Myogenesis and contraction in the early embryonic heart of the rainbow trout. An electron microscopic study.

Myogenesis in the embryonic heart of the rainbow trout, Salmo gairdneri (Rich.), was investigated electron microscopically from the 29th to the 41st somite stage. Thick and thin myofilaments are formed simultaneously as well as precursors of Z-lines, to which the thin filaments are attached. The genesis of filaments takes place in the region around the intracellular yolk droplets. The first myofibrils appear by the 33rd somite stage, probably formed by a mechanism of self-assembly in which the binding sites of actin and myosin participate. A- and I-bands do not develop before the 38th osmite stage. The contraction already begins during the 33rd somite stage in the middle of the tubular heart. Gradually, the peristaltic waves spread increasingly to other parts of the heart. In the 41st somite stage the entire heart is contractile and all myocytes contain myofibrils.