Refinement of innervation accuracy following initial targeting of peripheral gustatory fibers.

During development, axons of the chorda tympani nerve navigate to fungiform papillae where they penetrate the lingual epithelium, forming a neural bud. It is not known whether or not all chorda tympani axons initially innervate fungiform papillae correctly or if mistakes are made. Using a novel approach, we quantified the accuracy with which gustatory fibers successfully innervate fungiform papillae. Immediately following initial targeting (E14.5), innervation was found to be incredibly accurate: specifically, 94% of the fungiform papillae on the tongue are innervated. A mean of five papillae per tongue were uninnervated at E14.5, and the lingual tongue surface was innervated in 17 places that lack fungiform papillae. To determine if these initial errors in papillae innervation were later refined, innervation accuracy was quantified at E16.5 and E18.5. By E16.5 only two papillae per tongue remained uninnervated. Innervation to inappropriate regions was also removed, but not until later, between E16.5 and E18.5 of development. Therefore, even though gustatory fibers initially innervate fungiform papillae accurately, some errors in targeting do occur that are then refined during later embryonic periods. It is likely that trophic interactions between gustatory neurons and developing taste epithelium allow appropriate connections to be maintained and inappropriate ones to be eliminated.