Recent progress in vertebrate limb morphogenesis.

Morphogenesis of vertebrate limb, specifically that of the chick wing, has been recognized as a suitable model to study the cellular and molecular mechanisms of pattern formation. The importance of cellular inductive phenomena and the relevance of the processes such as cell division and cell death in the above model are discussed. These studies have revealed the retinoic acid (RA) and retinols as convincing candidates for vertebrate morphogens. The recent discovery that the RA receptors belong to the steroid hormone receptor superfamily might indicate the universality of the RA morphogen and might enlighten the possible mode of its action. Identification and characterization of the 1d locus genes associated with the mouse limb morphogenesis and the possible involvement of the homeobox proteins in chick wing development have opened new prospects in understanding the molecular mechanisms of vertebrate morphogenesis.