The myotome is a segmented skeletal muscle developing along the axis and is the first muscle to differentiate in every vertebrate. While fish and tadpole myotomes persist during development, myotomes of amniote embryos disappear during embryogenesis and are replaced by the long and complex epaxial muscles. Whereas the initial development of the myotome has been intensely investigated, very little is known about the fate of the myotome and the morphogenesis of the epaxial muscles in mammals.
This study firstly examined epaxial muscle morphogenesis. Myotomal fibres and cell death in muscle fibres were followed by immunohistochemistry during rat embryogenesis. Results showed that the morphogenesis of epaxial muscles occurs through the movement of the differentiated myotomal muscle fibres rather than by de novo fusion of myoblasts after apoptosis of the initial myotomal myofibres. The myotomal muscle masses undergo progressive transformation and segregation that result in the formation of the distinct groups of epaxial muscles.
Next, the mechanisms of epaxial muscle morphogenesis were investigated in rat embryos, by following muscle progenitor cells expressing the transcription factors Pax7 and Pax3 during epaxial muscle morphogenesis using immunohistochemistry. This demonstrated that the myoblasts responsible for epaxial muscle growth derive from a population of progenitors mingled within the epaxial muscle masses as they segregate from the myotome. No migration of precursors is involved.
Transgenic ScxGFP mouse embryos, carrying a marker green fluorescent protein under the control of scleraxis (a transcription factor specific to tendons and muscle connective tissues), permitted the tracing of the connective tissues during myotome transformation. Results strongly suggest that connective tissues associated with epaxial myofibres could be actively involved in creating the displacement of the myotomal myofibres during the transformation process.
Finally, to test whether the mammalian myotome has a function as a neurally-controlled muscle during development, innervation of the myotome was studied using immunohistochemistry in comparison with the innervation of the forelimb muscles of rat embryos. The results were striking, showing that whereas the migratory limb muscles are contacted by nerves from the beginning of their differentiation, the myotome differentiates and then develops over more than two days without nerves. As revealed by the appearance of acetylcholine receptors clusters, functional innervation only occurs in the epaxial muscles when the myotome has started its transformation. The true mammalian myotome is therefore never innervated and seems to have lost its role as a neurally-controlled muscle in contrast to the myotomal muscle of fish and amphibian tadpoles.
Overall the results indicate that the development of the epaxial muscles is strikingly different from that of the muscles originating from migratory myoblasts. Contrary to the migratory muscles, the myotome develops in the absence of nerves and its differentiated muscle fibres are transformed in position and orientation to create the epaxial muscles. The development of mammalian epaxial muscles upon a template of embryonic muscle resembles the development of some adult muscles in Drosophila, developing from the larval muscles. This suggests that the mammalian myotome could be of a larval nature but with the loss of innervation.
| Identifer | oai:union.ndltd.org:ADTP/243055 |
| Date | January 2009 |
| Creators | Deries, Marianne, n/a |
| Publisher | University of Otago. Department of Anatomy & Structural Biology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Marianne Deries |
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