Figure 6.

Model of apoptosis regulation in Xenopus embryos. Ionizing radiation promotes apoptosis in embryos irradiated before the MBT by triggering different proapoptotic factors, including the homodimerization of xBax, and caspase cascade activation. Apoptosis is prevented in embryos irradiated after the MBT by the coordinated action of antiapoptotic stimuli. Survival signaling pathways act at at least three levels: by promotion of cell cycle delay by increasing the amount of p27^Xic1 and its association with the complexes involved in the G₁/S transition, by inactivation of proapoptotic molecules by heterodimerization, and by activation of antiapoptotic pathways like Akt.