Reappraisal of histogenesis in the bursal lymphoid follicle of the chicken

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Abstract
The development of the bursal follicle and the appearance of the follicle-associated epithelial (FAE) cell and the reticuloepithelial (REp) cell were studied. The stadied. 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 folicle 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 surounded by the epithelium itself, which closes under the clump of mesenchymal cells. The epithelial cells lying upon the mesenchymal clump become necrotic, and anumber of mesenchymal cells bulge out, forming the FAE cells. The epithelial cells that hav closed under the mesenchymal nodule become stratified and form the REp cells; they become star-shaped because the mdedullarylymphoid cells grow between them. Finally, the cortex in formed, possibly as a result of the migration of medullary cells before they peripheralize. it is concluded that FAE cells are not specialized 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.