Our studies revealed that dissociated cells from medaka (the fresh-water fish, Oryzias latipes) blastula-stage embryos differentiate into many rhythmically contracting cells when incubated with a conditioned medium from a cell line. Analyses of these cells by immunostaining, electron microscopy, expression of marker genes, action potential recordings and pharmacological responses all showed the characteristics of myocardial cells. We succeeded in purifying the cardiac cell-inducing factor from the conditioned medium by 523,100-fold with 8% recovery of the protein. Analysis of its amino-acid sequence by mass spectrometry revealed the factor to be medaka activin A 2 (homodimer of inhibin βA2 subunit). Recombinant human activin A showed the same cardiac cell-inducing activity toward medaka embryonic cells. Our results demonstrate that activin A is a potent factor for medaka myocardial cell differentiation. This culture method should provide a novel and simple experimental system to study cardiomyocyte differentiation in vitro.
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