Regulation of EphA4 expression in paraxial and lateral plate mesoderm by ectoderm-derived signals

Somitogenesis in all vertebrates involves a mesenchymal to epithelial trans ition of segmental plate cells. Such a transition involves cells altering t heir morphology and their adhesive properties. The Eph family of receptor t yrosine kinases has been postulated to regulate cytoskeletal organization. In this study, we show that a receptor belonging to this family, EphA4, is expressed in the segmental plate in a region where cells are undergoing cha nges in cell shape as a prelude to epithelialization, We have identified th e ectoderm covering the somites and the midline ectoderm as sources of sign als capable of inducing EphA4, Loss of EphA4 results in cells of irregular morphology and somites fail to form. We also show that when somites fail to develop, expression of EphA4 in the lateral plate is also lost, We suggest that signaling occurs between the somites and the lateral plate mesoderm a nd provide evidence that retinoic acid is involved in this communication. ( C) 2001 Wiley-Liss, inc.