The epaxial–hypaxial subdivision of the avian somite

Abstract

In all jaw-bearing vertebrates, three-dimensional mobility relies on segregated, separately innervated epaxial and hypaxial skeletal muscles. In amniotes, these muscles form from the morphologically continuous dermomyotome and myotome, whose epaxial-hypaxial subdivision and hence the formation of distinct epaxial-hypaxial muscles is not understood. Here we show that En1 expression labels a central subdomain of the avian dermomyotome, medially abutting the expression domain of the lead-lateral or hypaxial marker Sim1. En1 expression is maintained when cells from the En1-positive dermomyotome enter the myotome and dermatome, thereby superimposing the En1-Sim1 expression boundary onto the developing musculature and dermis. En1 cells originate from the dorsomedial edge of the somite. Their development is under positive control by notochord and floor plate (Shh), dorsal neural tube (Wnt1) and surface ectoderm (Wnt1-like signalling activity) but negatively regulated by the lateral plate mesoderm (BMP4). This dependence on epaxial signals and suppression by hypaxial signals places En1 into the epaxial somitic programme. Consequently, the En1-Sim1 expression boundary marks the epaxial-hypaxial dermomyotomal or myotomal boundary. In cell aggregation assays, En1- and Sim1-expressing cells sort out, suggesting that the En1-Sim1 expression boundary may represent a true compartment boundary, foreshadowing the epaxial-hypaxial segregation of muscle.

Establishment of the epaxial–hypaxial boundary in the avian myotome

“…This finding suggests that En1 is the key marker for the beginning of epaxial-hypaxial segregation of the somite. Eventually, En1 expression also appears in the myotome, with the expression boundary aligned with the expression boundary in the overlying dermomyotome (Cheng et al, 2004). DiI-DiO labeling and quail-chick transplantation experiments indicated that all En1-expressing cells arise from the epaxial part of the somite (Cheng et al, 2004).…”

“…However, labeling experiments indicated that epaxial-hypaxial cells arise from distinct cell lineages during gastrulation (Selleck and Stern, 1991;Freitas et al, 2001;Eloy-Trinquet and Nicolas, 2002). Moreover, evidence from cell lineage restriction and cell sorting assays indicated that the mature dermomyotome and myotome comprises distinct epaxial and hypaxial compartments (Selleck and Stern, 1991;Freitas et al, 2001;Eloy-Trinquet and Nicolas, 2002;Cheng et al, 2004). Thus, an epaxial-hypaxial subdivision, although morphologically concealed, is present in the amniote somite.…”

“…Recent studies demonstrated that the epaxial-hypaxial boundary of the dermomyotome is molecularly defined by the differential expression of the homeobox containing transcription factor En1 and the basic helix-loophelix (bHLH) transcription factor Sim1, with En1 demarcating the epaxial and Sim1 the hypaxial domains (Cheng et al, 2004). Triggered by the lateral mesoderm, Sim1 is expressed in the entire lateral somite half from early stages onward (Pourquié et al, 1996).…”

“…Triggered by the lateral mesoderm, Sim1 is expressed in the entire lateral somite half from early stages onward (Pourquié et al, 1996). En1 expression, initiated by a complex regulatory network involving the notochord, neural tube, and surface ectoderm, appears in the dermomyotome at somite stages VIII-X (Olivera- Martinez et al, 2002;Cheng et al, 2004). At this stage, the En1-and Sim1-expressing parts of the dermomyotome form a compartment boundary (Cheng et al, 2004).…”

“…En1 expression, initiated by a complex regulatory network involving the notochord, neural tube, and surface ectoderm, appears in the dermomyotome at somite stages VIII-X (Olivera- Martinez et al, 2002;Cheng et al, 2004). At this stage, the En1-and Sim1-expressing parts of the dermomyotome form a compartment boundary (Cheng et al, 2004). This finding suggests that En1 is the key marker for the beginning of epaxial-hypaxial segregation of the somite.…”