Apical tubular network in the rat kidney proximal tubule cells studied by thick-section and scanning electron microscopy

Abstract

The apical cytoplasm of several absorbing epithelia contains well-developed apical tubules (AT) which contribute to membrane recycling from endocytic vacuoles to the apical cell membrane. In this study, we examined three-dimensional structures of the AT in rat kidney proximal tubule cells by transmission and scanning electron microscopy. In thin sections, the AT appeared as straight tubules with a rather constant diameter (70-90 nm), but others were curved and, occasionally, branching. No AT were labeled with the marker for the external cell surface (ruthenium red) or exhibited histochemical enzyme activity for lysosomal hydrolase (acid phosphatase). After intravenous injection of horseradish peroxidase, it was absorbed in the kidney proximal tubule cells and the AT were labeled with HRP reaction products. Stereo-viewing of the labeled AT in thick sections revealed that they formed an interconnected tubular network. Scanning electron microscopy allowed a three-dimensional view of the AT, in which a network of branching and anastomosing tubules was revealed. These observations indicate that the AT are intracellular endosomal compartments which form an extensive tubular network in the apical cytoplasm. The possibility that this apical tubular network serves as a large membrane store for membrane recycling is discussed.

Rapid downregulation of rat renal Na/Pi cotransporter in response to parathyroid hormone involves microtubule rearrangement

“…They reveal a coat with a characteristic substructure at their luminal side, whereas their cytoplasmic face is uncoated. Occasionally, the DATs carry small clathrin-coated buds (24)(25)(26).…”

Rab11a-positive compartments in proximal tubule cells sort fluid-phase and membrane cargo

“…1C). We speculate that these structures may represent the AVs described in vivo from which internalized apical cargo is recycled (3,16,17,29,30). A unique characteristic of PTCs is high capacity for apical endocytosis.…”

“…Much of our understanding of the apical endocytic pathway in these cells comes from electron microscopy studies examining the internalization pathway of fluid-phase and membrane tracers (3,16,17,29,30). These tracers are internalized into clathrin-coated vesicles that bud from the base of the brush border microvilli and are thought to fuse with a subapical network of tubules (30).…”

“…Fast recycling to the apical surface may occur from these sites. These fluid-phase and membrane tracers have access to large apical compartments, termed apical vacuoles (AVs), within 1-15 min after internalization (16,17,30). Tubule-like structures that are believed to carry recycling proteins back to the apical surface can be observed emanating from these AVs (11,16,17).…”

“…These fluid-phase and membrane tracers have access to large apical compartments, termed apical vacuoles (AVs), within 1-15 min after internalization (16,17,30). Tubule-like structures that are believed to carry recycling proteins back to the apical surface can be observed emanating from these AVs (11,16,17). Nonrecycling proteins are thought to be transported to lysosomes for degradation and/or transcytosed to the basolateral membrane (14,15).…”