Transient populations of terminal transferase positive (TdT+) cells in juvenile rats and mice.

Transient populations of TdT+ cells are present in peripheral blood, lung, liver, and spleen of rats and mice between birth and 7 wk of age. TdT+ blood cells appear to be post-thymic cells, as evidenced by their absence from nu/nu mice and their pattern of TdT fluorescence. TdT+ cells in lung, liver, and spleen appear to be prethymic cells. All transient populations of TdT+ cells are sensitive to dexamethasone. Only TdT+ cells in bone marrow and thymus persist into adult life.