increase in the number of eosinophilic and myoid cells. A clear distinction between the cortex and the medulla was lost. Immunoreactive POMC-related molecules and cytokines were demonstrated in thymic cells from 4-day-old chicken, and their number increased with ageing. These molecules were expressed in a few single epithelial cells and in interdigitating cells. With ageing, the number of immunoreactive interdigitating cells also increased, and these appeared in an activated phase. Histological modifications in the bursa of Fabricius appeared at 2 months and concerned a folding of the interfollicular surface epithelium covering the bursal plicae and a reduction in lymphoid follicle-associated epithelium. Fibrous tissue gradually increased, and large mucoid cysts were evident. The expression of POMC-derived peptide-and cytokine-like molecules differed during the development and involution phases of the organ. Cells of follicle associated epithelium and dendritic reticular cells of lymphoid follicles were immunoreactive to beta-endorphin between 4 days and 2 months, while ACTH-, alpha-MSH- and cytokine-like molecules were observed in follicles after 2 months. The findings indicate a physiological role of these molecules during the growth and involution of the two organs.