Enzyme-histochemical characterization of macrophages in the rat thymus, with special reference to metallophilic cells of the corticomedullary zone.

The cortex and medulla of the normal rat thymus are populated with scattered cells which are strongly positive for acid phosphatase and only weakly (cortex) or moderately (medulla) positive for nonspecific esterase. Corticomedullary zone is characterized, as a specific entity within the thymic tissue, by the presence of very large macrophages, highly positive for acid phosphatase and nonspecific esterase. Moreover, these cells, located exclusively in the corticomedullary zone, show very strong succinic dehydrogenase activity and contain the intracytoplasmic granules of varying size, which are aldehyde fuchsin, PAS, and oil red O positive. The presence of cholesterol was demonstrated within the lipid content.