In vivo cell cycle analysis in higher eukaryotes has been limited by the challenge of preserving the integrity of the living organism while visualizing dividing cells. Japanese medaka in order to visualize and manipulate the cell cycle progression in a live vertebrate. Our stable transgenic histone H2B-GFP medaka line allows fluorescence-based monitoring of the chromosomes. The system has a high specificity, with a strong GFP signal labeling the chromatin architecture. The subcellular resolution ensures detection of both normal and abnormal divisions in live recordings. This translates into the possibility to quantify temporal and spatial aspects of the cell cycle, such as length or nuclear size, as well as to expose drug toxicity at the earliest stage. We also show that acclimation to cold, a prominent feature of the eurytherm medaka, is a valuable natural way of inducing a reversible cell cycle arrest in the entire living organism. Our results suggest that this manipulation can be performed from the early stages of development, has no toxicity and does not alter the cell cycle profile of the embryo.
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