Properties of branchiomeric and somite-derived muscle development in Tbx1 mutant embryos

Vertebrate craniofacial and trunk myogenesis are regulated by distinct genetic programs. Tbx1, homologue of the del22q11.2 syndrome candidate gene TBX1, controls branchiomeric craniofacial muscle development. Here, we demonstrate using immunohistochemistry that myogenic regulatory factors are activated in Tbx1-positive cells within pharyngeal mesoderm. These cells are also Islet1 and Capsulin-positive and in the absence of Tbx1 persist in the core of the first arch. Sporadic hypoplastic mandibular muscles in Tbx1-/- embryos contain Pax7-positive myocytes with indistinguishable differentiation properties from wild-type muscles and have normal tendon attachments and fiber-type patterning. In contrast to TBX1 haploinsufficient del22q11.2 syndrome patients, no alteration in fiber-type distribution was detected in Tbx1+/- adult masseter and pharyngeal constrictor muscles. Furthermore, Tbx1-expressing limb muscles display normal patterning, differentiation, fiber-type growth, fiber-type distribution and fetal maturation in the absence of Tbx1. The critical requirement for Tbx1 during muscle development is thus in the robust onset of myogenic specification in pharyngeal mesoderm.     

The differentiation and morphogenesis of craniofacial muscles.

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.     

The formation of skeletal muscle: from somite to limb

During embryogenesis, skeletal muscle forms in the vertebrate limb from progenitor cells originating in the somites. These cells delaminate from the hypaxial edge of the dorsal part of the somite, the dermomyotome, and migrate into the limb bud, where they proliferate, express myogenic determination factors and subsequently differentiate into skeletal muscle. A number of regulatory factors involved in these different steps have been identified. These include Pax3 with its target c-met, Lbx1 and Mox2 as well as the myogenic determination factors Myf5 and MyoD and factors required for differentiation such as Myogenin, Mrf4 and Mef2 isoforms. Mutants for genes such as Lbx1 and Mox2, expressed uniformly in limb muscle progenitors, reveal unexpected differences between fore and hind limb muscles, also indicated by the differential expression of Tbx genes. As development proceeds, a secondary wave of myogenesis takes place, and, postnatally, satellite cells become located under the basal lamina of adult muscle fibres. Satellite cells are thought to be the progenitor cells for adult muscle regeneration, during which similar genes to those which regulate myogenesis in the embryo also play a role. In particular, Pax3 as well as its orthologue Pax7 are important. The origin of secondary/fetal myoblasts and of adult satellite cells is unclear, as is the relation of the latter to so-called SP or stem cell populations, or indeed to potential mesangioblast progenitors, present in blood vessels. The oligoclonal origin of postnatal muscles points to a small number of founder cells, whether or not these have additional origins to the progenitor cells of the somite which form the first skeletal muscles, as discussed here for the embryonic limb.     

The myogenic factor Myf5 supports efficient skeletal muscle regeneration by enabling transient myoblast amplification

The myogenic factor Myf5 defines the onset of myogenesis in mammals during development. Mice lacking both Myf5 and MyoD fail to form myoblasts and are characterized by a complete absence of skeletal muscle at birth. To investigate the function of Myf5 in adult skeletal muscle, we generated Myf5 and mdx compound mutants, which are characterized by constant regeneration. Double mutant mice show an increase of dystrophic changes in the musculature, although these mice were viable and the degree of myopathy was modest. Myf5 mutant muscles show a small decrease in the number of muscle satellite cells, which was within the range of physiological variations. We also observed a significant delay in the regeneration of Myf5 deficient skeletal muscles after injury. Interestingly, Myf5 deficient skeletal muscles were able to even out this flaw during the course of regeneration, generating intact muscles 4 weeks after injury. Although we did not detect a striking reduction of MyoD positive activated myoblasts or of Myf5-LacZ positive cells in regenerating muscles, a clear decrease in the proliferation rate of satellite cell-derived myoblasts was apparent in satellite cell-derived cultures. The reduction of the proliferation rate of Myf5 mutant myoblasts was also reflected by a delayed transition from proliferation to differentiation, resulting in a reduced number of myotube nuclei after 6 and 7 days of culture. We reason that Myf5 supports efficient skeletal muscle regeneration by enabling transient myoblast amplification. Disclosure of potential conflicts of interest is found at the end of this article.     

TBX1基因与染色体22q11.2微缺失综合征

染色体22q11·2微缺失综合征(22q11·2DS)是人类最常见的染色体微缺失综合征。TBX1基因作为一个T-BOX家族转录因子,其单拷贝缺失可能是22q11·2DS的主要原因之一,它对该综合征临床表征的出现可能有重要作用。TBX1在胚胎生长发育是胚胎咽部分节、咽弓和动脉弓形成、心脏流出道生长与排列及分隔等过程所必需的基因。TBX1受一系列调控基因调节,其本身也调控一系列基因。     

Presence of neurotrophic factors in skeletal muscle correlates with survival of spinal cord motor neurons.

To determine which combination of skeletal muscle-derived neurotrophic factors may be important for the survival of specific subpopulations of developing spinal cord motor neurons, we used Myf5 and MyoD (myogenic regulatory factors) knockouts, containing differentially committed myogenic precursor cells (MPCc) and immunohistochemistry against several muscle-secreted neurotrophic factors. At the peak of motor neuron cell death, skeletal muscle development is delayed in the back and body wall muscles of Myf5-/- embryos and in the limb muscles of MyoD-/- embryos. We hypothesized that, if the skeletal muscle was indeed an important source of survival factors for motor neurons, the back, the abdominal wall, and the forelimb MPCs of Myf5-/- or MyoD-/- embryos should produce at least some neurotrophic factors necessary for the survival of motor neurons. In this report, we demonstrate that (1) different MPCs lacking Myf5, MyoD, or Myf5/MyoD have different capabilities in providing factors potentially required for the survival of motor neurons and intramuscular nerve branching, (2) MPCs in double-mutant embryos do not contain neurotrophic factors in the absence of myogenic specification, and (3) different subpopulations of MPCs contain different combinations of neurotrophic factors potentially required for the survival of the specific subpopulations of innervating motor neurons.     

Myogenic waves and myogenic programs during Xenopus embryonic myogenesis

Although Xenopus is a key model organism in developmental biology, little is known about the myotome formation in this species. Here, we assessed the expression of myogenic regulatory factors of the Myod family (MRFs) during embryonic development and revealed distinct MRF programs. RESULTS: The expression pattern of each MRF during embryonic development highlights three successive myogenic waves. We showed that a first median and lateral myogenesis initiates before dermomyotome formation: the median cell population expresses Myf5, Myod, and Mrf4, whereas the lateral one expresses Myod, moderate levels of Myogenin and Mrf4. The second wave of myoblasts arising from the dermomyotome is characterized by the full MRF program expression, with high levels of Myogenin. The third wave is revealed by Myf5 expression in the myotome and could contribute to the formation of plurinucleated fibers at larval stages. Furthermore, Myf5- or Myod-expressing anlagen are identified in craniofacial myogenesis. CONCLUSIONS: The first median and lateral myogenesis and their associated MRF programs have probably disappeared in mammals. However, some aspects of Xenopus myogenesis have been conserved such as the development of somitic muscles by successive myogenic waves and the existence of Myf5-dependent and -independent lineages.     

Masticatory muscle defects in hemifacial microsomia: a new embryological concept

First arch syndromes correspond to a wide spectrum of human latero-facial congenital anomalies affecting cranial neural crest cells (CNCCs) derivatives of the first pharyngeal arch (PA1). The abnormal traits display variable quantitative expression and are often unilateral. Mandibular skeletal defects are invariably accompanied by hypoplasia or agenesis of masticatory muscles, but no explanation has been proposed for this association. Indeed, during embryonic development, CNCCs give only rise to skeletal components of the head while muscles derive from cephalic myogenic mesodermal cells (CMMCs). Recent studies on animal models have shown that communication between CNCCs and CMMCs is essential for the development of masticatory muscles: genetic lesions affecting only CNCCs can prevent muscularization of the jaws. To evaluate the involvement of CNCC/CMMC interactions in human craniofacial development, we performed a quantitative analysis of masticatory muscle and mandibular bone volumes on craniofacial CT-scans from 8 children, ages 3 months to 16 years, affected by hemifacial microsomia. We found that: (1) in seven patients the masseter muscle is absent in the affected side; (2) the absence of masseter is correlated neither with the age of the patients nor with the volume and shape of the affected ramus; and (3) in all cases the pterygoid and the temporal muscles are either reduced or absent. Our findings suggest that an early developmental event is the origin of the muscular defects in these patients. We propose that the hypoplasia or agenesis of masticatory muscles derives from a defect in the CNCCs/CMMCs communication during early embryonic development.     

Tbx1 mutation causes multiple cardiovascular defects and disrupts neural crest and cranial nerve migratory pathways

TBX1 is the major candidate gene for DiGeorge syndrome (DGS). Mouse studies have shown that the Tbx1 gene is haploinsufficient, as expected for a DGS candidate gene, and that it is required for the development of pharyngeal arches and pouches, as predicted by the DGS clinical phenotype. However, a detailed analysis of the cardiovascular phenotype associated with Tbx1 mutations has not been reported. Here we show that Tbx1 deficiency causes a number of distinct vascular and heart defects, suggesting multiple roles in cardiovascular development - specifically formation and growth of the pharyngeal arch arteries, growth and septation of the outflow tract of the heart, interventricular septation, and conal alignment. Comparison of phenotype and gene expression using a Tbx1-lacZ reporter allele supports a cell-autonomous function in the growth of the pharyngeal apparatus, and a cell non-autonomous function in the growth and early remodeling of the pharyngeal arch arteries. Our data do not support a direct role of neural crest cells in the pathogenesis of the Tbx1 mutant phenotype; however, these cells, and the cranial nerves, are misdirected. We hypothesize that this is due to the lack of a guidance role from the pouch endoderm, which is missing in these mutants.