The family of retinoids comprise an enormous number of compounds related to vitamin A. Many of these compounds are naturally occurring substances generated during the biological functioning of retinoids: the conversion of dietary sources of retinoids (P-carotenes, retinyl esters) to those that can be taken up by the absorptive epithelium of the gut (retinol); the conversion of absorbed forms to stored forms in the liver (retinyl esters); the conversion of stored forms to active forms as mediators of vision (retinals), skin differentiation (retinoic acids), and general cell differentiation and proliferation (retinoic acids). However, more of these compounds are synthetic, for example, the retinobenzoic acids, and have been generated in the desire to find more potent and less teratogenic retinoids for pharmaceutical use. This chapter will be concerned only with a very few retinoids, ones that have been used in an embryo-logical context.
The original definition of a retinoid was based on its chemical structure, but this soon became obsolete when these new compounds were synthesized. The current definition of a retinoid is a substance that can elicit specific biological responses by binding to and activating specific receptors or a set of receptors. This perceptive definition was formulated by Sporn and Roberts (1) before the discovery of the retinoid receptors and remains pertinent today with our knowledge of the retinoic acid receptors (RARs), whose ligand is all-trans-retinoic acid (tRA), and the retinoid X receptors (RXRs), whose ligand is 9-cis-retinoic acid (9-cis-RA) (2). In Table 1, the structure of tRA and 9-cis-RA is shown, as well as several other retinoids of embryological interest, and in the following section are a few comments on each of the compounds.
From: Methods in Molecular Biology, Vol 97: Molecular Embryology: Methods and Protocols Edited by: P. T. Sharpe and I. Mason © Humana Press Inc., Totowa, NJ
Chemical Formulae of the Retinoids That Are of Embryological Interest3
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