Re-utilization of Lymphocytes in Lymphopoiesis

I t is generally thought, but not proved, that the life span of the lymphocyte is short, probably only a few days. But in recent experiments, in which human lymphocytes were labelled in the DNA by p3~ (Osgood et al., 1952; Ottesen, 1954; Christensen and Ottesen, 1955) or by adenine-8-C 14 (Hamilton, 1954, 1956), it was found that labelled lymphocytes were still present in the blood after several months. In Hamilton's experiments, after 300 days the radioactivity of the lymphocyte DNA was still more than one-third of the peak activity. This means either that some lymphocytes live for more than a year, or, as Hamilton has suggested, that when lymphocytes die their nucleic acids are speciticaily re-utilized, without degradation, in the genesis of new lymphocytes. The purpose of this paper is to examine the reutilization hypothesis from the histological angle and to present some evidence in its support. In the terminology of Maximow the stages of lymphopoiesis are:--