Stress-induced apoptosis by heat shock, UV and γ-ray irradiation in zebrafish embryos detected by increased caspase activity and whole-mount TUNEL staining

Environmental stress-induced apoptosis in zebrafish Danio rerio embryos was characterized by assaying caspase-3-like activity and whole-mount terminal deoxynucleotidyl nick-end labeling (TUNEL) staining. Severe stress conditions, such as heat shock at 39 degreesC for 1 h, ultraviolet light at 10-100 mJ/cm(2) and gamma -ray irradiation at 5-20 Gy induced extensive apoptosis in embryos. Apoptotic cells were observed after the bud and 1-somite stages in normal embryos by TUNEL staining, and after stress treatment many TUNEL-positive cells were found in the enveloping and deep cell layers and the larval fin. The caspase-3-like activity increased severalfold during stress-induced apoptosis in a dose-dependent manner. These findings indicate that apoptotic pathways, mediated by caspase-3-like activity, play a major role in zebrafish embryogenesis under stress conditions.