In vertebrates, head and trunk muscles develop from different mesodermal populations and are regulated by distinct genetic networks. Neck muscles at the head-trunk interface remain poorly defined due to their complex morphogenesis and dual mesodermal origins. Here, we use genetically modified mice to establish a 3D model that integrates regulatory genes, cell populations and morphogenetic events that define this transition zone. We show that the evolutionary conserved cucullaris-derived muscles originate from posterior cardiopharyngeal mesoderm, not lateral plate mesoderm, and we define new boundaries for neural crest and mesodermal contributions to neck connective tissue. Furthermore, lineage studies and functional analysis of Tbx1- and Pax3-null mice reveal a unique developmental program for somitic neck muscles that is distinct from that of somitic trunk muscles. Our findings unveil the embryological and developmental requirements underlying tetrapod neck myogenesis and provide a blueprint to investigate how muscle subsets are selectively affected in some human myopathies.

Disrupted ERK1/2 signaling is associated with several developmental syndromes in humans. To understand the function of ERK2 (MAPK1) in the postmigratory neural crest populating the craniofacial region, we studied two mouse models: Wnt1-Cre;Erk2 fl/fl and Osr2-Cre;Erk2 fl/fl . Wnt1-Cre;Erk2 fl/fl mice exhibited cleft palate, malformed tongue, micrognathia and mandibular asymmetry. Cleft palate in these mice was associated with delay/failure of palatal shelf elevation caused by tongue malposition and micrognathia. Osr2-Cre;Erk2 fl/fl mice, in which the Erk2 deletion is restricted to the palatal mesenchyme, did not display cleft palate, suggesting that palatal clefting in Wnt1-Cre; Erk2 fl/fl mice is a secondary defect. Tongues in Wnt1-Cre;Erk2 fl/fl mice exhibited microglossia, malposition, disruption of the muscle patterning and compromised tendon development. The tongue phenotype was extensively rescued after culture in isolation, indicating that it might also be a secondary defect. The primary malformations in Wnt1-Cre;Erk2 fl/fl mice, namely micrognathia and mandibular asymmetry, are linked to an early osteogenic differentiation defect. Collectively, our study demonstrates that mutation of Erk2 in neural crest derivatives phenocopies the human Pierre Robin sequence and highlights the interconnection of palate, tongue and mandible development. Because the ERK pathway serves as a crucial point of convergence for multiple signaling pathways, our study will facilitate a better understanding of the molecular regulatory mechanisms of craniofacial development.

Cleft palate is one of the most common congenital birth defects. Tremendous efforts have been made over the last decades towards understanding hard palate development. However, little is known about soft palate morphogenesis and myogenesis. Finding an appropriate surgical repair to restore physiological functions of the soft palate in patients with cleft palate is a major challenge for surgeons, and complete restoration is not always achievable. Here, we first analyzed the morphology, orientation and attachments of the four muscles of the murine soft palate and found that they are very similar to their counterparts in humans, validating the use of mus musculus as a model for future studies. Our data suggests that muscle differentiation extends from the lateral region to the midline following palatal fusion. We also detected an epithelial seam in the fusing soft palatal shelves, consistent with the process of fusion of the posterior palatal shelves, followed by degradation of the epithelial remnants. Innervation and vascularization are present mainly in the oral side of the soft palate, complementing the differentiated muscles. Cell lineage tracing using Wnt1-Cre;Zsgreen fl/fl mice indicated that all the tendons and mesenchyme embedding the soft palate muscles are neural crest-derived. We propose that the posterior attachment of the soft palate to the pharyngeal wall is an interface between the neural crest- and mesoderm-derived mesenchyme in the craniofacial region, and thus can serve as a potential model for the study of boundaries during development. Taken together, our study provides a comprehensive view of the development and morphology of the murine soft palate and serves as a reference for further molecular analyses.

Craniofacial development depends on cell-cell interactions, coordinated cellular movement and differentiation under the control of regulatory gene networks, which include the distal-less (Dlx) gene family. However, the functional significance of in patterning the oropharyngeal region has remained unknown. Here, we show that loss of leads to a shortened soft palate and an absence of the levator veli palatini, palatopharyngeus and palatoglossus muscles that are derived from the 4th pharyngeal arch (PA); however, the tensor veli palatini, derived from the 1st PA, is unaffected. Dlx5-positive cranial neural crest (CNC) cells are in direct contact with myoblasts derived from the pharyngeal mesoderm, and disruption leads to altered proliferation and apoptosis of CNC and muscle progenitor cells. Moreover, the FGF10 pathway is downregulated in mice, and activation of FGF10 signaling rescues CNC cell proliferation and myogenic differentiation in these mutant mice. Collectively, our results indicate that plays crucial roles in the patterning of the oropharyngeal region and development of muscles derived from the 4th PA mesoderm in the soft palate, likely via interactions between CNC-derived and myogenic progenitor cells.