PRECURSORS OF 8-LYMPHOCYTES WITH SURFACE IgM AND THEIR POSSIBLE MIGRATION FROM BONE MARROW TO PERIPHERAL LYMPHATIC TISSUES*

The lymphocytes with surface immunoglobulins (slg) represent precursors of D·lymphocytes and were studied in ontogeny of pigs by autoradiography using iodinated monospecific anti- µ , anti-g or anti-α antibodies. The earliest detection of sIgM + small lymphocytes was at 44 days of gestation in liver, followed at 51 days in spleen and at 60 days in bone marrow, the thymocytcs being slg-negative. The first circulating small lymphocytes were detected at 38-40 days and are most probably thymus-derived. In adult pigs as well as after immunization of germ-free piglets IgM remains to represent the surface Ig on bone marrow lymphocytes while in peripheral lymphatic tissues there is appearance of lymphocytes labelled with anti-g or anti-α. The percentage of sIgM + lymphocytes in bone marrow is extremely small and also the density of sIgM per cell is low.