The differentiation and morphogenesis of craniofacial muscles

Abstract

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.

Tbx1 regulation of myogenic differentiation in the limb and cranial mesoderm

“…The extraocular eye muscle, the lateral rectus, which arises adjacent to the hindbrain and migrates to its final destination was also found to express Tbx1 (position indicated by white arrow in Fig. 1A, also see Noden and Francis-West, 2006). The expression of Tbx1 in myogenic cells was readily apparent by comparison with the expression of the myogenic factors capsulin, MyoR, and MyoD on adjacent sections at stages 20 and 24 (Fig.…”

“…This analysis revealed that Tbx1 transcripts are clearly apparent in the mesoderm adjacent to the caudal hindbrain from stage 7, encompassing the region that will give rise to the first and second arch muscles and the lateral rectus muscle (Fig. 3A, black arrow and data not shown; Evans and Noden, 2006; reviewed by Noden and Francis-West, 2006). In contrast, capsulin expression was undetectable in the cranial mesoderm ( Fig.…”

“…Tbx1 is not expressed in all myoblasts raising the question as to what is unique about these myoblasts and why do they require Tbx1 function? This highlights the novelties that are beginning to emerge between cranial, including subdivisions between cranial myoblasts, and axial myoblasts with each differentiating according to its own molecular program (reviewed by Noden and Francis-West, 2006). Future challenges will be to understand the exact role of Tbx1 during myogenic differentiation, its down-stream targets, and how Tbx1 regulates myocyte number.…”

“…The axial and appendicular muscles of the body develop from the segmented paraxial or somitic mesoderm, whereas the majority of those of the head are derived from the unsegmented paraxial mesoderm and prechordal plate (reviewed by Noden and Francis-West, 2006). Throughout the body, myogenic commitment is marked by the expression of the myogenic determination factors Myf5 and MyoD followed by the expression of MRF4 and myogenin.…”

Spatial relations between avian craniofacial neural crest and paraxial mesoderm cells

“…Typically, myogenic and connective tissue progenitors are induced and begin their differentiations independently (reviews, e.g., limbs: Duprez, 2002;Anakawe et al, 2003;Venters et al, 2004;head: FrancisWest et al, 2003;Noden and Trainor, 2005;Noden and Francis-West, 2006). However, the subsequent morphogenesis of muscles necessitates especially tight integration with their surrounding connective tissues.…”