Retinoic Acid in Ocular Growth Regulation

All-trans-retinoic acid (atRA) is a metabolite of vitamin A (retinol) and is required for growth and development of a variety of organ systems in all higher animals from fish to humans. Evidence is accumulating to suggest that atRA may also be an important molecular signal in the postnatal control of eye size. Choroidal synthesis of atRA is modulated during periods of visually-induced changes in ocular growth and has pronounced effects on eye growth and refraction in several animal models of myopia. Choroidal atRA synthesis is exclusively regulated by expression of the enzyme, retinaldehyde dehydrogenase 2 (RALDH2). In chicks and humans, RALDH2 is synthesized by a unique population of uncharacterized extravascular stromal cells concentrated in the proximal choroid. The identification of choroidal atRA and RALDH2 as visually induced ocular growth regulators provides the potential for new therapeutic targets for the treatment of childhood myopia. The objective of this chapter is to discuss what is presently known about atRA biosynthesis and transport in the eye during visually guided eye growth and how this research can contribute to a better understanding of the mechanisms underlying the development of myopia.