Phosphotungstic acid as a marker for the endocytic-lysosomal system (vacuolar apparatus) including transfer tubules of the lining cells of the sinusoids in the bone marrow and liver

Surface and intracellular components of endothelial cells in sinusoidal blood vessels of rat bone marrow and rat liver were studied after embedment in glycol methacrylate and staining with phosphotungstic acid (PTA) at pH 0.2–0.3. The results showed that under these conditions all components of the endocytic-lysosomal transport system (“vacuolar apparatus”), i.e., the large bristle-coated vesicles, the phagosomes, the transfer tubules, and the lysosomal dense bodies, were stained with PTA. Structures formed by fusion between components of the vacuolar system were also stained, as were Golgi cisternae at the mature face of the complex. The common staining property of these Golgi cisternae and the transfer tubules provides additional evidence for the origin of the transfer tubules from the Golgi apparatus. Endothelial cell nuclei, mitochondria, centrosomes, endoplasmic reticulum and ribosomes remained unstained. No conclusions can be reached regarding the precise chemical significance of the PTA staining of these functionally related cellular components.