Myotomal myogenesis inBombina variegata L

Everybody wants to move-Evolutionary implications of trunk muscle differentiation in vertebrate species

In our review we have completed current knowledge on myotomal myogenesis in model and non-model vertebrate species (fishes, amphibians, reptiles, birds and mammals) at morphological and molecular levels. Data obtained from these studies reveal distinct similarities and differences between amniote and anamniote species. Based on the available data, we decided to present evolutionary implications in vertebrate trunk muscle development. Despite the fact that in all vertebrates muscle fibres are multinucleated, the pathways leading to them vary between vertebrate taxa. In fishes during early myogenesis myoblasts differentiate into multinucleated lamellae or multinucleate myotubes. In amphibians, myoblasts fuse to form multinucleated myotubes or, bypassing fusion, directly differentiate into mononucleated myotubes. Furthermore, mononucleated myotubes were also observed during primary myogenesis in amniotes. The mononucleated state of myogenic cells could be considered as an old phylogenetic, plesiomorphic feature, whereas direct multinuclearity of myotubes has a synapomorphic character. On the other hand, the explanation of this phenomenon could also be linked to the environmental conditions in which animals develop. The similarities observed in vertebrate myogenesis might result from a conservative myogenic programme governed by the Pax3/Pax7 and myogenic regulatory factor (MRF) network, whereas differences in anamniotes and amniotes are established by spatiotemporal pattern expression of MRFs during muscle differentiation and/or environmental conditions.

Somite compartments in anamniotes

Somites are a common feature of the phylotypic stage of embryos of all higher chordates. In amniote species like mouse and chick, somite development has been the subject of intense research over many decades, giving insight into the morphological and molecular processes leading to somite compartmentalization and subsequent differentiation. In anamniotes, somite development is much less understood. Except for recent data from zebrafish, and morphological studies in Xenopus, very little is known about the formation of somite compartments and the differentiation of somite derivatives in anamniotes. Here, we give a brief overview on the development of myotome, sclerotome and dermomyotome in various anamniote organisms, and point out the different mechanisms of somite development between anamniotes and the established amniote model systems.

Mixed mesodermal and mesenchymal origin of myotomal muscles in pike (Esox lucius: Teleostei)

During the myotomal myogenesis in pike (Esox lucius) two phases of muscle differentiation can be distinguished. In the first phase, the somite cells-derived stock, the primary myoblasts (of mesodermal origin), fuse to form multinucleate myotubes. Participation of myotomal cells of mesodermal origin is insufficient for further muscle development. In the second stage mesenchymal cells migrate, via myosepts, into the myotome between myotubes. Immunocytochemical detection of proliferating cell nuclear antigen (marker of S phase of cell cycle) showed their mitotic activity. Transmission electron microscope analysis revealed that the differentiation of these cells depends on their position. Cells remaining in the myosepts develop into fibroblasts and produce collagen fibres, while those that have migrated into the myotomes transform into secondary myoblasts. Mesenchymal cells in the studied species are believed to participate in hypertrophy and hyperplasy of muscle fibres. Thus the muscle fibres in pike (E. lucius) are of mesodermal-mesenchymal origin.

The origin and morphogenesis of amphibian somites

The origin and development of the amphibian somitic mesoderm is summarized and reviewed with the goal of identifying issues most profitably pursued in these organisms. The location of the prospective somitic mesoderm as well as the cell movements bringing this tissue into its definitive position varies among amphibians. These variations have implications for the tissue interactions patterning the embryo, the design of the gastrulation movements, the role of the somitic mesoderm in early patterning and morphogenic processes, and the nature of the developmental pathway leading to somites. The presegmentation morphogenesis, the process of segmentation, and the subsequent, postsegmentation morphogenesis of the somitic mesoderm also varies considerably among amphibians. Although segmentation in amphibians shares what may be highly conserved and general patterning mechanisms with other vertebrates, the somitic developmental pathway as a whole is not conservative and has been capable of accommodating the use of a number of quite different morphogenic processes, all leading to very similar ends. The major challenges in studying amphibian somitogenesis are to develop molecular markers for major components of the somite, to determine the derivatives of the somite with better cell tracing experiments, and learning to work with the small dermatomal and sclerotomal cell populations found in most species. A potential advantage is that the diversity of somitogenesis among the amphibians makes this group ideal for studying the evolution of developmental processes. In addition, many amphibians allow direct observation of somitogenesis with great resolution and permit biomechanical analysis of tissues participating in morphogenesis, thus making it possible to analyze cellular mechanisms of morphogenesis in ways not possible in most other systems.