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The development of the bursal follicle and the appearance of the follicle-associated epithelial (FAE) cell and the reticuloepithelial (REp) cell were studied. The stages of development of the bursal follicle were observed by light and electron microscopy; an anticytokeratin monoclonal antibody was also used. At the beginning of follicle development, a mesenchymal cell cluster is observed in the tunica propria; the cluster becomes wedged in a niche of the surface epithelium, and gradually it is completely surrounded by the epithelium itself, which closes under the clump of mesenchymal cells. The epithelial cells lying upon the mesenchymal clump become necrotic, and a number of mesenchymal cells bulge out, forming the FAE cells. The epithelial cells that have closed under the mesenchymal nodule become stratified and form the REp cells; they become star-shaped because the medullary-lymphoid cells grow between them. Finally, the cortex is formed, possibly as a result of the migration of medullary cells before they peripheralize. It is concluded that FAE cells are not specialized epithelial cells, as they do not react to an anticytokeratin monoclonal antibody; on the contrary, they are formed by mesenchymal stemcells that bulge into the lumen and change their character after moving into the epithelium. The REp cells appear in the follicular primordium shortly after the bursal follicle begins to develop; the pronounced reactivity of the REp cells to an anticytokeratin monoclonal antibody supports the hypothesis of their epithelial origin.

The lymphoid-follicle associated epithelial (FAE) cells of the bursa Fabricii of chickens show enzymatic and micropinocytotic activities; they can perform functions reminiscent of those of macrophages. To test this relationship, we checked whether FAE cells are sensitive to carrageenan, a substance widely known to be toxic for macrophages. This substance was gently introduced into the bursal lumen and was left there for a period of 72 to 120 h. The bursae of Fabricius were then examined histologically and histochemically. The result was that the FAE areas had been reduced in number and that their structural pattern had been modified. This effect suggests that FAE cells may be sensitive to carrageenan. The immunological response to SRBC was studied in one group of animals that had received carrageenan. The antibody seemed to elicit an increase in the agglutinins and in the number of direct PFC/spleen against SRBC in comparison with control chickens. On the basis of the morphological and immunological results obtained, the origin and the role of the FAE cells of the bursa of Fabricius are discussed.