RETINOIC ACID IN DEVELOPMENT AND REGENERATION

Retinoids are low molecular weight, lipophilic derivatives of vitamin A which have profound effects upon the development of various embryoni c systems. Here I review the effects on developing and regenerating li mbs, regenerating amphibian tails and the developing central nervous s ystem (CNS). In the regenerating amphibian limb, retinoids can proxima lize, posteriorize and ventralize the axes of the blastema. In the chi ck limb bud retinoids can only posteriorize the tissue. In the regener ating amphibian tail retinoids can homeotically transform tail tissue into hindlimb tissue. In the developing and regenerating limb retinoic acid has been detected endogenously, confirming that this molecule pl ays a role in the generation of pattern and we have shown that limbs c annot develop in the absence of retinoic acid. In the developing CNS r etinoic acid specifically affects the hindbrain where it causes a tran sformation of anterior rhombomeres into more posterior ones. Again, en dogenous retinoic acid has been detected in the CNS and in the absence of retinoids the posterior hindbrain has been found to be affected. T he 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 b een proposed that the establishment of the head-to-tail axis in the me soderm is established by retinoic acid. These data show that retinoids play an important role in both the development and regeneration of va rious systems in the embryo and post-embryonically.