In Gallus gallus four FGFRs FGFR have been identified along

In Gallus gallus, four FGFRs (FGFR1-4) have been identified, along with 13 FGFs (FGF1–4, 8–10, 12, 13, 16, 18–20) (Thisse and Thisse, 2005). FGFRs are tyrosine kinase receptors that contain an extracellular ligand binding domain, a transmembrane domain and an intracellular tyrosine kinase domain. The extracellular domain is composed of three immunoglobulin-like domains (I, II and III). FGFRs1-3 can undergo alternative splicing at its carboxy-terminal in the immunoglobulin-like domain III, forming either ‘b’ or ‘c’ isoforms (Miki et al., 1992; Eswarakumar et al., 2005). These isoforms are often specific to either the epithelium (b) or the mesenchyme (c; reviewed in Ornitz and Itoh, 2001). The binding of FGF along with their cofactors heparan sulfate proteoglycans (HSPG) (Mohammadi et al., 2005) results in dimerization and auto-transphosphorylation of the receptor and activates further downstream intracellular signalling pathways (Itho and Ornitz, 2004; Sarabipour and Hristova, 2016).
Materials and methods
Results
Discussion The development of the conjunctival papillae share several similarities with the development of the feather placodes (Jourdeuil and Franz-Odendaal, 2017). The FGF pathway is required during feather placode formation (Mandler and Neubüser, 2004; Lin et al., 2006) and other morphogens such as BMP and HH pattern the feather bud and inter-bud regions. Furthermore, one of the first bromfenac expressed during feather development is β-catenin (Noramaly et al., 1999; Lin et al., 2006) similar to the conjunctival papillae (Jourdeuil and Franz-Odendaal, 2016). β-catenin is expressed in the epithelium prior to papillae formation and may be required for pre-patterning the epithelium, similar to other non-neurogenic placodes. Here we show that the FGFRs are expressed prior to and during conjunctival papillae development (phase 1). Thus the FGF pathway may be required for conjunctival papillae induction working together with β-catenin in a similar manner to feather formation. FGFRs 1 and 2 are expressed in mesenchymal skeletal condensations during intramembranous bone formation (Delezoide et al., 1998). Murray (1943) described the presence of condensations in the scleral mesenchyme at mid-papillae stage in the form of a “thick-disc”, a shape similar to that observed expressing FGFR1c in the mesenchyme at HH32 (Fig. 3C’). Furthermore, here we show that FGFR1 may also be involved in pre-osteoblast differentiation during scleral ossicle development and that FGFR2 may be required for the proliferation of osteoprogenitors as in other intramembranous bones (Iseki et al., 1999; Long, 2012). Indeed FGFR2 mutations in mice affects the shape of neural crest derived craniofacial bones (Heuzé et al., 2014). Thus, FGFRs may play an important role in skeletogenic condensation formation ultimately leading to the formation of the scleral ossicles. Transmembrane bound FGFR isoforms are usually present in a tissue specific manner, where the ‘b’ isoform is usually expressed in the epithelium and the ‘c’ isoform is expressed in the mesenchyme. However, studies in the eye, prostate and intestine have shown that the ‘c’ isoform can be found within epithelial cells (Wilson et al., 1994; Nickel, 1999; Alizadeh et al., 2003; Vidrich et al., 2004). Nishita et al. (2011) showed that this tissue specificity is not present during early developmental stages. Similarly, in the scleral ossicle system, the isoform tissue specificity does not appear to be very stringent. Previous studies in the chick scleral ossicle system have identified the expression of several genes within or surrounding the papillae (e.g. BMP2, SHH, IHH, β-catenin, VEGFa etc.) that are also expressed in the mesenchyme (Franz-Odendaal, 2008; Duench and Franz-Odendaal, 2012; Jabalee and Franz-Odendaal, 2015; Jourdeuil and Franz-Odendaal, 2016). The HH and BMP families have also been shown to interact during scleral ossicle development and are required for and during its development (Franz-Odendaal, 2008; Duench and Franz-Odendaal, 2012). FGFs interact with the BMP pathway (e.g. Noramly and Morgan, 1998; Yoon et al., 2006; Yanai et al., 2008; Guo and Wang, 2009) as well as with the HH pathway (e.g. Laufer et al., 1994; Crossley et al., 1996; Scherz et al., 2004; Tanaka et al., 2005) and it is therefore possible that FGFs are interacting with BMP and HH in the scleral ossicle system. Further research is required to elucidate these interactions.