The differentiation of monocytes into macrophages,epithelioid cells,and multinucleated giant cells in subcutaneous granulomas

Summary The peroxidatic (PO) activity of monocytes differentiating into macrophages, epithelioid cells, and multinucleated giant cells in subcutaneous granulomas was investigated with three different media for the demonstration of PO activity. Irrespective of the stage of differentiation, these cells did not show PO activity in the rough endoplasmic reticulum (RER) or nuclear envelope. In addition, it was found that the morphologically characteristic types of granule of the various cells of the monocyte line (the primary granules and secondary granules of monocytes, the macrophage granules, and the epithelioid cell granules), all have distinct cytochemical characteristics.Monocytes lose their primary and secondary granules during differentiation into mature macrophages. Simultaneously, the granules of both types become elongated and the secondary granules lose their halo. In contrast to monocytes, mature macrophages may contain a few microperoxisomes. During the differentiation of macrophages into epithelioid cells or multinucleated giant cells there is an increase in the number of microperoxisomes.