The floor plate is a group of specialized cells located in the ventral midline of the vertebrate neural tube. It plays important roles in neural tube development as a source of signalling molecules for dorsoventral patterning and axonal guidance

The floor plate is a group of specialized cells located in the ventral midline of the vertebrate neural tube. It plays important roles in neural tube development as a source of signalling molecules for dorsoventral patterning and axonal guidance (for reviews, see Jessell and Dodd, 1992; Dodd and Jessell, 1993). A floor plate-derived signalling molecule, Sonic hedgehog (Shh), which is a vertebrate homologue of Drosophila Hedgehog (Hh), induces differentiation of ventral type neurons (Ericson et al., 1995; Marti et al., 1995; Roelink et al., 1995). Another floor plate-derived signalling molecule, netrin-1, works as both a chemoattractant and a chemorepellent of axons, depending on neuronal cell types (Kennedy et al., 1994; Colamarino and Tessier-Lavigne, 1995). Differentiation of the floor plate is induced by a signal derived from the notochord or by Shh. High concentrations of Shh-N peptide or contact with the notochord are required for floor plate induction in neural explants (Marti et al., 1995; Roelink et al., 1995). A critical event of floor plate development is the expression of a winged-helix transcription factor, HNF-3β (Ang et al., 1993; Ruiz i Altaba et al., 1993; Sasaki and Hogan, 1993, 1994). Ectopic expression of HNF-3β in the neural tube induces ectopic expression of series of floor plate marker genes and prevents dorsal/lateral maker expression, indicating that HNF-3β acts as a regulator of floor plate development (Sasaki and Hogan, 1994; Ruiz i Altaba et al., 1995a). The HNF-3β expression in the floor plate is induced by a signal derived from the notochord or by Shh, and its induction does not require de novo protein synthesis (Roelink et al., 1995; Ruiz i Altaba et al., 1995b; Chiang et al., 1996). This makes HNF-3β a candidate for a direct target of Shh signalling. Previously, we have identified cis-regulatory regions of the mouse HNF-3β gene in transgenic mouse embryos. Enhancers for floor plate expression and node/notochord expression were 1313 Development 124, 1313-1322 (1997) Printed in Great Britain © The Company of Biologists Limited 1997 DEV4800