Specific helical-structure has been previously demonstrated in the lumen of so-called apical tubules (AT) of several absorbing epithelia (kidney proximal tubule, visceral yolk sac and ductuli efferentes); however, their chemical properties remain unknown. In this study, several cytochemical procedures were applied at electronmicroscopic levels to demonstrate the presence of carbohydrates and proteins in the AT of rat kidney proximal tubule cells. After staining for carbohydrates (periodic

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... hiocarbohydrazide-silver proteinate; PA-TCH-SP reaction), heavy reaction products were observed in the lumen of the AT, and the AT originating from the large endocytic vacuoles. Similar products were also shown at the cell surface coat, lysosomes and small portions of the Golgi apparatus. After staining for proteins (phosphotungstic acid or silicotungstic acid in acidic medium; PTA or STA reaction), the helical-structure in the AT was selectively stained, and the intensity of the staining was greatly reduced by treatment with protease. These observations indicate that the AT probably contain a high concentration of carbohydrates and the material which forms the luminal specializations is composed mainly of proteins, which are readily digested by a proteolytic enzyme.