The musculature of the body consists of an epaxial and hypaxial component. The epaxial component is composed of the deep back muscles which originate solely from the myotome. In contrast, the hypaxial component includes muscles of the ventrolateral body wall, girdle, limb and tongue. The hypaxial portion yields muscle in different ways. Muscle cells of the limb and tongue muscles and the lateral shoulder girdle muscles are derived from the migrating myogenic precursor cells from the somite. In contrast, the ventrolateral body wall muscles (intercostal and abdominal muscles) and the medial shoulder girdle muscles are formed from the myotome [1–6]. A part of the shoulder girdle muscles, the trapezius and sternocleidomastoideus muscle, originate from the lateral plate mesoderm .
The myotome, is the primitive skeletal muscle that forms within embryonic metameric structures called the somite. The somite is initially an epithelial sphere, surrounding a mesenchymal core [8, 9]. The mature somite compartmentalizes into a dorsal and a ventral part. The ventral part undergoes an epithelial to mesenchymal transition to form the sclerotome which gives rise to axial cartilages, bones, and tendons [10–12]. The dorsal portion remains as an epithelium and forms a cell sheath, called the dermomyotome, which contributes to the formationof the dorsal dermis and skeletal muscle. The four margins of the dermomyotome fold ventrally and form lip-like borders, called the dorsomedial (DML), ventrolateral (VLL), cranial and caudal dermomyotomal lips. The dermomyotomal lips contribute to myogenic cells in two ways. To form the limb muscle, undifferentiated muscle precursor cells delaminate from the VLL and undergo a long distance migration into the limb bud, where they differentiate into muscle cells . To form the myotome, muscle precursor cells delaminate from all four lips and differentiate into mononuclear myocytes immediately under the dermomyotome [13, 14].
The myotome morphogenesis has been extensively studied using the avian model. In a descriptive study using immunohistochemistry staining of myotomal cells, Kaehn et al. suggested that the myotome forms in a medial-to-lateral direction . This model postulates that the oldest myotome cells should be located at the extreme medial margin of the myotome and the youngest at the lateral margin. Based upon fluorescent cell lineage-tracing analysis, however, Denetclaw et al. [16, 17] advocated an incremental growth model in which the myotome is predicted grow in the opposite direction. According to this model, new cells are added to the myotome in a lateral-to-medial order. Further observations made by this group showed a two phase model of the myotome formation [18, 19]. In the first phase, myocytes from the DML form a thin layer of myotome. In the second phase, new myocytes derived from all four lips are recruited in a superficial-to-deep direction. In contrast, studies using thymidine dating suggested an intercalating growth model for myotome morphogenesis [20–24]. According to this model developed by Kalcheim et al, the first myotomal cells form pioneer cells which serve as a scaffold for the secondary myotomal cells. The pioneer cells translocate first from the DML to the ventral position under the dermomyotome. They then migrate to the cranial border, where they elongate towards the caudal border. The myotome cells of the second wave migrate from both cranial and caudal lip into the myotome between the pioneer cells. Using live imaging, Gros and colleagues provide a comprehensive two step model of myotome formation . In the first step, the myotome is formed by an incremental growth. Cells translocate from the DML and elongate bidirectionally, towards to cranial and caudal border of the dermomyotome. In the second step, the myotome is formed by an intercalating growth. Cells from the cranial and caudal border enter the scaffold made by the first myotomal cells originated from the DML.
Though the myotome is made up of the similar mononuclear myoblast, its epaxial and hypaxial part originate from two different sources and develop through different mechanisms. Cell lineage tracing experiments in chick embryos performed by Denetclaw and Ordahl  and Gros et al.  showed that the epaxial myotome is derived from the medial half of the somite, while the hypaxial myotome arises from the lateral half of the somite. After ablation of the DML, the medial myotome was truncated. Furthermore, the myotome formation in a DML-ablated somite could be restored by transplantation of a second DML from a donor embryo. Cell lineage tracing revealed that new myotome cells are derived from the donor DML. Based upon these observations Ordahl et al.  conclude that the DML drives the dorsal-to-medial growth of the dermomyotome which is essential for the medial extension of the myotome. Eloy-Trinquet and Nicolas characterised the formation of the epaxial and hypaxial myotome and concluded that they originate from distinct cell population . The VLL has always been assumed to execute the same function as the DML during the hypaxial mytome morphogenesis. However, this view is not supported by experimentally evidence. In this study, we analysed the myotome formation after surgical ablation of the VLL in chick embryos. Our findings demonstrate that the VLL is required for the hypaxial myotome formation.