Investigations on cell lineage and tissue interactions in the developing bursa of Fabricius through interspecific chimeras

Abstract

Embryonic rudiments of the bursa of Fabricius were grafted between quail and chick embryos at various developmental stages. The distinct morphological differences between the nuclei of the quail and those of the chick were used to determine the origin of cells in differentiated tissue at various times long after the initial graft. It was thus possible to demonstrate that the lymphocytes which develop in the bursa of Fabricius do not originate from the epithelium or from the mesenchyme of the rudiment but arise from precursor cells which reach the organ via the circulation. The inflow of hemopoietic stem cells takes place between 7 and 11 days of incubation in the quail embryo. In the chick the influx begins during the 8th day of incubation and extends until at least the 15th day. The basophilic cells which appear in the rudiment during bursal ontogeny were shown to be hemopoietic precursor cells of extrinsic origin. In the early stages of bursal development the basophilic cells seem to stay in the mesenchymal portion of the organ for a while before invading the epithelium. Some of them remain in the mesenchyme and undergo granulocytic differentiation. Only the cells which invade the epithelium develop into lymphocytes. Bursal histogenesis can occur only when the two primary components of the rudiment, i.e., the epithelium and the mesenchyme, are associated. The mesenchyme of the bursa proper cannot be replaced by another kind of mesenchymal substrate for promoting the development of the epithelium. When associated with the somatopleural mesoderm, the latter either remains in an undifferentiated state or even dedifferentiates.