APICAL TUBULAR NETWORK IN THE RAT-KIDNEY PROXIMAL TUBULE CELLS STUDIED BY THICK-SECTION AND SCANNING ELECTRON-MICROSCOPY

The apical cytoplasm of several absorbing epithelia contains well-deve loped apical tubules (AT) which contribute to membrane recycling from endocytic vacuoles to the apical cell membrane. In this study, we exam ined three-dimensional structures of the AT in rat kidney proximal tub ule cells by transmission and scanning electron microscopy. In thin se ctions, the AT appeared as straight tubules with a rather constant dia meter (70-90 nm), but others were curved and, occasionally, branching. No AT were labelled with the marker for the external cell surface (ru thenium red) or exhibited histochemical enzyme activity for lysosomal hydrolase (acid phosphatase). After intravenous injection of horseradi sh peroxidase, it was absorbed in the kidney proximal tubule cells and the AT were labelled with HRP reaction products. Stereo-viewing of th e labelled with AT in thick sections revealed that they formed an inte rconnected tubular network. Scanning electron microscopy allowed a thr ee-dimensional view of the AT, in which a network of branching and ana stomosing tubules was revealed. These observations indicate that the A T are intracellular endosomal compartments which form an extensive tub ular network in the apical cytoplasm. The possibility that this apical tubular network serves as a large membrane store fnr membrane recycli ng is discussed.