Cytochemical heterogeneity of human circulating monocytes.

Normal human monocytes, based on appearance after Romanowsky-type staining, have been considered to be a homogenous class of mononuclear leukocytes. Cytochemical studies, however, indicate that these cells are markedly heterogenous with respect to certain hydrolytic and oxidative lysosomal enzymes and to a lesser extent show variability in cytoplasmic carbohydrate and ribonucleic acid content. Individual monocytes were found to vary most widely in non-specific esterase activity and least in acid phosphatase activity. Variation in monocyte peroxidase activity was intermediate between these extremes. Using a cytochemical "scoring" technic, younger adults were found to have higher mean monocyte non-specific esterase scores than older adults. No significant age or sex differences were found for peroxidase or acid phosphatase activity. Furthermore, the overall activities of these monocyte enzymes in healthy adults, as estimated by intensity of staining, were unrelated to one another. Using consecutive double staining technics, individual monocytes were also found to vary in their content of these biochemical consituents. A statistically inverse relationship was found in individual monocytes between their nonspecific esterase and acid phosphatase activities (r = -0.68). Analysis of other combinations studied showed only random distribution with no significant correlations. These findings emphasize the biochemical individuality of circulating monocytes and the variations noted probably reflect differences in number and composition of lysosomes.