Palmitoylated Wnts bind Frizzleds, which are seven transmembrane proteins that serve as cognate Wnt receptors, with the help of the LRP5/6 co-receptor in certain cases ( figure 1 a). Present in metazoan species from cnidarians to primates, the Wnt family of secreted glycoproteins are well known for their diverse signalling functions. In this review, we focus on Wnts, an evolutionarily conserved family of morphogens that emerge as critical players in axon and dendrite development. Classical morphogens, such as Hedgehog (Hh), transforming growth factor β (TGF-β) and fibroblast growth factors (FGFs), also play important roles in guiding migrating neurons or axon growth cones that are distinct from their canonical functions in controlling cell fate. Morphogens are secreted proteins that specify cell fate depending on their concentration gradients. Representative examples include neurotrophic factors, axon guidance cues and cell adhesion molecules. Moreover, some molecules directly instruct the trajectory of axon and dendrites. These secreted or membrane-tethered molecules provide a permissive environment that allows neurites to develop and extend. In addition to genetic programmes governed by transcription factors that specify neuronal types and their wiring properties, extracellular signals are also instrumental in the construction of neural pathways. Proper functioning of the nervous system depends on highly accurate and specific connectivity of neuronal circuits.
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