This pheochromocytoma demonstrates the chromaffin reaction. This neoplasm of the adrenal medulla contains catecholamines (epinephrine and norepinephrine). The section of tumor at the bottom has been placed into a dichromate fixative which turns the tissue brown as the catecholamines are oxidized. Compare to the section of pink to yellow tumor at the top which has not been placed in dichromate fixative.