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 stain ing of cryosections and by now cytometry using cell-specific monoclonal ant ibodies 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-nega tive). Moreover, the ontogeny of the bursal structure was analyzed during e mbryogenisis. Using KUL05 and the CD57-specific monoclonal antibody HNK-1, two lymphoid c ell populations were differentiated in the embryonic bursa: a CD57(-), KUL0 5(+) stem cell population between the follicles, and a CD57(+),KUL05(-) pop ulation in the follicles, which arises from the stem cells. The localisatio n of IgM on a subpopulation of the latter follicular cells and its expressi on from embryonic day (ED) 15 onwards suggested that IgM in B cells was onl y expressed after gene conversion.