Monocytic differentiation of cluster cells in agar culture of normal human bone marrow. A cytochemical investigation.

A cytochemical investigation was carried out on the cells that make up the clusters forming in agar cultures of normal human bone marrow after 3, 4, and 5 days of incubation. All the clusters were found to comprise young monocytes that were PAS, peroxidase, Sudan Black, and alpha-naphthyl and naphthol-ASD-acetate esterase positive. Lysozyme activity, investigated by the cytobacterial test, was absent in all cases. After 3 days of incubation a large number of pairs of lymphomonocytes or monocytes with the same cytochemical characteristics were observed. When a method of double esterase incubation (alpha-naphthyl or naphthol-ASD-acetate and naphthol-ASD-chloroacetate) was performed on the same slide, no cells of the granulocytic line or intermediate cells between these and the monocytes were observed in any of the clusters. These results indicate that there is direct monocytic differentiation of the colony-forming cells cultured. The origins and stages observed in the maturation of the monocytes are discussed.