Biochemistry. A fat-soluble vitamin that plays an essential role in metabolic functioning of the retina, the growth and differentiation of epithelial tissue, the growth of bones, reproduction, and the immune response. It is produced by animals and humans from carotinoides (provitamin A) found in deep green and orange vegetables and fruits. It is enriched in egg yolks, liver, whole milk, cream, and cheese. Vitamin A deficiency is associated with night blindness and dry skin.