The growth of the dermomyotome and formation of early myotome lineages in thoracolumbar somites of chicken embryos

Myotome formation in the epaxial and hypaxial domains of thoraco-lumbar somites was analyzed using fluorescent vital dye labeling of dermomyotome cells and cell-fate assessment by confocal microscopy. Muscle precursor cells for the epaxial and hypaxial myotomes are predominantly located in the dorsomedial and ventrolateral dermomyotome lips, respectively, and expansion of the dermomyotome is greatest along its mediolateral axis coincident with the dorsalward and ventralward growth directions of the epaxial and hypaxial myotomes. Measurements of the dermomyotome at different stages of development shows that myotome growth begins earlier in the epaxial than in the hypaxial domain, but that after an initial lag phase, both progress at the same rate. A combination of dye injection and/or antibody labeling of early and late-expressed muscle contractile proteins confirms the myotome mediolateral growth directions, and shows that the myotome thickness increases in a superficial (near dermis) to deep (near sclerotome) growth direction. These findings also provide a basis for predicting the following gene expression sequence program for the earliest muscle precursor lineages in mouse embryos: Pax-3 (stem cells), myf-5 (myoblast cells) and myoD (myocytes). The movements and mitotic activity of early muscle precursor cells lead to the conclusion that patterning and growth in the myotome specifically, and in the epaxial and hypaxial domains of the body generally, are governed by morphogenetic cell movements.     

Sonic hedgehog inhibits the terminal differentiation of limb myoblasts committed to the slow muscle lineage

The proliferation, differentiation, and fusion of a small number of myogenic precursor cells must be precisely regulated during development to ensure the proper size, organization, and function of the limb musculature. We have examined the role of Sonic hedgehog (Shh) in these processes by both augmentation and inhibition of the Shh-mediated signaling pathway. Our data show that Shh regulates muscle development by repressing the terminal differentiation of early myogenic precursor cells and does not function as a myoblast mitogen. Shh function in hypaxial muscle appears to be spatially restricted to the early myoblast population within the ventral muscles of the posterior region of the limb. Furthermore, Shh appears to act as a permissive, rather than an inductive, signal for slow MyHC expression in myoblasts. Our data thus provide the foundation for a new hypothesis for Shh function in hypaxial skeletal muscle development.     

Differential expression of two glucuronyltransferases synthesizing HNK-1 carbohydrate epitope in the sublineages of the rat myogenic progenitors

HNK-1 epitope is a cell-surface carbohydrate mediating various cell-cell or cell-substrate interactions. We found HNK-1 epitope in longitudinally arrayed fibers in the subpopulation of the epaxial myotome, and hypaxial myoblasts migrating into the limb bud in the rat embryo. We next investigated the expression patterns of genes encoding two glucuronyltransferases (GlcAT-P, GlcAT-D) and sulfotransferase (Sul-T), which are required for biosynthesis of HNK-1 epitope. GlcAT-P gene was expressed in the non-migrating longitudinal fibers, whereas GlcAT-D gene was expressed in the migrating myoblasts in the limb bud. Sul-T gene expression was ubiquitously observed in all these myogenic populations. Thus, differential expression of GlcAT genes may relate to the epaxial/hypaxial or migrating/non-migrating myoblast lineages.     

Medial pioneer fibers pattern the morphogenesis of early myoblasts derived from the lateral somite

The first wave of myoblasts which constitutes the post-mitotic myotome stems from the medial epithelial somite. Whereas medial pioneers extend throughout the entire mediolateral myotome at cervical and limb levels, at flank regions they are complemented laterally by a population of early myoblasts emerging from the lateral epithelial somite. These myoblasts delaminate underneath the nascent dermomyotome and become post-mitotic. They are Myf5-positive but express MyoD and desmin only a day later while differentiating into fibers. Overexpression of Noggin in the lateral somite triggers their premature differentiation suggesting that lateral plate-BMP4 maintains them in an undifferentiated state. Moreover, directly accelerating their differentiation by MyoD overexpression prior to arrival of medial fibers, generates a severely mispatterned lateral myotome. This is in contrast to medial pioneers that have the capacity for self-organization. Furthermore, inhibiting differentiation of medial pioneers with dominant-negative MyoD also disrupts lateral myoblast patterning and differentiation. Thus, we propose that medial pioneers are needed for proper morphogenesis of the lateral population which is kept as undifferentiated mesenchyme by BMP4 until their arrival. In addition, medial pioneers also organize dermomyotome lip-derived fibers suggesting that they have a general role in patterning myotome development.     

A novel role for lbx1 in Xenopus hypaxial myogenesis

We have examined lbx1 expression in early X. laevis tadpoles. In contrast to amniotes, lbx1 is expressed in all of the myoblasts that contribute to the body wall musculature, as well as in a group of cells that migrate into the head. Despite this different expression, the function of lbx1 appears to be conserved. Morpholino (MO) knockdown of lbx1 causes a specific reduction of body wall muscles and hypoglossal muscles originating from the somites. Although myoblast migratory defects are observed in antisense MO injected tadpoles targeting lbx1, this results at least in part from a lack of myoblast proliferation in the hypaxial muscle domain. Conversely, overexpression of lbx1 mRNA results in enlarged somites, an increase in cell proliferation, but a lack of differentiated muscle. The control of cell proliferation is linked to a strong downregulation of myoD expression in gain-of-function experiments. Co-injection of myoD mRNA with lbx1 mRNA eliminates the overproliferation phenotype observed when lbx1 is injected alone. The results indicate that a primary function of lbx1 in hypaxial muscle development is to repress myoD, allowing myoblasts to proliferate before the eventual onset of terminal differentiation.     

Stac3 Inhibits Myoblast Differentiation into Myotubes

The functionally undefined Stac3 gene, predicted to encode a SH3 domain- and C1 domain-containing protein, was recently found to be specifically expressed in skeletal muscle and essential to normal skeletal muscle development and contraction. In this study we determined the potential role of Stac3 in myoblast proliferation and differentiation, two important steps of muscle development. Neither siRNA-mediated Stac3 knockdown nor plasmid-mediated Stac3 overexpression affected the proliferation of C2C12 myoblasts. Stac3 knockdown promoted the differentiation of C2C12 myoblasts into myotubes as evidenced by increased fusion index, increased number of nuclei per myotube, and increased mRNA and protein expression of myogenic markers including myogenin and myosin heavy chain. In contrast, Stac3 overexpression inhibited the differentiation of C2C12 myoblasts into myotubes as evidenced by decreased fusion index, decreased number of nuclei per myotube, and decreased mRNA and protein expression of myogenic markers. Compared to wild-type myoblasts, myoblasts from Stac3 knockout mouse embryos showed accelerated differentiation into myotubes in culture as evidenced by increased fusion index, increased number of nuclei per myotube, and increased mRNA expression of myogenic markers. Collectively, these data suggest an inhibitory role of endogenous Stac3 in myoblast differentiation. Myogenesis is a tightly controlled program; myofibers formed from prematurely differentiated myoblasts are dysfunctional. Thus, Stac3 may play a role in preventing precocious myoblast differentiation during skeletal muscle development.     

The epaxial-hypaxial subdivision of the avian somite

In all jaw-bearing vertebrates, three-dimensional mobility relies on segregated, separately innervated epaxial and hypaxial skeletal muscles. In amniotes, these muscles form from the morphologically continuous dermomyotome and myotome, whose epaxial-hypaxial subdivision and hence the formation of distinct epaxial-hypaxial muscles is not understood. Here we show that En1 expression labels a central subdomain of the avian dermomyotome, medially abutting the expression domain of the lead-lateral or hypaxial marker Sim1. En1 expression is maintained when cells from the En1-positive dermomyotome enter the myotome and dermatome, thereby superimposing the En1-Sim1 expression boundary onto the developing musculature and dermis. En1 cells originate from the dorsomedial edge of the somite. Their development is under positive control by notochord and floor plate (Shh), dorsal neural tube (Wnt1) and surface ectoderm (Wnt1-like signalling activity) but negatively regulated by the lateral plate mesoderm (BMP4). This dependence on epaxial signals and suppression by hypaxial signals places En1 into the epaxial somitic programme. Consequently, the En1-Sim1 expression boundary marks the epaxial-hypaxial dermomyotomal or myotomal boundary. In cell aggregation assays, En1- and Sim1-expressing cells sort out, suggesting that the En1-Sim1 expression boundary may represent a true compartment boundary, foreshadowing the epaxial-hypaxial segregation of muscle.     

Sonic hedgehog is a survival factor for hypaxial muscles during mouse development

Sonic hedgehog (Shh) has been proposed to function as an inductive and trophic signal that controls development of epaxial musculature in vertebrate embryos. In contrast, development of hypaxial muscles was assumed to occur independently of Shh. We here show that formation of limb muscles was severely affected in two different mouse strains with inactivating mutations of the Shh gene. The limb muscle defect became apparent relatively late and initial stages of hypaxial muscle development were unaffected or only slightly delayed. Micromass cultures and cultures of tissue fragments derived from limbs under different conditions with or without the overlaying ectoderm indicated that Shh is required for the maintenance of the expression of myogenic regulatory factors (MRFs) and, consecutively, for the formation of differentiated limb muscle myotubes. We propose that Shh acts as a survival and proliferation factor for myogenic precursor cells during hypaxial muscle development. Detection of a reduced but significant level of Myf5 expression in the epaxial compartment of somites of Shh homozygous mutant embryos at E9.5 indicated that Shh might be dispensable for the initiation of myogenesis both in hypaxial and epaxial muscles. Our data suggest that Shh acts similarly in both somitic compartments as a survival and proliferation factor and not as a primary inducer of myogenesis.     

Isolation of the avian homologue of the homeobox gene Mox2 and analysis of its expression pattern in developing somites and limbs

We have isolated the cDNA of avian Mox2 and analyzed its expression pattern during somitogenesis and limb bud formation. Mox2 plays an important role in limb muscle differentiation in the mouse. Mox2 is expressed in the somites of developing chick embryos and in presumptive migrating myoblasts from the dermomyotome to the limb buds. It is also expressed in the ventral and dorsal part of limb buds and is associated with non-proliferating myoblasts. Significant differences were observed in chick and mouse expression patterns, namely in the chick dermomyotome and limb.     

Lbx1 is required for muscle precursor migration along a lateral pathway into the limb

In mammalian embryos, myogenic precursor cells emigrate from the ventral lip of the dermomyotome and colonize the limbs, tongue and diaphragm where they differentiate and form skeletal muscle. Previous studies have shown that Pax3, together with the c-Met receptor tyrosine kinase and its ligand Scatter Factor (SF) are necessary for the migration of hypaxial muscle precursors in mice. Lbx1 and Pax3 are co-expressed in all migrating hypaxial muscle precursors, raising the possibility that Lbx1 regulates their migration. To examine the function of Lbx1 in muscle development, we inactivated the Lbx1 gene by homologous recombination. Mice lacking Lbx1 exhibit an extensive loss of limb muscles, although some forelimb and hindlimb muscles are still present. The pattern of muscle loss suggests that Lbx1 is not required for the specification of particular limb muscles, and the muscle defects that occur in Lbx1(-/-) mice can be solely attributed to changes in muscle precursor migration. c-Met is expressed in Lbx1 mutant mice and limb muscle precursors delaminate from the ventral dermomyotome but fail to migrate laterally into the limb. Muscle precursors still migrate ventrally and give rise to tongue, diaphragm and some limb muscles, demonstrating Lbx1 is necessary for the lateral, but not ventral, migration of hypaxial muscle precursors. These results suggest that Lbx1 regulates responsiveness to a lateral migration signal which emanates from the developing limb.     

Sonic hedgehog controls epaxial muscle determination through Myf5 activation.

Sonic hedgehog (Shh), produced by the notochord and floor plate, is proposed to function as an inductive and trophic signal that controls somite and neural tube patterning and differentiation. To investigate Shh functions during somite myogenesis in the mouse embryo, we have analyzed the expression of the myogenic determination genes, Myf5 and MyoD, and other regulatory genes in somites of Shh null embryos and in explants of presomitic mesoderm from wild-type and Myf5 null embryos. Our findings establish that Shh has an essential inductive function in the early activation of the myogenic determination genes, Myf5 and MyoD, in the epaxial somite cells that give rise to the progenitors of the deep back muscles. Shh is not required for the activation of Myf5 and MyoD at any of the other sites of myogenesis in the mouse embryo, including the hypaxial dermomyotomal cells that give rise to the abdominal and body wall muscles, or the myogenic progenitor cells that form the limb and head muscles. Shh also functions in somites to establish and maintain the medio-lateral boundaries of epaxial and hypaxial gene expression. Myf5, and not MyoD, is the target of Shh signaling in the epaxial dermomyotome, as MyoD activation by recombinant Shh protein in presomitic mesoderm explants is defective in Myf5 null embryos. In further support of the inductive function of Shh in epaxial myogenesis, we show that Shh is not essential for the survival or the proliferation of epaxial myogenic progenitors. However, Shh is required specifically for the survival of sclerotomal cells in the ventral somite as well as for the survival of ventral and dorsal neural tube cells. We conclude, therefore, that Shh has multiple functions in the somite, including inductive functions in the activation of Myf5, leading to the determination of epaxial dermomyotomal cells to myogenesis, as well as trophic functions in the maintenance of cell survival in the sclerotome and adjacent neural tube.     

Expression of patched, prdm1 and engrailed in the lamprey somite reveals conserved responses to Hedgehog signaling

SUMMARY In the zebrafish embryo, expression of the prdm1 and patched1 genes in adaxial cells is indicative of their specification to give rise to slow twitch muscle fibers in response to Hedgehog (Hh) signaling. Subsets of these slow twitch muscle progenitors activate engrailed ( eng ) strongly in response to high-level Hh signaling, and differentiate into muscle pioneer cells, which are important for subsequent development of the horizontal myoseptum. In addition, eng is expressed more weakly in medial fast fibers in response to lower Hh levels. Somite morphology in the lamprey, an agnathan (jawless) vertebrate, differs significantly from that of teleosts. In particular, the lamprey does not have clear epaxial/hypaxial domains, lacks a horizontal myoseptum, and does not appear to possess distinct populations of fast and slow fibers in the embryonic somite. Nevertheless, Hh is expressed in the midline of the lamprey embryo, and we report here that, as in zebrafish, homologues of patched and prdm1 are expressed in adaxial regions of the lamprey somite, and an eng homologue is also expressed in the somite. However, the lamprey adaxial region does not exhibit the same distinct adaxial cell morphology as in the zebrafish. In addition, the expression of follistatin is not excluded from the adaxial region, and eng is not detected in discrete muscle pioneer-like cells. These data suggest the presence of conserved responses to Hh signaling in lamprey somites, although the full range of effects elicited by Hh in the zebrafish somite is not recapitulated.