Fig. 2. Whole-mount TUNEL staining of normal and hypomethylated Xenopus embryos. Normal and methylation-deficient embryos were assayed by TUNEL for the appearance of apoptotic cells. Late blastula (A, E and I), gastrula (B, F and J), neurula (C, G and K) and tailbud (D, H and L) were assayed in WT (A–D), MD (E–H) and ZD (I–L) embryos. An MD gastrula embryo with a high number of TUNEL-positive cells, corresponding to +++ in Table I, is shown in (F). A ZD gastrula with a few TUNEL-positive cells, + in Table I, is shown in (J). A ZD neurula with the highest levels of TUNEL-positive cells, ++++, is shown in (K). Embryos in (A), (B), (C), (D) and (I) are TUNEL negative. Note that the positive staining (purple) is subsequent to, or coincident with loss of DNA methylation (Figure 1F) in MD and ZD embryos, respectively. Abbreviations: An, animal pole; bp, blastopore; DM, dorsal mesoderm; VM, ventral mesoderm; PHE, presumptive head ectoderm; NF, neural fold; NP, neural plate; FB, forebrain; NT, neural tube; Ant., anterior; and Post., posterior.