Precursors for Cholesterol Synthesis (7‐Dehydrocholesterol, 7‐Dehydrodesmosterol, and Desmosterol): Cholesterol/7‐Dehydrocholesterol Ratio as an Index of Development and Aging in PNS but Not in CNS

: In rat sciatic nerve, the 7‐dehydrocholesterol content decreased dramatically during the postnatal period and slowly during adulthood and aging. In contrast, the 7‐dehydrodesmosterol content peaked at 14 days and was nearly undetectable after 60 days. The desmosterol content peaked at 21 days and was nearly undetectable after 1 year. The cholesterol content increased up to 21 days and remained nearly constant thereafter. In brain (in contrast to sciatic nerve), 7‐dehydrodesmosterol and desmosterol contents decreased dramatically during development and slightly during adulthood and aging; the 7‐dehydrocholesterol content peaked at 21 days and remained constant during aging. Only 7‐dehydrocholesterol was dramatically more concentrated in PNS than in CNS. In brain, the cholesterol/7‐dehydrocholesterol ratio increased during development and remained stable after 6 months. In contrast, in sciatic nerve, this ratio continuously increased during development and aging (950‐fold between 5 days and 18 months). Thus, the cholesterol/7‐dehydrocholesterol ratio is a useful biochemical index of development and aging in the PNS.