The third wave of myotome colonization by mitotically competent progenitors: regulating the balance between differentiation and proliferation during muscle development

Follistatin complexes Myostatin and antagonises Myostatin-mediated inhibition of myogenesis

Follistatin is known to antagonise the function of several members of the TGF-beta family of secreted signalling factors, including Myostatin, the most powerful inhibitor of muscle growth characterised to date. In this study, we compare the expression of Myostatin and Follistatin during chick development and show that they are expressed in the vicinity or in overlapping domains to suggest possible interaction during muscle development. We performed yeast and mammalian two-hybrid studies and show that Myostatin and Follistatin interact directly. We further show that single modules of the Follistatin protein cannot associate with Myostatin suggesting that the entire protein is required for the interaction. We analysed the interaction kinetics of the two proteins and found that Follistatin binds Myostatin with a high affinity of 5.84 x 10(-10) M. We next tested whether Follistatin suppresses Myostatin activity during muscle development. We confirmed our previous observation that treatment of chick limb buds with Myostatin results in a severe decrease in the expression of two key myogenic regulatory genes Pax-3 and MyoD. However, in the presence of Follistatin, the Myostatin-mediated inhibition of Pax-3 and MyoD expression is blocked. We additionally show that Myostatin inhibits terminal differentiation of muscle cells in high-density cell cultures of limb mesenchyme (micromass) and that Follistatin rescues muscle differentiation in a concentration-dependent manner. In summary, our data suggest that Follistatin antagonises Myostatin by direct protein interaction, which prevents Myostatin from executing its inhibitory effect on muscle development.

A Two-Step Mechanism for Myotome Formation in Chick

The study of the morphogenetic cell movements underlying myotome formation in the chick embryo has led to the emergence of highly controversial models. Here we report a real-time cell lineage analysis of myotome development using electroporation of a GFP reporter in newly formed chick somites. Confocal analysis of cell movements demonstrates that myotome formation involves two sequential steps. In a first phase, incremental myotome growth results from a contribution of myocytes derived solely from the medial border of the dermomyotome. In a second phase, myocytes are produced from all four borders of the dermomyotome. The relative distribution of myocytes demonstrates that the medial and the lateral borders of the somite generate exclusively epaxial and hypaxial muscles. This analysis also identified five myotomal regions, characterized by the origin of the myocytes that constitute them. Together, our results provide a comprehensive model describing the morphogenesis of the early myotome in higher vertebrates.

Myotome meanderings. Cellular morphogenesis and the making of muscle

The formation of muscles within the vertebrate embryo is a tightly orchestrated and complex undertaking. Beyond the initial specification of cells to become muscle are several complex cellular movements and migrations, which lead to the positioning of muscle precursors at specific locations within the embryo. The consequent differentiation, elongation and striation of these cells results in the formation of individual muscles. Investigation of the in vivo morphogenesis of individual vertebrate muscle cells has only recently begun, and is being approached through the use of sophisticated cell labelling and lineage analysis techniques. However, a consensus about the mechanisms involved has yet to be achieved. This review outlines vertebrate embryonic muscle formation in chick, fish and mice, focusing on the embryonic myotome, which generates both the axial musculature and the appendicular muscle of the fins and limbs. We highlight the points of consensus about, and the complexity of, this developmental system, and propose an evolutionary context for the basis of these understandings.

A somitic compartment of tendon progenitors

We demonstrate that the tendons associated with the axial skeleton derive from a heretofore unappreciated, fourth compartment of the somites. Scleraxis (Scx), a bHLH transcription factor, marks this somitic tendon progenitor population at its inception, and is continuously expressed through differentiation into the mature tendons. Two earlier-formed somitic compartments, the sclerotome and myotome, interact to establish this fourth Scx-positive compartment. The tendon progenitors are induced at the sclerotome's edge, at the expense of skeletogenic Pax1 positive cells and in response to FGF signaling in the adjacent myotome. The tendon primordia thus form in a location abutting the two tissues that the mature tendons must ultimately connect. Tendon progenitor formation may reveal a general mechanism for the specification of other somitic subcompartments.

The regulation and action of myostatin as a negative regulator of muscle development during avian embryogenesis

Myostatin is a potent inhibitor of muscle growth. Genetic deletion of Myostatin leads to massive hyperplasia and hypertrophy of skeletal muscle. However, the overall muscle pattern is preserved. We show that, during chick embryonic development, Myostatin is expressed at stages and positions unlikely to influence qualitative muscle development. In the somites, Myostatin is predominantly expressed in a central domain of the dermomyotome but not at the dorsomedial and ventrolateral lips, where most cells for myotomal elongation are recruited. During limb bud development, Myostatin is transiently expressed at early stages in both myogenic and nonmyogenic regions. Myostatin is reexpressed during limb bud development at a time when splitting of muscle is underway. Heterotopically developed wing buds that fail to form muscle still express Myostatin. This demonstrates that, in the limb, not all Myostatin-expressing cells are of myogenic origin. Ectoderm and Sonic hedgehog have different effects on the expression of Myostatin dependent on stages at which the operation was performed and the length of the postoperative period. Finally, we show that application of Myostatin protein into the developing limb bud results in a down-regulation of Pax-3 and Myf-5, both genes associated with proliferation of myogenic cells; and, furthermore, Myostatin also prevents the expression of MyoD, a gene associated with muscle differentiation. The long-term effect of Myostatin treatment leads to a deficiency of limb muscle. Therefore, Myostatin negatively affects gene expression of transcription factors, which are necessary for establishing myogenic cell identity.