Hindbrain respecification in the retinoid-deficient quail

We report here the development and rescue of the truncated hindbrain of ret inoid-deprived quail embryos. The embryo is completely rescued by an inject ion of retinol into the egg; this confirms retinol, or a related retinoid, as a required molecule in hindbrain development. Staging the retinoid repla cement enabled us to determine that the 3-4 somite stage is the period when retinoids are required for normal development. Analysis of the development of the retinoid-deprived hindbrain phenotype through somitogenesis has rev ealed a pathway of retinoid action in early hindbrain regionalisation, The hindbrain of the retinoid-deprived embryo is normal in size, during early s omitogenesis, but has a respecified pattern of Knox-20 expression. From the earliest expression of Krox-20, at the 5 somite stage, the rhombomere 3 st ripe fills the caudal third of the developing hindbrain to the level of the first somite. Morphologically only 2, instead of the normal 5, rhombomere bulges form. These 2 bulges express genes and, later, develop morphology ch aracteristic of rhombomeres 1 and 2 and rhombomere 3. Posterior hindbrain s pecific genes, Hoxb-1, Fgf3, MafB, and the rhombomere 5 stripe of Krox-20 a re never expressed in the head neuroepithelium of these embryos. From the i nitial formation of the: neural plate, there is no evidence of rhombomere 4 -7 specific characteristics. These results indicate the specification of th e posterior hindbrain is lost and its cells participate in the formation of an enlarged anterior hindbrain. In our previous study, we reported the abs ence of the posterior hindbrain in retinoid-deprived quails (Maden, M., Gal e, E., Kostetskii, L, Zile, M., 1996. Vitamin A-deficient quail embryos hav e half a hindbrain and other neural defects. Curr. Biol. 6, 417-426). Here, we show this phenotype to be the result of respecification of the hindbrai n cells. This provides evidence for a region specific response to a single stimulus, retinol, which suggests a pre-rhombomeric regionalisation of the hindbrain. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.