Kidney convoluted tubules stained with Mallory's phosphotungstic haematoxylin, light micrograph. Mallory's phosphotungstic haematoxylin selectively stains mitochondria. In the kidney, the convoluted tubules have a large number of mitochondria at their basal pole, where they are associated with folds of the cell membrane (bale infoldings). The mitochondria provide the energy necessary for the intense reabsorption processes that take place in these tubes. This low magnification micrograph of the renal cortex shows a violet border in the basal area of the convoluted tubules, which corresponds to the mitochondria associated with the basal infoldings characteristic of these tubes. The renal corpuscles visible do not show such staining.