SummaryThe somitic compartment that gives rise to trunk muscle and dermis in amniotes is an epithelial sheet on the external surface of the somite, and is known as the dermomyotome. However, despite its central role in the development of the trunk and limbs, the evolutionary history of the dermomyotome and its role in non-amniotes is poorly understood. We have tested whether a tissue with the morphological and molecular characteristics of a dermomyotome exists in non-amniotes. We show that representatives of the agnathans and of all major clades of gnathostomes each have a layer of cells on the surface of the somite, external to the embryonic myotome. These external cells do not show any signs of terminal myogenic or dermogenic differentiation. Moreover, in the embryos of bony fishes as diverse as sturgeons (Chondrostei) and zebrafish (Teleostei) this layer of cells expresses the pax 3 and 7 genes that mark myogenic precursors. Some of the pax7-expressing cells also express the differentiation-promoting myogenic regulatory factor Myogenin and appear to enter into the myotome. We therefore suggest that the dermomyotome is an ancient and conserved structure that evolved prior to the last common ancestor of all vertebrates. The identification of a dermomyotome in fish makes it possible to apply the powerful cellular and genetic approaches available in zebrafish to the understanding of this key developmental structure.

A technique for the demonstration of myofibrillar adenosine triphosphatase activity (ATPase) used for mammalian muscle has been modified to suit fish muscle. The mammalian method involves selectively inhibiting fiber types by preincubation at either alkaline (pH 10.4) or acid (pH 4.3) pH before incubation for myofibrillar (ATPase) activity. Fish muscle fibers were found to be generally inactivated under these conditions. Preincubation at an acid pH was found to be unsuitable for fish muscle because of the indiscriminate inactivation of the fibers. The effects of preincubating at pH 10.4 and incubating tissue sections for different time periods and at different pH's and temperatures have been investigated. A differential staining of fiber types correlated with biochemical data on myofibrillar ATPase for red and white muscles was obtained by preincubating sections for short periods (1–2 min) at pH 10.4. Under these conditions the intermediately positioned pink fibers were found to stain similarly to the white fibers of high myofibrillar ATPase activity. An investigation has been made of the qualitative distribution of fiber types in the myotomal muscle of live teleost species: coalfish (Gadus virens), grey mullet (Mugil cephalus), crucian carp (Carassius carassius), black mollie (Mollienesia sp), and glassfish (Chanda ranga). The pink fibers were found to be abundant in all the species examined with the exception of the glassfish.

A qualitative histochemical study has been made of the myotomal muscles of five teleost fish (glass fish, Chanda raaga; carp, Curassius carussius; coalfish, Gadus vivens; black mollie, Molliensia sp. and grey mullet, Mugil cephalus). Three or four main fibre types were distinguished in these species on the basis of the distribution and relative activities of glycogen, lipid, aglycerophosphate dehydrogenase, phosphorylase, and succinic dehydrogenase. The so-called red and white fibre types were found to have similar histochemical properties to previously investigated species. All the species studied, with the exception of the glass fish, C/zandu runga, were found to have one or two types of pink fibre situated between the red and white fibre regions. In the carp, coalfish and mullet, the pink fibres were found to be composed of small and large diameter fibres which were similar to red and white fibres respectively, except for their staining for succinic dehydrogenase. Considerable differences were found in the relative amounts of pink muscles between species. Minor fibre components were found in several species. These consisted of very small diameter fibres which did not stain well with any of the histochemical procedures used. It is suggested that these fibres represent areas of continuing muscle growth. The results obtained are discussed in relation to the division of labour between myotomal muscles during swimming.

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