Temperature and myogenesis in embryos of the Atlantic herring Clupea harengus

The development of axial muscles has been investigated in spring-spawning Atlantic herring (Clupea harengus L.) reared at 5, 8, 12 and 15 &deg;C. In 1994, around 90 % of embryos hatched after 28 days at 5 &deg;C, 16 days at 8 &deg;C, 9 days at 12 &deg;C and 8 days at 15 &deg;C. The somites were formed from cells of the paraxial mesoderm in a rostral to caudal direction, starting at the neural plate stage. Somites were added at rates ranging from one every 3 h at 5 &deg;C to one every 52 min at 15 &deg;C. A small number of myoblasts, located adjacent and lateral to the notochord, elongated to span the somite to form mononuclear myotubes. The majority of muscle fibres were formed by the fusion of 2&shy;5 myoblasts to produce multinucleated myotubes that subsequently differentiated into either superficial or inner fibre types. The timing of myogenesis with respect to somite stage and the initial appearance of the gut, pectoral fin buds and pronephric tubules was found to vary with development temperature. For rostral myotomes, the synthesis of contractile filaments and myofibril assembly were first observed at the 42-, 38- and 27-somite stages at 5, 8 and 12 &deg;C, respectively. The differentiation of myotubes into morphologically recognisable muscle fibre types first occurred at the 62-somite stage at 5 &deg;C, at the 48-somite stage at 8 &deg;C and as early as the 40-somite stage at 12 &deg;C. Cell proliferation experiments with 5-bromo-2'-deoxyuridine showed that another population of myoblasts was activated on the surface of muscle fibres just prior to hatching. Development temperature also affected muscle cellularity; there were 43 % more inner muscle fibres in the myotomes of 1-day-old larvae reared at 12 &deg;C than at 5 &deg;C (P<0.02).