An area of T cell localization in the cloacal bursa of White Leghorn chickens.

Within the cloacal bursa (bursa of Fabricius), there is a diffusely infiltrated area (DIA) of lymphoid cells just dorsal to the bursal duct opening. Because this region displayed certain histologic characteristics of thymus-dependent areas found in mammals, such as high endothelial venules, heavy reticular fiber components, and epithelium infiltrated with lymphocytes, experiments were conducted to determine whether thymus-derived cells (T cells) were present. Two independent methods substantiated the presence of T cells in the DIA. After the acid alpha-naphthyl acetate staining technique, the characteristic T cell focal esterase activity could be demonstrated in significant numbers of cells found in the DIA. Also autologous radioactively labeled thymocytes could be identified in the DIA by autoradiography a few hours after IV injection. This establishment of a definite T cell area within the bursa gives additional credence to the contention that the bursa functions as a secondary lymphatic organ.