Xenopusmuscle development: From primary to secondary myogenesis

Abstract

Xenopus myogenesis is characterized by specific features, different from those of mammalian and avian systems both at the cellular level and in gene expression patterns. During early embryogenesis, after the initial molecular signals inducing mesoderm, the myogenic determination factors XMyoD and XMyf-5 are activated in presomitic mesoderm in response to mesoderm-inducing factors. After these first inductions of the myogenic program, forming muscles in Xenopus can have different destinies, some of these resulting in cell death before adulthood. In particular, it is quite characteristic of this species that, during metamorphosis, the primary myotomal myofibers completely die and are progressively replaced by secondary "adult" multinucleated myofibers. This feature offers the unique opportunity to totally separate the molecular analysis of these two distinct types of myogenesis. The aim of this review is to summarize our knowledge on the cellular and molecular events as well as the epigenetic regulations involved in the construction of Xenopus muscles during development. Developmental Dynamics 226:12-23, 2003.

Myogenic regulatory factors: Redundant or specific functions? Lessons from Xenopus

“…In fact, during early Xenopus development, the nAChR (acetylcholine receptor) subunit gene expression pattern proves to be correlated with that of the MRFs (Charbonnier et al, 2003 and Fig. 1) and we have pointed out that Xmyogenin is always coexpressed with the adult structural genes, adult type myosin heavy chain (MHC), and ␤-tropomyosin (␤-TM), during secondary myogenesis (Nicolas et al, 1998;Chanoine and Hardy, 2003).…”

“…Each of them is expressed according to a specific spatiotemporal sequence, which varies according to muscle origin (e.g., myotome vs. limb muscles), the type of myogenesis (embryonic vs. adult myogenesis) and between species (for reviews, see Sassoon, 1993;Nicolas et al, 1996Nicolas et al, , 2000Chanoine and Hardy, 2003). Although the very existence of different patterns of expression might be indicative of specific functions and even if in many cases expression coincides with a clearly defined step of myogenesis, no biologically relevant model can account for all the available experimental data and attribute a specific physiological role for each MRF in the course of muscle development.…”

Myogenic waves and myogenic programs during Xenopus embryonic myogenesis

“…In some cases, specific target gene transactivation for each MRF has been reported (Ridgeway et al, 2000;Charbonnier et al, 2002Charbonnier et al, , 2003Suelves et al, 2004). Ectopic expression of Myf5, Myod (Ludolph et al, 1994), or Mrf4 (Chanoine and Hardy, 2003) indicates that MRFs are able to induce ectopic myogenesis in Xenopus. We performed gain of function experiments by injection of synthetic mRNA coding for each MRF at a relative low dose (100 pg per embryo) into a single blastomere at the two-cell stage.…”

“…Xenopus myogenesis differs from those of mammalian and avian models, due to the biphasic development characterized by the radical reorganization of the body at metamorphosis (Chanoine and Hardy, 2003;Elinson, 2007). Embryonic and larval myogenesis have been defined as primary myogenesis and are followed by secondary myogenesis, initiated during metamorphosis.…”

“…It is quite characteristic of this species that, during metamorphosis, the primary myotomal myofibers completely die and are progressively replaced by secondary ''adult'' multinucleated myofibers. Although the cellular and molecular events associated with specific developmental fates of larval-and adulttype muscles during metamorphosis are beginning to be understood, numerous aspects of primary myogenesis remain to be elucidated (Chanoine and Hardy, 2003). Development of embryonic and fetal muscle fibers in vertebrates takes place by the appearance of successive myogenic waves (Tajbakhsh, 2009).…”