Lymphoid follicle formation in the bursa of Fabricius of the chick embryo.

The colonisation of lymphoid stem cells and the following lymphoid follicle formation in the bursa of Fabricius of the chick embryo was studied histochemically and morphologically. Most basophilic haematopoietic stem cells first appeared in the mesenchyme of the distal bursa at 7 and 8 days. Basophilic cells were also seen in the loose connective tissue near the distal bursa, and in the vessels which were located dorsal to the distal bursa. They began to invade the epithelium at 10 and 11 days of incubation by digesting the basal laminar components which the fluorescent lectin probes stained. Haematopoietic stem cells were in close association with the extracellular matrix in the mesenchyme and contained abundant F-actin. F-actin also increased in the epithelial cells surrounding basophilic haematopoietic stem cells during lymphoid follicle formation. F-actin in migrating haematopoietic stem cells and epithelial cells might be involved in the migration of haematopoietic cells and the histogenesis of the lymphoid follicles. Granulopoiesis occurred mainly in the bursal mesenchyme, but some granulocytes were seen in the epithelium. The results support the extrinsic origin of the bursal lymphoid stem cells and their active migration towards the epithelium in the chick embryo. Cell-cell interactions and tissue interactions in the lymphoid follicle formation are also discussed.