Muscle Development

Abstract

Compared to the development of the trunk and limb muscles, where the complex set of tissue and molecular interactions has been unravelled in great detail, there is an extreme paucity of data about craniofacial myogenic differentiation. For example, in the limb we know that ectodermal signals, mediated by Bmps, maintain proliferation and prevent myogenic differentiation whilst the signalling factors scatter factor and Fgfs control migration and differentiation (reviewed by Buckingham 2001). However, this knowledge cannot be used to dissect the cascade of craniofacial myogenesis as these cells usually do not express gene markers, such as Pax3 and Lbxl, which mark uncommitted myogenic cells in other regions of the body, suggesting that their development is distinct. Expression of the myogenic determination factor, Myf5, is also under the control of distinct promoters in the head and trunk (Hadchouel et al. 2000; Summerbell et al. 2000; Carvajal et al. 2001). The proposed differences have been further supported by studies in which undifferentiated craniofacial myogenic cells have been grafted into the trunk and vice versa, where the cells have been shown to maintain their original identity (Hacker and Guthrie 1998; Mootoosamy and Dietrich 2002). However, whether these studies provide conclusive proof that cranial and trunk myogenic cells are intrinsically different is still far from clear. These grafting experiments transpose relatively large pieces of tissue and hence the premyogenic cells are grafted with surrounding tissue. As with the cranial neural crest there may be a community effect or the surrounding mesoderm may maintain the appropriate genetic code (see Sect. 5.1). Hints at this come from experiments by Hacker and Guthrie (1998)—where the somitic cells become detached from the main graft of cells, they switch off the expression of Pax3 in accordance to the local environment. These questions will only be answered by clonal analysis in which individual myogenic cells are transferred into an ectopic environment.