INITIAL INNERVATION OF EMBRYONIC RAT TONGUE AND DEVELOPING TASTE PAPILLAE - NERVES FOLLOW DISTINCTIVE AND SPATIALLY RESTRICTED PATHWAYS

The rat tongue has an extensive, complex innervation from four cranial nerves. However, the precise developmental time course and spatial ro utes of these nerves into the embryonic tongue are not known, although this knowledge is crucial for studying mechanisms that regulate devel opment and innervation of the lingual taste organs, gustatory papillae and resident taste buds. We determined the initial spatial course of nerves in the developing tongue and papillae, and tested the hypothesi s that sensory nerves first innervate the tongue homogeneously and the n retract to more densely innervate papillae and taste buds. Antibodie s to GAP-43 and neurofilaments were used to label nerve fibers in rat embryo heads from gestational day 11 through 16 (E11-E16). Serial sagi ttal sections were traced and reconstructed to follow paths of each ne rve. In Ell rat, geniculate, trigeminal and petrosal ganglia were labe led and fibers left the ganglia and extended toward respective branchi al arches. At E13 when the developing tongue is still a set of tissue swellings, the combined chorda/lingual, hypoglossal and petrosal nerve s approached the lingual swellings from separate positions. Only the c horda/lingual entered the tongue base at this stage. At E14 and E15, t he well-developed tongue was innervated by all four cranial nerves. Ho wever, the nerves maintained distinctive entry points and relatively r estricted mesenchymal territories within the tongue, and did not follo w one another in common early pathways. Furthermore, the chorda/lingua l and glossopharyngeal nerves did not set up an obvious prepattern for gustatory papilla development, but rather seemed attracted to develop ing papillae which became very densely innervated compared to surround ing epithelium at E15. To effect this dense papilla innervation, senso ry nerves did not first innervate the tongue in a homogeneous manner w ith subsequent retraction and/or extensive redirection of fibers into the taste organs. Results contribute to a set of working principles fo r development of tongue innervation. Points of entry and initial neura l pathways are restricted from time of tongue formation through morpho genesis, suggesting distinctive lingual territories for each nerve. Th us, sensory and motor nerves distribute independently of each other, a nd sensory innervation to anterior and posterior tongue remains discre te. For taste organ innervation, gustatory papillae are not induced by a prepatterned nerve distribution. In fact, papillae might attract de nse sensory innervation because neither chorda/lingual nor glossophary ngeal nerve grows homogeneously to the lingual epithelium and then red istributes to individual papillae.