Recent studies on the role of the thymus in early stages of lymphopoiesis and immune differentiation.

The influence of thymus deprivation and thymus restoration was studied: a) on the cycling capacity of colony forming cells (CFU-S) in the bone marrow and b) on the establishment of tolerance in liver radiation chimeras. After neonatal thymectomy a reduction in the number of CFU-S in the bone marrow was observed. This reduction was accompanied by a striking decrease in the proportion of cycling cells in the bone marrow of thymus deprived mice. On the other hand, restoration of thymus function by thymic hormone (THF), by implantation of thymus in semi-impermeable cellophane bags, or by pregnancy, raised the number of cycling cells in the bone marrow to that of normal controls. Parental embryonic liver cells reconstitute lethally irradiated mice, and permit establishment of tolerance to further challenges of immunocompetent cells syngeneic to liver donors. We found here that adult thymectomy prevents the establishment of permanent tolerance in liver chimeras. Again, restoration of thymic function by THF permitted liver chimeric mice to resist the immunologic attack of parental spleen lymphocytes syngeneic to donor liver cells.