Identification of the bursa-dependent and thymus-dependent areas in the tonsilla caecalis of chickens.

In this report, on the basis of the difference in lymphocytic make-up, the lymphatic tissue of the tonsilla caecalis was divided into two parts, the mid and deep portion of the lamina propria and its superficial portion which was referred to as a subepithelial zone. In the former there were accumulated small lymphocytes, while in the latter lymphocytes in various sizes, small to large, and cells of plasmocytic series. The germinal centers always appeared in the mid and deep portion and never in the subepithelial zone. Chickens which were bursectomized and X-irradiated at hatching showed a marked depletion of lymphocytes in the subepithelial zone while an increasing accumulation of small lymphocytes was persistent in the mid and deep portion. On the other hand, in chickens which received repeated X-irradiation of the thymus, lymphocyte depletion was striking in the mid and deep portion but not in the subepithelial zone. In both groups of treated chickens the germinal centers were markedly decreased in number or essentially lacking. In the bursectomized-irradiated chickens, foci of reticular cell aggregation usually occurred in the mid and deep portion and were deemed as an incomplete form of the germinal center. It was concluded that in the tonsilla caecalis the bursa-dependent area was the subepithelial zone and the thymus-dependent area was the mid and deep portion of the lamina propria, Possible relation to the germinal center development of three factors, i.e. the formation of the focus of reticular cell aggregation in the thymusdependent area, the accumulation of bursa-derived lymphoid cells and a certain thymus-dependent factor, was discussed.