Muscle development and differentiation in the urodele Ambystoma mexicanum.

Skeletal muscle in vertebrates is the most representative tissue and it is composed by different types of fibers which differ for anatomical and physiological features. These traits permit to distinguish them basing on the speed of twitch and the load resistance. The characteristics of a fiber depend on the amount of glycogen and mitochondria in their cytoplasm, as well as by the presence of glycolitic and oxidative enzymes. In particular, slow fibers are specific for a long-lasting contraction, they express a MyHC isoform having an ATPase activity at acidic pH (pH 4.0) and an aerobic oxidative metabolism. In contrast, fast fibers are larger in size, with few mitochondria. They are specialized in rapid and short-lasting contractions and express a MyHC isoform, having an ATPase activity at basic pH (pH 10.0) and an anaerobic glycolitic metabolism. These different types of fibers, both in amniotes and in lower vertebrates such as fish and anuran amphibians s come from distinct populations of myoblasts which appear in the somites during the embryonic development. Even if substantial data concerning the muscle differentiation and the characterization of the different types of fibers in zebrafish and Xenopus are available, nothing is still known about the mechanisms regulating these processes in amphibian urodeles. Since these animals present some anatomical features and life style between fish and anurans, the study of myogenic processes in these animals could be useful to clarify the evolutionary changes which lead to the formation of skeletal muscle in the trunk of land vertebrates. To shed light on the myogenic processes in urodele amphibians we focused our studies on the axolotl Ambystoma mexicanum and we analysed several embryonic stages of this amphibian in order to identify the different types of fibers and their pattern of distribution during the myogenic process. Using morphological analysis, enzymatic hystochemistry and immunohystochemistry we showed that in A. mexicanum, as in zebrafish, the first differentiating fibers are the slow ones deriving from myoblasts localized close to the notochord. These fibers then migrate towards the somitic surface and here they give rise to a uperficial layer of slow fibers, while the myoblasts forming the medial part of the somite differentiate into fast fibers. Further, in order to evaluate the possible involvement of muscle-specific transcriptional factors and protein signalling in the regulation of myogenic process of this amphibian, we used molecular biological approaches to identify and clone the muscle-specific transcriptional factor (Myf5) and the Sonic hedgehog signalling protein, known to regulate the muscle development and differentiation both in amniotes and lower vertebrates.