Computer-aided three-dimensional reconstruction of serial ultrathin sections revealed that freshly prepared monocytes from human blood contained endogenous peroxidase (PO) not only in cytoplasmic granules, but also in long contorted tubules and in complex elements, which consisted of both tubular and granular components. The various PO- positive elements formed an intricate system, which was separate from all other cytoplasmic structures, including the endoplasmic reticulum and the Golgi apparatus. Because the PO-positive elements in monocytes are known to be primary lysosomes, which are involved in host defense mechanisms, we suggest that the antimicrobial functions of human blood monocytes are exerted by functionally and morphologically diverse subcompartments of a complex system rather than by separate uniform granules.

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