7-Dehydrocholesterol

7-Dehydrocholesterol is a zoosterol that functions in the serum as a cholesterol precursor, and is converted to vitamin D₃ in the skin, therefore functioning as provitamin-D₃. The presence of this compound in human skin enables humans to manufacture vitamin D₃ from ultra-violet rays in the sun light, via an intermediate isomer pre-vitamin D₃. It is also found in the milk of several mammalian species. In insects it is a precursor for the hormone ecdysone, required for reaching adulthood. It was discovered by Nobel-laureate organic chemist Adolf Windaus.