Ultrastructural differentiation of cardiomyocytes of the zebrafish during the 8-26-somite stages.

We report cellular hypertrophy, mitochondrial proliferation and differentiation, myofibrillogenesis, and junctional maturation in cardiac progenitor cells between the 8 and 26-somite stages of the zebrafish, Danio rerio. However, coordinated contraction in embryonic cardiomyocytes did not occur until 26-somite stage when 'developed' intercalated discs, rooted sarcomeres, and a well-established sarcoplasmic reticulum had differentiated.