Abstract
Five antigenic lymphoid cell surface determinants (LCSD) were detected in hatched chickens using specific antisera. These LCSD were: thymus-specific surface determinants, bursa-specific non-immunoglobulin determinants, IgM-specific determants, IgG-specific determinants, and IgA-specific determinants (ASD). Viable cell suspension of embryonic yolk sac, bursa, thymus and spleen were tested by means of indirect or direct immunofluorescent staining procedures for the presence and frequency of LCSD during maturation. Experiments performed with liver cells. brain cells and red blood cells of embryos confirmed the specificities of the antisera used for determinants present on cells of lymphoid tissues. The results showed LCSD to occur on yolk sac cells on the 5th to 7th embryonic day (ED). This suggests the presence of a stem cell pre-committed for the lymphoid cell line already in the yolk sac. Furthermore, findings are reported indicating the presence of distinct lympoid stem cell populations or maturation stages in the yolk sac, which may be responsible for either populating the thymus or the bursa. The finding of ASD-bearing cells early in ontogenesis of the lymphoid system suggests the presence of two specificities in anti-chicken IgA sera, one of which may be directed against an antigenic site on a rudimentary immunoglobulin molecule, which becomes lost or hidden in later maturation. Studies on the bursa and the thymus show that covering, hiding, or loss of antigenic determinants plays an important role in lymphoid cell differentiation. Furthermore, the spleen is reached by B-determined stem cells as early as the bursa, but these stem cells seem not to proliferate in the former to any considerable extent until hatching. Finally, the sequence of the appearance of immunoglobulin classes as proposed by other authors is confirmed with reservaitons concerning IgA, and it is suggested that immunoglobulins are detectable earlier on cell surfaces than intracytoplasmatically.
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