The Bursa of Fabricius of 15 day, 1-, 3-, and 6 month-old adult chickens (White Leghorn strain) were studied by histological and histochemical staining, histoenzymatic reactions (LDH, SDH, α-GPDH, NAD, NADPH, Ca ++ -dependent ATP-ase, pH 8.5) and by anti-thymostimulin immunoreaction. Positive reactions for mucopolysaccharides and enzymatic activities were located in the epithelia of the follicles, i.e. in follicleassociated-epithelium (FAE), inter-follicle-epithelium (IFE) and in different epithelial compartments of cortical and medullary zones. Positive reaction for thymostimulin-like (TSlike) substance was restricted to FAE cells and weakly to the basal lamina of IFE. In 6-month-old chickens, the FAE cells disappeared; the phenomenon of bursal regression was evident, although not all the follicles were involved. In the few still normal follicles, the good reactivity to the enzymes tested suggests that residual physiological activity is still present, even if reduced. , 1621) is a primary lymphoid organ peculiar to birds. The bursa is responsible for the maturation and differentiation of B-lymphocytes to produce humoral immunological responses. The other primary lymphoid organ, the thymus, instead, is responsible for the maturation and differentiation of T-lymphocytes, leading to cellular immunological responses (Dent and Good, 1965;Schaffner et al., 1974a;Le Douarin et al., 1984).The structure of the bursa has been described both during the ontogenetic period (Jolly, 1911a(Jolly, , 1914Cordier, 1969;Edwards et al., 1975;Le Douarin et al., 1975;Le Douarin et al., 1976;Le Douarin et al., 1984;Ritter and Lebacq, 1977) and during the posthatching period up to its involution (Jolly, 1911b(Jolly, ,1914Mc Connachie and Ruth, 1973;Glick, 1974Glick, , 1983Bellamy and Mohamed, 1982;Davenport and Allen, 1985).Since the discovery that the bursa was implicated in the differentiation of B-lymphocytes for humoral immunity (Chang et al., 1955;Glick et al., 1956;Papermaster and Good, 1962;Peterson and Good, 1965), attention was focused, on the one hand, on lymphoid cell populations (Ackermann and Knouff, 1959;Ackermann, 1962;Woods and Linna, 1965;Cooper et al., 1966; Toivanen et al., 1972 a,b;Toivanen et al., 1974;Back et al., 1973;Grossi et al., 1974;Grossi et al., 1976;Glick, 1974Glick, , 1983Glick, , 1995Glick et al., 1975;Eerola et al., 1982;Brand et al., 1983;Pink et al., 1985;Lassila, 1989;Nowak et al., 1990;Motyka and Reynolds, 1991;Petrini et al., 1991;Paramithiotis and Ratcliffe, 1993, 1994 a,b, 1996Reynaud and Weill, 1993;Masteller and Thompson, 1994;Masteller et al., 1995;Paramithiotis et al., 1995;Obranovich and Boyd, 1996;Funk and Thompson, 1996) and, on the other hand, on the epithelial stroma providing the bursal microenvironment within which B-cell maturation and differentiation occurs.
