Responses of epibranchial placodes to disruptions of FGF-signaling in embryonic mice

Washausen S, Knabe W

Abstract

Epibranchial placodes (EPs) are major contributors to the development of cranial sensory ganglia. In zebrafish, induction and neurogenesis of EPs depend on fibroblast growth factor (FGF)-signaling. In FGFR1 hypomorphic mice, neurogenesis is exclusively reduced in EP1. Here we investigate how neurogenesis of EPs is affected by three different other ways of disrupting FGF-signaling in mice. C57BL/6N embryos (E8.5/E9) were cultured for 24h in the presence of the FGFR inhibitors SU5402 and PD173074, respectively. Furthermore, blocking experiments with anti-FGFR3 antibodies were carried out. For negative controls, culture medium was applied with DMSO only. Serially sectioned embryos were qualitatively and quantitatively analysed using immunohistochemistry with antibodies against the neuronal differentiation markers neurogenin (Ngn) 1 and Ngn2. Application of SU5402 or PD173074 reduces the numbers of Ngn2+ (EP1>>EP2>EP3) and Ngn1+ neuroblasts (EP1, EP2), whereas the amount of Ngn1+ neuroblasts remains constant in EP3. Furthermore, exposure to PD173074 but not SU5402 results in increased Ngn1/Ngn2 ratios in EP1 and EP3. Different from SU5402 or PD173074 treatments, incubation with anti-FGFR3 antibodies causes decreases of Ngn1+ and Ngn2+ neuroblasts in all EPs. As another point of difference, the reduction rates of Ngn1+ and Ngn2+ neuroblasts are equal in the three EPs. Disrupting FGF-signaling with pharmacological inhibitors (SU5402, PD173074) or anti-FGFR3 antibodies differentially affects total neuroblast numbers and/or Ngn1/Ngn2 ratios in the different EPs of embryonic mice. Contrary to earlier views, FGFR3 appears to be active in EP1. Whether neurogenesis in the different EPs is regulated by different subsets of FGFRs and/or by FGF-independent pathways is discussed.