When dilute and exposed to light, oxygen and high temperatures, it is polymerized, oxidized and isomerized. The major site of lipid hydrolysis is the upper small bowel. In terms of cell differentiation, hypovitaminosis A causes mucous secreting cells to be replaced by keratin producing cells in many tissues. The converse has been demonstrated in tissue culture. There is data that also suggests that vitamin A mediates the effects of hormones.
Reproduction is particularly dependent on normal vitamin A levels.
Vitamin A also influences embryogenesis. All appear to be secondary to reductions in mucous secreting cells of the cornea and conjunctiva. Yellow skin also goes along with increased carotenoids. Vitamin D concentrates rapidly in the liver where is it oxidized to 25-hydroxy vitamin D 3 . The biosynthesis of vitamin D begins in the skin with 7-dehydrocholesterol.