Fig. 8. Induction of OLPs in ventral spinal cord or ventral forebrain cultures is independent of FGFR-TK activity. (A) Ventral spinal cord explants from Hamilton and Hamburger stage 12 (E2) chicks developed O4-positive OLPs when cultured without exogenously added growth factors. Their development was strongly inhibited by cyclopamine but only weakly by PD173074. FGF2 in the absence of complementary Shh activity (i.e. in the presence of cyclopamine) was unable to induce O4-positive OLPs. (B-E) Dissociated cells from mouse E10.5 ventral forebrain (boxed region) were cultured in the presence of the cyclopamine or PD173074 for 10 days, then the cultures were fixed and immunolabelled with anti-NG2. Control cultures developed numerous NG2-positive cells (C). Their production was inhibited by cyclopamine (D) but not by PD173074 (E). (F) The experiment was quantified by counting NG2-positive cells in more than 10 randomly selected fields on each of two separate coverslips (x63 objective) and the results displayed as mean±s.d. Similar data were obtained in at least two independent experiments.