[The participation of retinoic acid in embryonic and postembryonic development]

Retinoids are low molecular weight, lipophilic derivatives of vitamin A which have profound effects upon the development of various embryonic systems. Here I review the these effects on developing and regenerating limbs, regenerating amphibian tails and the developing central nervous system (CNS). In the regenerating amphibian limb, retinoids can proximalise, posteriorise and ventralise the axes. In the chick limb bud retinoids can only posteriorise the tissue. In the regenerating amphibian tail retinoids can homoetically transform tail tissue into hindlimb tissue. In the developing and regenerating limb retinoic acid has been detected endogenously, confirming that this molecule plays a role in the generation of pattern and limbs cannot develop in the absence of retinoic acid. In the developing CNS retinoic acid specifically affects the hindbrain where it causes a transformation of anterior rhombomeres into more posterior ones. Again, endogenous retinoic acid has been detected in the CNS and in the absence of retinoids the posterior hindbrain has been found to be affected. The effects of retinoids on the CNS are most likely to be mediated via the Hox genes acting in the mesoderm after gastrulation. It has also been proposed that the establishment of the head-to-tail axis in the mesoderm is established by retinoic acid. These data show that retinoids play an important role in both the development and regeneration of various systems in the embryo and post-embryonically.