REVIEW Chaya Kalcheim Æ Nitza Kahane Æ Yuval Cinnamon Raz Ben-Yair Mechanisms of lineage segregation in the avian dermomyotome Accepted: 18 August 2006 / Published online: 12 September 2006 Ó Springer-Verlag 2006 Abstract The somite and its intermediate derivatives, sclerotome and dermomyotome (DM), are composed of distinct subdomains based on lineage analysis and gene expression patterns. This sets the grounds for elucidating the mechanisms underlying differential cell specification and morphogenesis. By examining the in vivo roles of N-cadherin on discrete domains of the somitic epithe- lium at various times, our recent studies highlight the existence of a regional and temporal heterogeneity in cellular responsiveness. As examples of this assortment, we document a coupling between asymmetric cell divi- sion and fate segregation in the DM sheet, sequential effects of N-cadherin-mediated adhesion on early myo- genic specification compared to later myofiber pattern- ing, and a differential behavior of pioneer myoblasts compared to later myogenic waves. Keywords Adherens junctions Æ Avian embryo Æ Cell adhesion Æ Dermomyotome Æ Desmin Æ Epithelial to mesenchymal transition Æ Muscle Æ MyoD Æ Myf5 Æ N-cadherin Æ Pioneer myoblasts Æ Somite Introduction The paraxial mesoderm of vertebrate embryos segments into metameric units, the somites. Even at the epithelial stage, somites are not homogeneous structures, differing in properties at least between medial vs. lateral, and rostral vs. caudal domains (Brent and Tabin 2002; Christ et al. 2004; Scaal and Christ 2004). An additional distinction, between dorsal and ventral regions, becomes apparent shortly before overt mesenchymalization of the ventral domain to generate the sclerotome while the dorsal somite half remains epithelial and becomes the dermomyotome (DM). Sclerotomal cells undergo fur- ther morphogenetic changes and form the vertebrae and ribs (Brand-Saberi and Christ 2002; Christ et al. 2004). The DM comprises a central sheet delimited by four contiguous lips that bend toward the sclerotome. DM precursors segregate into a variety of derivatives that include the epaxial muscles of the back, the hypaxial muscles of the body wall and limbs (Scaal and Christ 2004), a set of mitotic myoblasts (Ben-Yair and Kalcheim 2005) part of which generate satellite cells (Gros et al. 2005; Kassar-Duchosoy et al. 2005; Relaix et al. 2005), the dorsal dermis (Ben-Yair et al. 2003; Huang and Christ 2000), the scapula blade at flank levels of the axis (Huang et al. 2000) and endothelial cells (reviewed in Scaal and Christ 2004). Recent interest focused on the cellular and molecular mechanisms underlying formation, growth and segregation of the DM into its derivatives. Three phases of DM develop- ment were distinguished, an early patterning phase, a growth phase and then a stage of dissociation into its derivatives (reviewed in Kalcheim and Ben-Yair 2005). Inspite of regional differences measured in cell prolifer- ation and morphology during the expansion phase of this epithelium, the overall growth of the DM was found to be coherent in the medial-to-lateral extent (Ben-Yair et al. 2003). Lineage tracing analysis has demonstrated that all four lips of the DM generate mononucleated fibers (Cinnamon et al. 1999, 2001; Gros et al. 2004; Huang and Christ 2000; Kahane et al. 1998a). In addition to the proven myogenic capacity of the DM, an earlier myo- genic domain comprising the medial epithelial somite was recognized a few years ago. It consists of a spe- cialized group 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 Yuval Cinnamon and Raz Ben-Yair have equally contributed data presented in this study. C. Kalcheim (&) Æ N. Kahane Æ Y. Cinnamon Æ R. Ben-Yair Department of Anatomy and Cell Biology, Hebrew University-Hadassah Medical School, P.O. Box 12272, Jerusalem 91120, Israel E-mail: kalcheim@nn-shum.cc.huji.ac.il Tel.: +972-2-6758438 Fax: +972-2-6757451 Anat Embryol (2006) 211 (Suppl. 1): S31–S36 DOI 10.1007/s00429-006-0116-y