INTRODUCTION The paraxial mesoderm of vertebrate embryos segments into metameric units, the somites, which develop stepwise to give rise initially to the ventral mesenchymal sclerotome and the dorsal epithelial dermomyotome (DM). Sclerotomal cells then undergo further morphogenetic changes and form the vertebrae and ribs (Brand-Saberi and Christ, 2000; Huang et al., 2000b) (see Kato and Aoyama, 1998). DM precursors give rise to the epaxial muscles of the back, the hypaxial muscles of the body wall and limbs, and the dorsal dermis (Christ et al., 1983; Kalcheim et al., 1999; Huang and Christ, 2000), yet the mechanism by which these derivatives form from the DM is not fully resolved. In the past few years, the existence of an additional cellular domain in the medial epithelial somite has been described. It consists of a specialized subset of early post- mitotic progenitors which in avian embryos, express both MyoD and Myf5. During the process of somite dissociation, these cells bend underneath the forming DM and upon delamination and polarized caudorostral migration, differentiate into the first unit-length myofibers that span the entire mediolateral extent of each segment (Kahane et al., 1998a; Kahane et al., 2002). Notably, these early fibers were suggested to provide a scaffold for further addition of myogenic cells that arise in the overlying DM. These pioneer myoblasts, together with DM-derived myofibers, constitute the myotome from which vertebral muscles of the back and the body wall (intercostal and abdominal) will form (Kalcheim et al., 1999). At variance, limb muscles derive exclusively from the ventrolateral lip (VLL) of the DM (Chevallier et al., 1977) and bear no contribution of pioneer myofibers (N.K. and C.K., unpublished). A wealth of information is available on early patterning of somite derivatives that involves opposite and mutually inhibitory gradients of factor activity (reviewed by Brent and Tabin, 2002). Yet, the mechanisms by which these basic gradients are translated into morphogenetic processes remain incompletely understood. Ordahl and colleagues (Denetclaw et al., 1997; Denetclaw and Ordahl, 2000) have proposed that growth of both the myotome and DM is driven by two stem cell systems restricted to the dorsomedial and ventrolateral lips (DML and VLL, respectively) of the DM. The DML was suggested to drive medial growth of the myotome and the VLL to be responsible for expansion in a ventral direction (Denetclaw et al., 1997; Denetclaw and Ordahl, 2000; Ordahl et al., 2001). Consequently, both myotome and DM expand incrementally in opposite medial and lateral directions, with the youngest cells approaching the medial and lateral extremes 4325 Development 130, 4325-4336 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00667 We have previously shown that overall growth of the myotome in the mediolateral direction occurs in a coherent and uniform pattern. We asked whether development of the dermomyotome and resultant dermis follow a similar pattern or are, alternatively, controlled by restricted pools of stem cells driving directional growth. To this end, we studied cellular events that govern dermomyotome development and the regional origin of dermis. Measurements of cell proliferation, nuclear density and cellular rearrangements revealed that the developing dermomyotome can be subdivided in the transverse plane into three distinct and dynamic regions: medial, central and lateral, rather than simply into epaxial and hypaxial domains. To understand how these temporally and spatially restricted changes affect overall dermomyotome growth, lineage tracing with CM-DiI was performed. A proportional pattern of growth was measured along the entire epithelium, suggesting that mediolateral growth of the dermomyotome is coherent. Hence, they contrast with a stem cell view suggesting focal and inversely oriented sources of growth restricted to the medial and lateral edges. Consistent with this uniform mediolateral growth, lineage tracing experiments showed that the dermomyotome- derived dermis originates from progenitors that reside along the medial as well as the lateral halves of somites, and whose contribution to dermis is regionally restricted. Taken together, our results support the view that all derivatives of the dorsal somite (dermomyotome, myotome and dermis) keep a direct topographical relationship with their epithelial ascendants. Key words: Avian embryo, Cell delamination, Dermis, Dermomyotome, Desmin, Epithelial-mesenchymal conversion, Myotome, Somite SUMMARY Coherent development of dermomyotome and dermis from the entire mediolateral extent of the dorsal somite Raz Ben-Yair, Nitza Kahane and Chaya Kalcheim* Department of Anatomy and Cell Biology, Hebrew University-Hadassah Medical School, Jerusalem 91120, PO Box 12272, Israel *Author for correspondence (e-mail: kalcheim@nn-shum.cc.huji.ac.il Accepted 9 June 2003