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Diagram showing how cholecalciferol (vitamin D3) is synthesized in the human epidermis. In a two-stage process, 7-dehydrocholesterol (7-DHC) present in the cell membranes of keratinocytes is converted into 9,10-secosteroid (pre-cholecalciferol) by UV-B radiation in sunlight (wavelengths of 290–320 nm). Then, through thermal isomerization at body temperature, pre-cholecalciferol is transformed into cholecalciferol (vitamin D3).
