Abstract
Lymphocytes were isolated from five fetal livers (13-17 weeks) and examined for different surface markers. Immunoglobulin M was found on 0.5-4.0% of lymphocytes. No membrane-bound surface IgD, IgA. or IgG was detected. Mouse erythrocyte rosette-forming lymphocytes ranged from 0.5 to 6.0%. Thymus-processed (T) lymphocytes, which were defined as those forming spontaneous rosettes with sheep erythrocytes, as well as lymphocytes with IgG Fc receptors, were present in a small proportion. No complement-receptor-bearing lymphocytes were found in the two cell populations of fetal liver studied. It is evident that during the ontogeny of bone-marrow-derived (B) lymphocytes IgM is the first surface immunoglobulin to appear. Receptors for the binding of mouse erythrocytes are present at the same time as surface IgM. The slight excess of mouse erythrocyte rosette-forming cells over cells having surface immunoglobulin M could mean that IgM appears later than do the receptors for the binding of mouse erythrocytes.