MICHAEL W. DAVIDSON / SCIENCE PHOTO LIBRARY MICHAEL W. DAVIDSON / SCIENCE PHOTO LIBRARY
Retinoic acid. Polarised light micrograph of crystals of retinoic acid, one of the physiological forms of Vitamin A (retinol). Vitamin A is fat-soluble. It is essential to vertebrates and some invertebrates, being vital to the maintenance of skin and epithelial cells and the production of rhodopson ('visual purple'). Deficiency of Vitamin A can lead to vision problems such as nyctalopia or systemic problems such as hyperkeratosis and keratomalacia. Good dietary sources are animal liver or fish liver oils, and plants rich in the precursor carotene (carrots, spinach, etc.). Magnification x4 at 35mm size.
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