Lymphocyte recirculation in the sheep fetus.

The numbers of circulating thymus-derived and surface Ig-bearing lymphocytes in the fetal lamb increase exponentially over the last third of gestation. Experiments in which [3H]thymidine was continuously infused into fetal lambs have established that these cells are long-lived in the fetus. The migration of 51Cr-labelled autologous lymphocytes from intestinal or prescapular lymph was compared in fetal lambs and adult sheep. A subpopulation of thymus-derived lymphocytes present in intestinal lymph of adults which migrated preferentially to the small intestine was not found in fetal intestinal lymph. There were marked differences in the migration of fetal and adult lymphocytes to the lungs and liver. In spite of the absence of circulating antibodies or immunoglobulins and of extrinsic antigen in the immunologically virgin sheep fetus, the circulation of lymphocytes through the spleen and lymph nodes of fetal lambs was more intense than in the adult, indicating that the pathways of recirculation and the capacity of cells to recirculate arise as a physiological process independently of antigenic stimulation.