Glossopharyngeal Nerve Function In Vallate Taste Bud Formation (development, Trophic, Induction, Circumvallate, Rat).

Abstract

The neural requirements for circumvallate taste bud formation were characterized in the rat. Taste buds are first present at birth (0 days post-partum, 0d p.p.) and, by 5d p.p;, begin an exponential increase to produce maximal values of 610 vallate taste buds by 90d p.p. The formation of these taste buds requires neural induction during a sensitive period which extends through the first 10-20d p.p. Interruption of the glossopharyngeal (C.N. IX) nerve supply during this time leads to permanent and severe taste bud deficits. In these, and all other operations, the taste bud deficits are the result of a loss of the nerve supply and not adverse changes in papilla size, morphology, taste bud distribution or taste bud overcrowding within the papilla. When unilateral IXth nerve removal (avulsion) is performed at 3d p.p., only 228 taste buds form while after unilateral avulsion at 75d p.p., 496 taste buds are maintained. A unilateral nerve crush at 3d p.p., which permits nerve regeneration, results in 340 taste buds at 90d p.p., only partially reversing the effects of 3d p.p. unilateral avulsion. Combined right IXth nerve avulsion and left IXth nerve crush at 3d p.p. results in only 30 taste buds at 90d p.p., this is not significantly different from the 0 to 15 taste buds found after bilateral IXth nerve avulsions at 75d and 3d p.p., respectively. After combined avulsion and crush at 75d p.p., however, 405 taste buds regenerated in response to the return of a single IXth nerve; this is significantly more than with any of the surgical protocols performed at 3d p.p. Neural induction of taste bud precursors during the sensitive period, therefore, is needed if de novo taste bud formation is to occur. Once induced, however, the neurotrophic maintenance and regeneration of taste buds is more easily accomplished than is this initial induction.