Fine structure and peroxidatic activity of rat blood monocytes.

Rat blood monocytes contain granules of two types, primary and secondary granules, which can be distinguished on morphological grounds. Secondary granules have a halo between the limiting membrane and the matrix; primary granules have no halo and are generally more elongate than the secondary granules. In addition, there are differences in the distribution of the diameters of these two types of granules. The cytochemical characteristics of the peroxidatic activity of these granules were investigated by using different prefixation conditions in combination with a number of DAB media differing in pH and H2O2 concentration. Prolongation of the prefixation and increasing the pH of the incubation medium increased the staining intensity of the secondary granules and decreased the staining intensity of the primary granules. In unfixed cells peroxidatic activity could be demonstrated only in the primary granules. It is concluded from the present findings that peroxidase occurs in primary granules and not in secondary granules, whereas catalase is present in secondary granules but probably not in primary granules. No evidence was found for the presence of microperoxisomes in rat blood monocytes.