of incorporatcd tritium lahellecl thymidine is distributed to thc daughter (.ell at mitosis( 1 ) one can exlmt that 1 y atlministering tritiutn labelled thymidine at 6 hour intervals, virtually all cells dividing during the period between booster injection of antigen and sacrifice would be exposed to, and incorporate labelled thymidine. The value of 92% stated as the labelling index of the plasma cell series here is probably conservative and may actually approach 100%. This low value might result )from the initiation and coinpletion of DNA synthesis in some cells between esposures to isotope or from technical limitations in the sensitization of the radiographic emulsion by the label in some cells. These factors, alone or in combination, would tend to lower the observed incidence af labelling. The results presented are in general agreement with those of other investigators employing similar technics (8,10,11) who have implicated a rapidly proliferating population of immature lymphablasts and plasmablasts as the chief source of plasma cells arising after antigenic stimulation. As a corollary. the large percentage of labelled antibody containing cells found in these experiments indicate that most plasma cells do not arise by direct, non-mitotic differentiation from small lymphocytes. The present studies could not offer quantitative data concerning the relative mitotic activity of the various stages leading to development of the mature plasma cell. However, by the use of colchicine and by short in vitro incubation of cells with thymidine it has been shown that functionally differentiated antibody producing cells can divide, indicating that mitotic activity within the family of antibody producing cells is not a n exclusive property of the immature or pre-CII rsor f orins.Suiiiiiinry. 14 technical approach for the study of the kinetics of antibody producing cells is possible by combination of autoradiography and the fluorescent antibody technic. Results show that: 1. Almost all antibody containing cells present by day 4 of a secondary response are newly formed cells arising ifrom mitotic division of a precursor sometime after antigenic stimulation; 2. Most plasma cells do not arise by direct, non mitotic differentiation from lymphocytes; and 3. Functionally differentiated antibody containing cells can divide.
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