An investigation has been made into the mitotic and premitotic activity of the circulating atypical mononuclear cells which occur in infectious mononucleosis. Dividing and binucleate cells were found in 2.4 per cent of blood films and in 63.9 per cent of white cell concentrates. The number of cells in mitosis was small and they were most commonly encountered during the first three weeks of the disease; cells in prophase and metaphase predominated. The incidence of dividing cells in white cell concentrates was increased by a preliminary period of incubation with colchicine. The uptake of 3H-thymidine was shown to be increased during the first four weeks of the disease, with peak labeling in the second and third weeks; in one autoradiograph, 5.4 per cent labeling was observed. The label was confined to the nuclei of certain atypical mononuclear cells, most of which showed rather marked cytoplasmic basophilia. Karyotype analyses were consistently normal. The occurrence of similar large mononuclear cells, capable of taking up 3H-thymidine, in other pathological conditions and in normal subjects is stressed. Their origin, function and potentialities are ill-understood at the present time.

Two further autoradiographic studies21,22 dealing with the mitotic activity of atypical mononuclear cells in infectious mononucleosis have recently appeared, both of which record increased DNA synthesis. Epstein and Brecher21 observed peak labeling during the first two weeks of the disease; most of the labeled cells showed enhanced cytoplasmic basophilia. No variation in labeling at different phases of the disease was found by Schmid et al.,22 who, unlike previous workers, characterize the labeled cells in some detail as plasmacytoid, monocytoid and lymphocytoid elements.

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