Analysis of filament population in different stages of fish and amphibian myogenesis.

In order to determine the participation of different types of filaments in particular phases of myogenic cell differentiation, analysis of filament population was done in embryonic somite cells of the trout (S. trutta) and in the cells of developing limb of the salamander larvae (S. salamandra L.). The following types of myogenic units were selected for the observation: undifferentiated mesenchymal cells (presumptive myoblasts), myoblasts prior to and post fusion and myotubules. Basing on morphological features four types of filaments were encountered: skein (4 nm), actin (8 nm), intermediate (12...14 nm) and myosin (14, 17, 21, 25 nm) filaments. Within the myosin filament population three different classes of filaments differing significantly from one another in their thickness (p less than 0.01) were found. In fish myogenic cells we have found myosin filaments of a thickness 17, 21, 25 nm, whereas in amphibian's cells, myosin filaments measured in thickness 14, 17, 21 nm respectively. The observed quantitative differences in the composition of myosin filament population might reflect their maturation process during sarcomere formation. As the process of differentiation of contractile units - sarcomeres and microfibrils develops, a distinctive quantitative predominance of 17 nm myosin filaments in fishes and 14 nm myosin filaments in amphibians becomes established.