Ultrastructural aspects of the lymphoid follicle-associated cells of the cloacal bursa after treatment with silica or carrageenan.

The present paper describes the ultrastructural changes in the follicle-associated epithelial cells of the cloacal bursae, which were observed after treating chickens with silica or with carrageenan. These substances, which are widely known to be toxic to macrophages, were used to establish whether the behaviour of the follicle-associated epithelial cells is the same as that of the lining epithelial cells, which are not affected by the action of silica and carrageenan, or whether the follicle-associated epithelial cells behave in the same way as macrophages. Experiments showed that silica and carrageenan cause extensive damage to the structure of the follicle-associated epithelial cells. The results, especially those obtained with the endobursal administration of silica, would seem strongly to support the hypothesis that follicle-associated epithelial cells are of mesenchymal origin, because silica is selectively toxic for mesenchymal cells with histiocytic activity.