Fig. 3. Dose-dependent induction of NKX2.1 and albumin by FGF2. Ventral endoderm (VE) explants were initiated with a single dose of FGF2 and cultured for 24 or 48 hours; similar results were obtained from 1 or 2 days of exposure. (A-H) Immunostaining for NKX2.1 and albumin in serial sections (A,B) or the same explant (C-H). Bright signal indicates positive staining. (I) RT-PCR analysis of RNA from cultured ventral endoderm with heparin sulfate (50 ng/ml) and the designated concentration of FGF2. Control explants (embryonic neural tissue, midsection, tail) failed to express albumin or NKX2.1 in the presence of FGF2, at all doses. (J) Number of explants (positive signal/total tested), at indicated concentration of FGF2, which expressed NKX2.1 or albumin protein (immunostaining, I) or mRNA (RT-PCR, R).