Branching of Spiral Ganglion Neurites Is Induced by Focal Application of Fibroblast Growth Factor‐1

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Abstract
Objectives/Hypothesis During the terminal innervation of the developing organ of Corti, fibroblast growth factor‐1 (FGF‐1) messenger RNA has been shown to be transiently expressed in the sensory epithelium of the rat, suggesting that this growth factor may contribute to developmental processes such as innervation and synaptogenesis of the inner and outer hair cells. In a previous study it has been demonstrated that exogenous FGF‐1 supports rat spiral ganglion neurite extension in vitro, whereas a secreted form of FGF‐1 produced by transfected fibrocytes induces neurite branching and targeting.Study Design Response of spiral ganglion neurites to FGF‐1–coupled beads was compared with the response to noncoupled control beads.Methods Effects of multiple focal sources of FGF‐1 to outgrowing spiral ganglion neurites were investigated on explants from postnatal day 4 rat spiral ganglion samples that were cultured in the presence of FGF‐1 covalently coupled to polybead microspheres. After fixation and immunocytochemical labeling of the explants the growth patterns of the extending neuronal processes were evaluated.Results When spiral ganglion neurites were observed near clusters of FGF‐1 beads, they formed a plexus‐like network characterized by significantly higher branching in the vicinity of the beads. However, fibers did not appear to terminate on the beads. Plexus‐like formations were not seen at a distance from FGF‐1 coupled beads or in the vicinity of control beads lacking FGF‐1 on their surface.Conclusion The results of the study indicate that spiral ganglion neurites branch in response to focal sources of FGF‐1, suggesting an important role of this growth factor for hair cell innervation during the terminal development of the sensory epithelium.