Retinoids. A window into vertebrate development

Vitamin A is required for maintaining healthy epithelial tissues, e.g., skin and gut, normal reproductive capacity, and vision. Vitamin A deficiency also causes premalignant changes in epithelial tissues. These observations led to the use of retinoids (vitamin A-related compounds) in treating various skin diseases and more recently to inhibit cancer growth. Retinoids are extremely teratogenic to developing vertebrate embryos, a fact which limits their clinical usefulness. In particular, specific malformations of skeletal structures are often observed; for instance, retinoic acid induces craniofacial and limb deformities in avian and mammalian embryos. Recent technological advances have increased our understanding of how retinoids affect vertebrate development. Some newly discovered mechanisms underlying these actions are reviewed.