Immunohistochemical analysis of the development of the bursa of Fabricius

The structure and the distribution of specific cell phenotypes in the bursa of Fabricius of the chicken were investigated by immunohistochemical staining of cryosections and by now cytometry using cell-specific monoclonal antibodies as markers. In post-hatch chickens the follicle-associated epithelium (KUL17(+), KUL18(-), non-specific esterase-positve) could readily be differentiated from the interfollicular epithelium (KUL17(+), KUL18(-), non-specific esterase-negative). Moreover, the ontogeny of the bursal structure was analyzed during embryogenisis. Using KUL05 and the CD57-specific monoclonal antibody HNK-1, two lymphoid cell populations were differentiated in the embryonic bursa: a CD57(-), KUL05(+) stem cell population between the follicles, and a CD57(+),KUL05(-) population in the follicles, which arises from the stem cells. The localisation of IgM on a subpopulation of the latter follicular cells and its expression from embryonic day (ED) 15 onwards suggested that IgM in B cells was only expressed after gene conversion.