Fig. 2

Effects of UV exposure on zebrafish embryos. (A-D) The three classes of embryos generated by UV exposure of zygotes (A-C) and an untreated control embryo (D) at 12 hours of development. In the type-A embryo (A) the blastoderm has covered the yolk cell up to 80% and shows a well-formed anterior axis rudiment on the dorsal side. In type-B embryos (B) the blastoderm rounded up forming a vesicle on top of the yolk mass. Type-C embryos (C) show retardation of epiboly but do not form an axis rudiment. (E,F) UV-treated (E) and control embryo (F) at 8 hours of development. UV treatment results in strong retardation of epiboly. Control embryos (F) are at the 80% epiboly stage, whereas UV-exposed embryos (E) have just covered the yolk sphere by 40% at 8 hours after fertilisation. (G-K) UV-irradiated embryos (H-K) and a control embryo (G) at 24 hours of development. UV-treated embryos at this stage show various degrees of posterior defects ranging from subtle tail defects (H,I) to complete failure to finish epiboly (J,K) leaving the yolk cell uncovered by the blastoderm at the vegetal pole. The arrowhead in J indicates the open blastopore. Embryos were exposed to 1.8 mJ/cm² at the late zygote stage. Blastoderm margins are indicated by black or white dots. Embryos are oriented anterior up and dorsal to the right with the exception of panel G where anterior is to the left and dorsal up.