AN ELECTRON-MICROSCOPIC INVESTIGATION INTO THE POSSIBLE SOURCE OF NEWMUSCLE-FIBERS IN TELEOST FISH

This study is based on transmission electron microscopic (TEM) investi gations of deep (fast, 'white') teleost fish muscle proliferation in e arly developmental stages of three European cyprinid species and the r ainbow trout. Our fine structural findings provide evidence that early myotomal growth in these animals may utilize different mechanisms tha t are activated in close succession during early life history. First, initial enlargement of the deep muscle bulk in the embryo seems to be due to hypertrophy of the somite-cell derived stock of muscle fibres. Second, we suggest that deep muscle growth becomes additionally powere d by attachment of presumptive myogenic cells that originate from and proliferate within the adjacent mesenchymal tissue lining. Third, mese nchyme-derived muscle cell precursors are thought to enter the myotome s via the myosepta. After migration between the pre-established muscle fibres these cells may function as myosatellite cells, thus at least partly providing the stem cell population for subsequent rapid hyperpl astic growth. Finally, there is evidence that presumptive deep muscle satellite cells also proliferate by mitotic division in situ. A simila r process of myogenic cell migration and proliferation may foster inte rmediate fibre differentiation. The model of myogenic cell migration i s discussed in view of in vitro and in vivo data on satellite cell mig ratory power and with respect to temperature-induced and species depen dent differences. As for the latter, our results indicate that pattern s of muscle differentiation may diverge between a fast growing salmoni d species and a moderately growing cyprinid species of similar final s ize. The model is compatible with the well-established idea that teleo st muscle growth may rely on different subclasses of myosatellite cell s.