Investing the role of FGF signaling in left-right determination

Left-right patterning is set up early in development, at embryonic day 8.5 in the mouse. Cilia are present on the cells of a transient structure, termed the node, the location where L/R symmetry is broken. In this pit-like structure, motile cilia generate a leftwards flow of extracellular fluid, which is translated into a left-specific signal by immotile cilia on crown cells at the edge of the node. How exactly this flow is translated remains unknown, with one hypothesis proposing that nodal vesicular parcels (NVPs) within the node transport signalling molecules to the left side. FGF signalling was first implicated in L/R patterning when an Fgf8 mutant mouse was shown to exhibit situs defects. The precise role of FGFs in L/R patterning is unclear, but they have been proposed to regulate NVP release. Other hypotheses include them functioning in the regulation of node cilia length, and in the formation and maintenance of the node structure. These hypotheses are explored in this thesis, through the re-assessment of the Fgf8 mouse mutant and through the use of FGF signalling receptor inhibitors in embryo culture. My findings confirm a role for FGF signalling in L/R determination, and support a role for FGF signalling in specifying, maintaining and/or patterning the cells that make up the embryonic node. My data also casts doubt on the evidence previously used to argue for the existence of NVPs. Analysis of the location of FGFRs in the embryo and within cells shows the presence of FGFR2 in the node cilia, but whether ciliary localisation of FGFRs is important in L/R patterning remains unclear. I present evidence that the major mechanism by which FGF signalling affects L/R determination is through the formation/patterning of the node. However, I also present evidence for a lesser role FGFs may play in controlling mammalian cilia length. Intriguingly, my data also indicates a strong influence of genetic background on the Fgf8 mutants.