A role for vitamin A in living organisms has been known throughout human history. In the last 100 years, the biochemical nature of vitamin A and its active derivative, retinoic acid, its physiological impact on growth processes and the essential details of its mechanism of action have been revealed by investigations carried out by researchers using vertebrate and more recently invertebrate models to study a multiplicity of processes and conditions, encompassing embryogenesis, postnatal development to old age. A wealth of intercellular interactions, intracellular signaling systems and...