Naphthol AS D chloroacetate esterase-positive macrophages in the cortico-medullary zone of the normal rat thymus.

Naphthol AS D chloroacetate esterase (NASDCE)-positive macrophages are positioned in the cortico-medullary zone (CMZ) of the normal rat thymus. These cells contain the very strongly NASDCE-positive granules of varying size in the cytoplasm. An identical distribution within the thymic parenchyma and an identical morphological appearance is observed in CMZ macrophages after staining with aldehyde fuchsin. The incubation of thymic sections in 10% formalin at pH 7.0 for 48 h does not inhibit the activity of NASDCE in CMZ macrophages. The activity of nonspecific esterase is almost totally abolished by this treatment: only the single, largest globular inclusion within some of the cells remains weakly or moderately positive. The granular content of the CMZ macrophages does not stain metachromatically with toluidine blue and these cells are endogenous peroxidase-negative. NASDCE-positive thymic macrophages are easily distinguished from: a) NASDCE-positive mast-cells, which are confined to the capsular and septal connective tissue and contain densely packed metachromatic granules, and b) NASDCE-positive neutrophilic granulocytes, which have a specific morphological appearance and show very strong endogeneous peroxidatic activity.