Oxygen deprivation causes suspended animation in the zebrafish embryo

Continuous exposure to oxygen is essential for nearly all vertebrates. We f ound that embryos of the zebrafish Danio rerio can survive for 24 h in the absence of oxygen (anoxia, 0% O-2) In anoxia. zebrafish entered a state of suspended animation where all microscopically observable movement ceased, i ncluding cell division, developmental progression, and motility. Animals th at had developed a heartbeat before anoxic exposure showed no evidence of a heartbeat until return to terrestrial atmosphere (normoxia, 20.8% O-2) In analyzing cell-cycle changes of: rapidly dividing blastomeres exposed to an oxia. we found that no cells arrested in mitosis. This is in sharp contrast to similarly staged normoxic embryos that consistently contain more than 1 5% of cells in mitosis. Flow cytometry analysis revealed that blastomeres a rrested during the S and G(2) phases of the cell cycle. This work indicates that survival of oxygen deprivation in vertebrates involves the reduction of diverse processes, such as cardiac function and cell-cycle progression, thus allowing energy supply to be matched by energy demands.