Inhibitory effects of the graft versus host reaction upon the development of the bursa of Fabricius. Early modifications in the subepithelial mesenchyma.

The graft versus host reaction elicited in the chick embryo by lymphoid cells from a homologous adult donor is known to alter the histogenesis of the bursa of Fabricius. This paper reports the first scanning and transmission electron microscope studies of this phenomenon. Of the three main bursal components (lymphoid lineage, endodermic epithelium and mesenchyma), the subepithelial mesenchyma (SEM) exhibits the earliest and the most obvious modifications. There is a concomitant decrease of SEM alkaline phosphatase activity. It is suggested that disorganization of the SEM, which is a normal pathway of migration for the lymphoid precursors, reduces the inflow of these cells into the epithelial compartment.