Nucleolar morphology and cell proliferation kinetics of thymic lymphocytes

Proliferation kinetics of cells of the lymphocytic series were studied in mouse thymuses using ³H-methyl-thymidine (³H-TdR) as a tracer of DNA synthesis and employing autoradiographic technique. Cells were allocated arbitrarily to several compartments according to their degree of maturity and nucleolar morphology. Serial sampling after continuous ³H-TdR injection showed that thymic cells of the lymphocytic series constitute a small highly proliferating pool that feeds into a large nonproliferating pool. Lymphoblasts with dense and trabeculate nucleoli and prolymphocytes with trabeculate nucleoli represent multiplicative compartments and belong to the proliferating pool. A fraction of multiplicative precursors enters a ‘dormant’ state and these immature lymphocytes are morphologically characterized by ring-shaped nucleoli. Multiplicative compartments and non-dividing compartments of immature lymphocytes differ significantly in labeling indices, kinetics of labeling in serial samples and in other kinetic parameters, namely in the efflux from the unlabeled pool, labeling increment and efflux from the labeled pool. Serially connected compartments of lymphoblasts with dense and trabeculate nucleoli, prolymphocytes with trabeculate nucleoli and mature lymphocytes, represent the main stream of cell differentiation and maturation. At least a portion of mature lymphocytes proceeds during maturation from the compartment of cells with trabeculate nucleoli to the compartment of cells with ring-shaped nucleoli. The presence of proliferating and ‘dormant’ precursors suggests that lymphopoiesis in thymuses may correspond to the advantaged logarithmic system of multiplication.