A Two-Step Mechanism for Myotome Formation in Chick

Abstract

The study of the morphogenetic cell movements underlying myotome formation in the chick embryo has led to the emergence of highly controversial models. Here we report a real-time cell lineage analysis of myotome development using electroporation of a GFP reporter in newly formed chick somites. Confocal analysis of cell movements demonstrates that myotome formation involves two sequential steps. In a first phase, incremental myotome growth results from a contribution of myocytes derived solely from the medial border of the dermomyotome. In a second phase, myocytes are produced from all four borders of the dermomyotome. The relative distribution of myocytes demonstrates that the medial and the lateral borders of the somite generate exclusively epaxial and hypaxial muscles. This analysis also identified five myotomal regions, characterized by the origin of the myocytes that constitute them. Together, our results provide a comprehensive model describing the morphogenesis of the early myotome in higher vertebrates.

Establishment of the epaxial–hypaxial boundary in the avian myotome

“…The myotome is laid down in waves (Christ et al, 1983;Denetclaw et al, 1997;Kahane et al, 1998a;Cinnamon et al, 1999;Denetclaw and Ordahl, 2000;Huang and Christ, 2000;Cinnamon et al, 2001;Gros et al, 2004). The first muscle precursors are postmitotic and stem from the medial wall of the epithelial somite (Kahane et al, 1998b;Cinnamon et al, 1999).…”

“…Here, the somite differentiates along the dorsal-ventral axis into a dorsally located dermomyotome, the source of dorsal dermis and muscle, and the ventrally located sclerotome, the source of the vertebral column and ribs (Christ and Ordahl, 1995;Gossler and Hrabé de Angelis, 1998;Pourquié, 2001). The dermomyotome forms the first embryonic muscle, the myotome, by depositing myoblasts underneath in waves (Christ et al, 1983;Denetclaw et al, 1997;Kahane et al, 1998a;Cinnamon et al, 1999;Denetclaw and Ordahl, 2000;Huang and Christ, 2000;Cinnamon et al, 2001;Gros et al, 2004). The first wave contains postmitotic cells and arises from the dorsomedial wall of the epithelial somite (Kahane et al, 1998b;Cinnamon et al, 1999).…”

“…The first wave contains postmitotic cells and arises from the dorsomedial wall of the epithelial somite (Kahane et al, 1998b;Cinnamon et al, 1999). According to Kalcheim and colleagues, these cells spread underneath the dorsomedial-ventrolateral extent of the dermomyotome (Kahane et al, 1998b;Cinnamon et al, 1999), and act as a scaffold for the intercalation and alignment of a second wave of postmitotic cells that arises from all four edges of the dermomyotome (Kahane et al, 1998a;Cinnamon et al, 1999;Gros et al, 2004). In this second wave, the cells remain true to origin in that the dorsomedial lip contributes to the dorsomedial, the ventrolateral lip to the ventrolateral aspect of the myotome, and cells from the rostrocaudal lips project into the myotome directly underneath (Denetclaw et al, 1997;Kahane et al, 1998a;Cinnamon et al, 1999;Denetclaw and Ordahl, 2000;Gros et al, 2004;Huang and Christ, 2000).…”

“…According to Kalcheim and colleagues, these cells spread underneath the dorsomedial-ventrolateral extent of the dermomyotome (Kahane et al, 1998b;Cinnamon et al, 1999), and act as a scaffold for the intercalation and alignment of a second wave of postmitotic cells that arises from all four edges of the dermomyotome (Kahane et al, 1998a;Cinnamon et al, 1999;Gros et al, 2004). In this second wave, the cells remain true to origin in that the dorsomedial lip contributes to the dorsomedial, the ventrolateral lip to the ventrolateral aspect of the myotome, and cells from the rostrocaudal lips project into the myotome directly underneath (Denetclaw et al, 1997;Kahane et al, 1998a;Cinnamon et al, 1999;Denetclaw and Ordahl, 2000;Gros et al, 2004;Huang and Christ, 2000). The first two waves create a primary, postmitotic myotome, which is then populated by a third wave of progenitor cells that enter from the rostral and caudal lips of the dermomyotome, are mitotically active, and express Fgfr4 (Marcelle et al, 1995;Sechrist and Marcelle, 1996;Kahane et al, 2001).…”

The pattern of MyoD and contractile protein localization in primary epaxial myotome reflects the dynamic progression of nascent myoblast differentiation

“…Limb muscle cells develop from the lateral (hypaxial) half of the somite while epaxial muscle (muscle directly attached to vertebrae) is derived from the medial half of the somite (Ordahl and Le Douarin, 1992). The epaxial myotome, which is the first to develop within each somite, involves at least two phases; in the primary phase, the myotome expands along the somite dorso-medial-to-ventro-lateral axis; a subsequent secondary phase involves expansion of the myotome in a superficial-to-deep direction Kahane et al, 2002;Gros et al, 2004). Both phases of myotome growth occur subjacent to the somite dorsal epithelium, the dermomyotome, from which myotomal precursor cells are known to be derived (Christ and Ordahl, 1995).…”