Abstract
Leukocyte exchange between the hemopoietic tissues of parabiotic rats was studied subsequent to giving multiple injections of 3H-thymidine to one member of each pair while arresting the cross-circulation. Cell types that migrated from one parabiont to the other were segmented granulocytes, small, medium and large lymphocytes, immunoblasts, monocytoid cells, macrophages or their immediate precursors, and plasma cells. Evidence for the transformation of circulating cells to other cell types was rarely seen. The long-lived small lymphocytes were equilibrated between parabionts, suggesting that this is a single pool of cells with respect to kinetic behavior and recirculation. There was no evidence for a trephocytic function of lymphocytes. A small number of bone marrow lymphocytes coursed directly to lymph nodes and spleen. Evidence is given for a limited recirculation of short-lived lymphocytes of thoracic duct lymph (TDL), as well as for long-lived cells. Only a few immunoblasts of TDL recirculated. The majority of cells that entered the white pulp of the spleen were long-lived small lymphocytes, while the majority of immigrant cells to the red pulp were monocytoid cells and granulocytes. Many small lymphocytes originated in splenic red pulp and entered the blood. No immigrant cells to the thymic cortex were noted, although some small lymphocytes and monocytoid cells entered the medullary areas. Immigrant cells to the bone marrow (less than 2% of the cells in marrow) included monocytoid cells, small lymphocytes, and plasma cells. Evidence for the direct transformation of a circulating cell into a committed blast, based on reduction in grain count, was noted only in bone marrow.
