Graded retinoid responses in the developing hindbrain.

The purpose of this study was to make an explicit test of the idea that a retinoid could act as a morphogen, differentially activating genes and specifying anteroposterior (a-p) level in the developing vertebrate central nervous system (CNS). Our approach was to characterize the concentration-dependent effects of retinoic acid (RA) on the neural expression of a set of a-p patterning genes, both in vivo and in an in vitro system for neural patterning. Our results indicate that a retinoid is unlikely to specify a-p level along the entire CNS. Instead, our data support the idea that the developing hindbrain may be patterned by a retinoid gradient. Sequentially more posterior hindbrain patterning genes were induced effectively by sequentially higher RA concentration windows. The most posterior CNS level induced under our RA treatment conditions corresponded to the most posterior part of the hindbrain.