Single and multiple injections of tritiated thymidine were combined with radioautography to study the rates of proliferation and interrelationships of the various cell lines in the mesenteric lymph node of the rat. The appearance and labeling patterns of the different cells are described from studies of both smears and tissue sections. Reticular cells exhibit wide variations in labeling intensity, phagocytize labeled lymphocytes, and become labeled in high percentages only when TTH is administered over a period of many days. Other slowly proliferating cell types include small lymphocytes, fat cells, endothelial cells and mast cells.

Rapidly proliferating cell lines include plasmablasts, hemohistioblasts, proplasmacytes and large lymphocytes. The generation time of plasmablasts and hemohistioblasts was determined to be approximately 9 and 12 hours respectively. Mature plasma cells constitute a non-dividing population which is renewed in lymph node in not more than 5 days.

Evidence is presented that the most primitive cells in the lymphocyte and plasma cell lines are the hemohistioblasts and plasmablasts respectively. Reticular cells most probably are not stem cells. No evidence is found to support previous reports that plasma cells derive from lymphocytes.

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