Fig. 7. FGFs act downstream of or in parallel to han. Labelling is as in Fig. 1. Grafting of FGF-soaked beads significantly increases the number of han^s6 mutant embryos with detectable slc5a5 expression. (A) Example of a han^s6 mutant implanted with an FGF1-soaked bead. Implantation of FGF-soaked beads significantly increased the probability that han^s6 mutants expressed detectable thyroid markers (arrow). Counterstaining against MF20 enables the identification of han^s6 mutant homozygotes. (B) Diagram showing the complete data set. (C) Schematic drawing, summarising scenarios of how FGF signalling and Hand2 might influence thyroid development. han-expressing cells (blue) influence early thyroid development either directly (I,II) or indirectly (III). (I) Hand2-expressing tissue signals directly to the thyroid precursor cells (green) via the FGF pathway (arrow) to promote the development of the precursor cells. (II) Hand2-expressing tissue has a direct influence on thyroid development (not via FGF signalling), with neighbouring tissue providing additional FGF signals. In this scenario, increased levels of FGFs can compensate for the missing influence of Hand2-expressing cells. (III) In the case of indirect influence of Hand2-expressing tissue on thyroid development, FGF signals can be involved at different levels (i.e. on the level of the upper or the lower arrow, or on the level of both arrows). Green cells: thyroid precursor cells; blue cells: Hand2-expressing cells; red arrows: potential contribution of FGF signals. b, bead; h, heart.