Study of thymus and peripheral lymphocyte populations characterized by the dominant nucleolar type

In neonatally thymectomized mice and in "B" mice obtained by adult thymectomy, lethal irradiation and repopulation with foetal liver cells, blood lymphocytes were classified according to their nucleolar morphology as revealed by toluidine blue staining. A majority of the "active", i.e., RNA-synthesizing cells were found to be thymus-dependent. Direct evidence was obtained by testing the "active" lymphocytes for their sensitivity to anti- serum in vitro. During postnatal ontogenesis, the frequency distribution of individual nucleolar types of lymphocytes tends to reflect the developmental changes in the organ tested. The decline of "active" lymphocytes in the thymus has its counterpart in their increase in the lymph nodes and peripheral blood. In hydrocortisone-treated mice, the most affected lymphocytes were those from the thymus cortex -- both the prevalent population of cells with micronucleoli and the "active" lymphocytes -- whereas the active small lymphocytes in the medulla (resembling those from peripheral blood and probably also their precursors) were relatively hydrocortisone-resistant.