Redifferentiation of Fungiform Papillae after Axonotmesis

Abstract

To elucidate the trophic function of neurons on redifferentiation of the denervated fungiform papillae, unilateral axonotmesis (nerve crush) of the chorda tympani-lingual nerve of the rat was performed at day 1 and weeks, 1, 2, 3, 4, 7 and 10 after birth by using watchmakers forceps. Specimens were obtained at weeks 6 and 8, and months 3 and 4 after nerve crush for examination with scanning electron microscope. As revealed by nerve transection experiment, process of morphological changes of the fungiform papilla consisted of degeneration and redifferentiation phase. Sequential alterations of the morphology of redifferentiating fungiform papillae were divided into 4 stages:(1) quasifiliform papillae indistinguishable from normal filiform papillae ;(2) filiform-like papillae similar to, but distinguishable from, the normal filiform papilla ;(3) undifferentiated fungiform papillae with small shape and no taste pore ;(4) redifferentiated fungiform papillae same as the normal fungiform papilla. Using these criteria, the number of each type of fungiform papillae was determined for a given sampling area, and compared with those of nerve transection experiment. Appearance of the redifferentiated fungiform papilla in axonotmesis (nerve crush) experiments was much earlier than in nerve transection experiments. All of fungiform papillae denervated at 1 day after birth changed to the quasi-filiform papilla and did not return to the normal fungiform papilla. Some of fungiform papillae denervated at weeks 1 and 2 after birth changed to the quasi-filiform papilla and remains changed to the filiform.-like papilla. According to extension of the period after nerve crush, redifferentiated fungiform papillae were increased and quasi-filiform papillae were decreased in number. Most fungiform papillae denervated at 3 or more weeks after birth changed to the filiform-like papilla and then returned to the normal fungiform papilla. These findings suggest that (1) axonotmesis is better than nerve transection in regard to regeneration of axon ;(2) there are two kinds of quasi-filiform papillae concerned with redifferentiation ability ;(3) ability to redifferentiate is determined by degree of maturation of the fungiform papilla at denervation and regeneration speed of the damaged axon.