The first cellular evidence of dorsoventral patterning occurs as the ventral half of the somite undergoes an epithelial-mesenchymal transition to form the sclerotome. Ventral axial signals from the notochord and floor

Somites, paired epithelial spheres on either side of the axial neural tube and notochord, form by condensation of the segmental plate mesoderm at its cranial end, producing, in the case of chick, 50 somite pairs at a rate of one somite every 90 minutes (Palmeirim et al., 1997). The sequential formation of somites in a caudo-cranial direction imparts a progressive cranial directed maturation gradient which can be defined in terms of somite stages to mark somite axial position with specific developmental events during early embryo development (Ordahl, 1993). The newly formed, stage 1 somite (ss1) is epithelial and is patterned along its initial dorsoventral, cranio-caudal, and mediolateral axes in response to local signals from surrounding tissues (Christ and Ordahl, 1995; Christ et al., 1992). The first molecular evidence of dorsoventral patterning is the suppression of Pax-3 gene expression in the ventral half of the epithelium of the newly formed somite (Williams and Ordahl, 1994; Goulding et al., 1994; Bober et al., 1994). The first cellular evidence of dorsoventral patterning occurs as the ventral half of the somite undergoes an epithelial-mesenchymal transition to form the sclerotome. Ventral axial signals from the notochord and floor plate of the neural tube have been implicated in this patterning, inducing both the cellular transformation (Brand-Saberi et al., 1993; Koseki et al., 1993; Pourquie et al., 1993) and associated changes in gene expression (Fan and Tessier-Lavigne, 1994; Dietrich et al., 1997). A major signal moiety from these axial structures is the enigmatic Sonic Hedgehog gene product that is sufficient in vitro to elicit activation of the Pax-1 gene in somite explant culture (Fan and Tessier-Lavigne, 1994) but remains controversial as to whether it acts as an inducer, in the classical embryological sense, or as a growth/survival factor necessary for the expression of endogenous cell fate programs (Teillet and Le Douarin, 1983; Teillet et al., 1998). Dorsally, the neural tube and the surface ectoderm signals also act locally, through secretion of Wnt proteins and intercellular contacts, to maintain the dorsal somite epithelium, the dermomyotome, and to maintain its potential for myogenesis (Meunsterberg et al., 1995; Stern et al., 1995; Spence et al., 1996; Fan et al., 1997; Marcelle et al., 1997; Dietrich et al., 1997). In histological section, the simple pseudostratified columnar epithelium of the dermomyotome recurves at the interfaces with the ventral sclerotome forming boundaries or lips, where dermomyotome cells lose their columnar morphology but remain contained within the basal lamina (Tosney et al., 1994). As tissues grow and expand, the medial lip of the dermomyotome maintains its close 893 Development 127, 893-905 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV2501