Immunocytochemical localization of cathepsin D in rat junctional epithelium

Localization of cathepsin D was studied in the junctional epithelium (JE) of healthy rat gingivae by immuno-light and -electron microscopy, by means of both the avidin-biotin-peroxidase complex method and a colloidal gold IgG method. At the light-microscopic level, cathepsin D was demonstrated in the JE and oral sulcular epithelium (OSE). Cathepsin D immunoreactivity was remarkable in the coronal portion of the JE and decreased toward its apical portion. However, cathepsin D immunoreactivity in the basal cell layer of the JE was negligible or negative. In the OSE, the granular layer was positive for cathepsin D. In the adjacent connective tissue, many macrophage-like cells (not clear at this level) close to the basal cell layer showed strong immunoreactivity. At the electron microscopic level, cathepsin D was found in the primary lysosomes and trans-cisternae of Golgi apparatus in the JE cells. These lysosomes were often fused together or were fused with cathepsin D-negative intracytoplasmic vacuoles to form secondary lysosomes, which indicated that intracellular digestion may have been in progress. However, neutrophils contained few gold particles based on cathepsin D. It is likely that the amounts of cathepsin D contained in the JE cells and macrophages are larger than those of cathepsin D contained in the neutrophils. These findings provided morphological evidence that JE cells have the same endocytotic capacity as macrophages and neutrophils, and that JE cells participate in the intracellular digestion that is carried out by lysosomal enzymes such as cathepsin D. It is suggested, in addition, that maximum intracellular digestion occurs in the coronal portion of the JE.