In this high magnification LM, the epithelia are all simple (one layer) and range from squamous to columnar. This is a transverse section through the medulla (inner region) of the kidney. Since there are few apical modifications on these epithelia, you can often see a sharp edge of the free surface facing the lumen. Although not visible with most histological preparations, the special stain, PAS, which binds to polysaccharides, has stained pink the basement membrane that surrounds each tubule and blood vessel (bv). The lumens of the tubules of the nephron are clear because they contain the urinary filtrate. In several locations, between tubules, you can see small blood vessels that contain blood cells. The simple squamous epithelium that lines all blood vessels is endothelium.