Renal cortex of an experimental animal stained with trypan blue, light micrograph. The filtrate produced in the kidney's glomerulus is almost entirely reabsorbed as it passes through the nephron. Most of this reabsorption takes place in the proximal convoluted tubule, the cells of which contain an endocytic apparatus capable of reabsorbing all the complex molecules that have passed the glomerular filter. This endocytic apparatus can be revealed by administering vital dyes, such as trypan blue, which is taken up from the glomerular filtrate by the cells of the proximal convoluted tubule and sent to the phagocytic vacuoles or endosomes that form therein. The blue particles mark the position of the proximal convoluted tubes. The renal corpuscle and distal convoluted tubule are free of marking.