Computer analysis of defined populations of lymphocytes irradiated in vitro: I. Evaluation of murine thoracic duct lymphocytes.

Computer-assisted morphometric analysis of murine lymphocytes obtained by thoracic duct cannulation demonstrates two populations of cells; the larger population (73 percent) appears to be thymus-derived and the remaining 27 percent is of bone marrow origin. Following exposure to varying amounts of x-radiation, morphologic alterations in both populations are evident. The smaller cell populations are evident. The smaller cell population exhibits some of these changes at lower dose levels than does the larger population. In addition, the character of the radiation-induced changes appears to be different for the two populations of lymphocytes. After 500 rad, the nuclei of the larger population appears unchanged; the nuclei of the population representing 27 percent of the cells have become enlarged and vacuolated and are thought to be edematous. After 2000 rad, the nuclei of the larger population appear pyknotic with coarsely clumped chromatin. In the examined set of cells, the smaller population could no longer be detected after 2000 rad. Such disparate responses to radiation-induced injury may correlate with known differences in immunologic function which serve to distinguish thymic-dependent and bone marrow-derived small lymphocytes.