Fig. 2. FGF8b is a robust mesoderm inducer. (A) Diagram of explant assay. (B) RT-PCR analysis of explants injected as indicated at the one-cell stage, excised at stage 9 and cultured to stage 11; EF1 was used as a loading control. Lane 1, whole embryo (WE) positive control; lane 2 negative control minus reverse transcriptase (RT-); lane 3, uninjected explant lane. FGF8b and FGF8f robustly induce xbra expression (lanes 6-10). (C-H) Animal caps injected as indicated and cultured to stage 19. (I-T) Overexpression of Xenopus FGF8b expands xbra in whole embryos. Embryos were injected with mRNA, as indicated, into the marginal zone of one cell at the two-cell stage and cultured until stage 10.5. Embryos in the top row are shown from blastoporal views; the bottom row of images show the same embryos as the respective one above but from a lateral view with the blastopore down. ß-galactosidase mRNA was injected as a lineage tracer and detected using Red-Gal substrate. (I,J) Control uninjected embryos. Neither (K,L) XlFGF8a (15/15 embryos) or (M,N) MmFGF8a (14/15 embryos) affects xbra expression; (O,P) XlFGF8b (15/15 embryos), (Q,R) HsFGF8b (20/20) and (S,T) MmFGF8f (18/18 embryos) robustly expand the xbra expression domain in a non-cell-autonomous manner. (U-FF) FGF8a and FGF8b have separable activities. Embryos were injected as indicated into one cell at the two-cell stage and processed by in situ hybridization for expression of myoD (top row) or neuronal ß-tubulin (ntub) (bottom row) at stage 20. All are dorsal views with anterior towards the left. Effects on mesoderm and production of ectopic neurons is scored below the images; - indicates no effect. Overexpression of XlFGF8a and MmFGF8a results in massive ectopic ntub expression without affecting mesodermal development (W-Z). A minimum of eight embryos were examined and they showed consistent phenotypes for each injection.