The pattern of retinoic acid receptor gamma (RAR gamma) expression in normal development of Xenopus laevis and after manipulation of the main body axis.

Nuclear receptors are a prerequisite for the transduction of retinoid signals in vertebrate embryogenesis. We have raised an antiserum against the X. laevis retinoic acid receptor gamma (xRAR gamma), which specifically detects a polypeptide with an M(r) of 54 x 10(3) in embryos at gastrula and neurula stages. The antiserum reveals xRAR gamma as a nuclear protein in the head endomesoderm, the neuroectoderm of the hindbrain region, and the entire posterior region during neurula stages. Explanted animal caps express low amounts of xRAR gamma in the absence of mesoderm induction. Treatment of animal caps with activin leads to an increase of the amount and to a regionalization of the xRAR gamma expression. In exogastrulae and in Keller sandwiches, the pattern of expression in the endomesoderm is as expected by analogy to the normal embryo and therefore independent of the vertical contact between the germ layers. A planar signal coming from the dorsal mesoderm is sufficient to impose region-specific expression of xRAR gamma in the neuroectoderm of Keller sandwiches.