- Email: [email protected]
Morphological changes of taste buds and fungiform papillae following long-term neurectomy
Brain Research, 533 (1990) 321-323 Elsevier
321
BRES 24384
Morphological changes of taste buds and fungiform papillae following long-term neurectomy Takeshi Nakashima 1, Kuniaki Toyoshima 2, Akitatsu Shimamura 2 and Nagayoshi Yamada 1 Departments of 1Oral Surgery and 2Oral Anatomy, Kyushu Dental College, Kokurakita-ku, Kitakyushu (Japan) (Accepted 7 August 1990) Key words: Fungiform papilla; Taste bud; Neurectomy; Chorda tympani-lingual nerve; Rabbit
Long-term neurectomy of chorda tympani-lingual nerves results in a complete disappearance of taste buds from rabbit fungiform papillae. This supports the view that taste buds of mammalian fungiform papillae are neurally dependent. Furthermore, the covering epithelium of denervated fungiform papillae develops a characteristic keratinization pattern corresponding to that of filiform papillae. It is known that fungiform papillae are doubly innervated by sensory endings of chorda tympani and lingual nerve fibers 7-9A1. The fibers of the chorda tympani innervate taste buds and contribute gustatory sensation 4. The lingual nerve endings, on thi~ other hand, innervate both the epithelium outside the taste buds and subepithelial sensory corpuscles, and subserve somatosensation11. It has long been believed that the total denervation of the gustatory papillae results in a disappearance of taste buds along with an atrophy of the papillae 1'2'1°. Insofar as fungiform papillae are concerned, however, there still remains a research question whether taste buds in these papillae can survive without sensory innervation. Recently, Whitehead et al. 9 concluded, in their denervation study of the hamster fungiform papillae, that there was no absolute neural requirement for taste bud survival, although taste bud structure was dependent on innervation. This report is in contrast to the result by Ganchrow and Ganchrow ] that taste buds in rat fungiform papillae are critically neurally dependent. In view of this discrepancy, we have further examined the morphological effects on the denervation of fungiform papillae in rabbits. So far as we know, no denervation study of rabbit fungiform papillae has yet been carried out. Rabbits were anesthetized by intravenous injection of sodium pentobarbital (40 mg/kg), and the combined chorda tympani-lingual nerves were severed unilaterally. The proximal and distal cut ends were turned backward along the course of the stumps. After survival periods of 24 h to 18 months, the animals were anesthetized with an
overdose of sodium pentobarbital, and then perfused transcardially with half-strength Karnovsky 3 fixative buffered with phosphate. Anterior fungiform papillae on the denervated side were dissected free and postfixed for 2 h in phosphate-buffered 1% osmium tetroxide. The specimens were then dehydrated in ascending concentration of ethanol, passed through propylene oxide and embedded in epoxy resin. Serial semithin sections were stained with toluidine blue for light microscopic examinations. The fungiform papillae of the rabbit are numerous near the tip of the tongue and show a flattened hemispherical upper part. Three to seven taste buds are located in the nonkeratinized covering epithelium of their superior surface (Fig. 1A,B). Detailed morphology of taste buds in rabbit fungiform papillae was described previously 6. In contrast to the finding 9, taste buds in rabbit fungiform papillae disappeared by the 10th day after denervation. The disappearance of taste buds was confirmed by completely serial semithin sections of the papillae. Of interest was the fact that the dorsal surface of the fungiform papilla began to display filiform-like structures 15 weeks after denervation (Figs. 2A,B and 3A,B). The covering epithelium showed morphological changes indicative of modification of the pattern of keratinization of filiform papillae. The keratinized epithelium tapered into a pointed end which was curved backward with the concave side facing the root of the tongue. This epithelial peak was always associated with the connective tissue papilla. When observed by scanning electron microscopy, remarkable changes were seen on the dorsal surface of
Correspondence: T. Nakashima, First Department of Oral Surgery, Kyushu Dental College, Kokurakita-ku, Kitakyushu 803, Japan.
322 the tongue since 15 weeks after denervation, that is,
tongue showed a repeating pattern of keratinization with
fungiform papilla could hardly be found on the dorsal surface, even on the tongue tips where the greatest
epithelial peaks corresponding to that of filiform papillae (Figs. 2A,B and 3A,B).
accumulation of fungiform papillae was seen. All of the lingual papillae locating in the anterior portion of the
The present study has revealed that the taste buds in rabbit fungiform papillae are neurally dependent. This is
Fig. 1. A: surface view of the anterior part of the tongue. A fungiform papilla is seen among filiform papillae. B: light micrograph showing a control fungiform papilla. A taste bud is seen in the dorsal epithelium. Bars = 100/~m. Fig. 2. A: lingual surface of experimental side 15 weeks after combined chorda tympani and lingual nerve resection. Fungiform papillae are difficult to find because of unusual keratinization. B: dorsal epithelium of fungiform papilla 15 weeks after neurectomy shows a filiform-like keratinization pattern with curved epithelial peak. Bars = 100/*m. Fig. 3. A: lingual surface of experimental side 18 months after neurectomy. No fungiform papilla can be found in the lingual surface. Bar = 400/~m. B: morphology of fungiform papilla (arrow) 18 months after neurectomy closely resembles that of adjacent filiform papillae. Bar = 100 ~m.
323 in accord with the result by Ganchrow and Ganchrow 1. A n o t h e r interesting point in the present study is the morphological changes which appeared in the covering epithelium of denervated fungiform papillae. These changes are characterized by unusual keratinization patterns which are reminiscent to those of filiform papillae. Similar findings were also described briefly in long-term denervated fungiform papillae of rats 1, in which dorsal surface of fungiform papillae was unusually keratinized and bore a filiform-like slight peak. Although nothing can as yet be said, a clue to understand these epithelial changes may be gained from a series of works by Mackenzie's group 5. They examined heterotypic recombination of epithelium and connective tissue from various sources and described that the epithelium acquired features of spatial organization or keratinization characteristic of the epithelium normally associated with the connective tissue component of the
recombinant. When buccal epithelium was recombined with tongue connective tissue, the epithelium developed a repeating unit pattern of organization with keratinized epithelial peaks corresponding to the normal distribution of tongue filiform papillae. Same was true in palatal epithelium recombined with tongue connective tissue. Thus, the possibility can be considered that the tongue connective tissue potentially has a t r o p h i c factor to develop and maintain filiform papillae as well as their characteristic keratinization pattern. It is not unreasonable to speculate that this potential ability of the tongue connective tissue may be regulated by sensory nerves or taste buds, or both. Denervation and following disappearance of taste buds may promote morphological changes of a keratinization pattern of fungiform papillae into filiform-like structures. Further studies into this problem present an intriguing field for research.
1 Ganchrow, J.R. and Ganchrow, D., Long-term effects of gustatory neurectomy on fungiform papillae in the young rat, Anat, Rec., 225 (1989) 224-231. 2 Guth, L., Histological changes following partial denervation of the circumvailate papilla of the rat, Exp. Neurol., 8 (1963) 336-349. 3 Karnovsky, M.J., A., formaldehyde-glutaraldehyde fixative of high osmolarity for use in electron microscopy, J. Cell Biol., 27 (1965) 137A. 4 Kimura, K. and Beidler, L.M., Microelectrode study of taste receptors of rat and hamster., J. Cell Comp. Physiol., 58 (1961) 131-139. 5 Mackenzie, I.C., Epithelial-connective tissue relationships and the development and maintenance of structure. In J. Meyer, C.A. Squier and S.J. Gerson (Eds.), The Structure and Function of Oral Mucosa, Pergamon, Oxford, 1984, pp. 119-139. 6 Murray, R.G., The ultrastructure of taste buds. In I. Friedmann (Ed.), The Ultrastructure of Sensory Organs, North-Holland/ American Elsevier, Amsterdam/New York, 1973, pp. 1-81.
7 Nishimoto, T., Akai, M., Inagaki, S., Shiokawa, S., Shimizu, Y., Yamamoto, K., Senba, E., Sakanaka, M., Takatsuki, K., Hara, Y., Takagi, H., Matsuzaki, T., Kawai, Y. and Tohyama, M., On the distribution and origins of substance P in the papillae of the rat tongue: an experimental and immunohistochemical study, Anat. Rec., 207 (1982) 85-92. 8 Whitehead, M.C., Beeman, C.S. and Kinsella, B.A., Distribution of taste and general sensory nerve endings in fungiform papillae of the hamster, Am. J. Anat., 173 (1985) 85-201. 9 Whitehead, M.C., Frank, M.E., Hettinger, T.P., Hou, L.-T. and Nah, H.-D., Persistence of taste buds in denervated fungiform papillae, Brain Research, 405 (1987) 192-195. 10 Zalewski, A.A., Changes in phosphatase enzymes following denervation of the circumvallate papilla of the rat, Exp. Neurol., 22 (1968) 40-51. 11 Zahm, D.S. and Munger, B.L., The innervation of the primate fungiform papilla - Development, distribution and changes following selective ablation, Brain Res. Rev., 9 (1985) 147-186.
321
BRES 24384
Morphological changes of taste buds and fungiform papillae following long-term neurectomy Takeshi Nakashima 1, Kuniaki Toyoshima 2, Akitatsu Shimamura 2 and Nagayoshi Yamada 1 Departments of 1Oral Surgery and 2Oral Anatomy, Kyushu Dental College, Kokurakita-ku, Kitakyushu (Japan) (Accepted 7 August 1990) Key words: Fungiform papilla; Taste bud; Neurectomy; Chorda tympani-lingual nerve; Rabbit
Long-term neurectomy of chorda tympani-lingual nerves results in a complete disappearance of taste buds from rabbit fungiform papillae. This supports the view that taste buds of mammalian fungiform papillae are neurally dependent. Furthermore, the covering epithelium of denervated fungiform papillae develops a characteristic keratinization pattern corresponding to that of filiform papillae. It is known that fungiform papillae are doubly innervated by sensory endings of chorda tympani and lingual nerve fibers 7-9A1. The fibers of the chorda tympani innervate taste buds and contribute gustatory sensation 4. The lingual nerve endings, on thi~ other hand, innervate both the epithelium outside the taste buds and subepithelial sensory corpuscles, and subserve somatosensation11. It has long been believed that the total denervation of the gustatory papillae results in a disappearance of taste buds along with an atrophy of the papillae 1'2'1°. Insofar as fungiform papillae are concerned, however, there still remains a research question whether taste buds in these papillae can survive without sensory innervation. Recently, Whitehead et al. 9 concluded, in their denervation study of the hamster fungiform papillae, that there was no absolute neural requirement for taste bud survival, although taste bud structure was dependent on innervation. This report is in contrast to the result by Ganchrow and Ganchrow ] that taste buds in rat fungiform papillae are critically neurally dependent. In view of this discrepancy, we have further examined the morphological effects on the denervation of fungiform papillae in rabbits. So far as we know, no denervation study of rabbit fungiform papillae has yet been carried out. Rabbits were anesthetized by intravenous injection of sodium pentobarbital (40 mg/kg), and the combined chorda tympani-lingual nerves were severed unilaterally. The proximal and distal cut ends were turned backward along the course of the stumps. After survival periods of 24 h to 18 months, the animals were anesthetized with an
overdose of sodium pentobarbital, and then perfused transcardially with half-strength Karnovsky 3 fixative buffered with phosphate. Anterior fungiform papillae on the denervated side were dissected free and postfixed for 2 h in phosphate-buffered 1% osmium tetroxide. The specimens were then dehydrated in ascending concentration of ethanol, passed through propylene oxide and embedded in epoxy resin. Serial semithin sections were stained with toluidine blue for light microscopic examinations. The fungiform papillae of the rabbit are numerous near the tip of the tongue and show a flattened hemispherical upper part. Three to seven taste buds are located in the nonkeratinized covering epithelium of their superior surface (Fig. 1A,B). Detailed morphology of taste buds in rabbit fungiform papillae was described previously 6. In contrast to the finding 9, taste buds in rabbit fungiform papillae disappeared by the 10th day after denervation. The disappearance of taste buds was confirmed by completely serial semithin sections of the papillae. Of interest was the fact that the dorsal surface of the fungiform papilla began to display filiform-like structures 15 weeks after denervation (Figs. 2A,B and 3A,B). The covering epithelium showed morphological changes indicative of modification of the pattern of keratinization of filiform papillae. The keratinized epithelium tapered into a pointed end which was curved backward with the concave side facing the root of the tongue. This epithelial peak was always associated with the connective tissue papilla. When observed by scanning electron microscopy, remarkable changes were seen on the dorsal surface of
Correspondence: T. Nakashima, First Department of Oral Surgery, Kyushu Dental College, Kokurakita-ku, Kitakyushu 803, Japan.
322 the tongue since 15 weeks after denervation, that is,
tongue showed a repeating pattern of keratinization with
fungiform papilla could hardly be found on the dorsal surface, even on the tongue tips where the greatest
epithelial peaks corresponding to that of filiform papillae (Figs. 2A,B and 3A,B).
accumulation of fungiform papillae was seen. All of the lingual papillae locating in the anterior portion of the
The present study has revealed that the taste buds in rabbit fungiform papillae are neurally dependent. This is
Fig. 1. A: surface view of the anterior part of the tongue. A fungiform papilla is seen among filiform papillae. B: light micrograph showing a control fungiform papilla. A taste bud is seen in the dorsal epithelium. Bars = 100/~m. Fig. 2. A: lingual surface of experimental side 15 weeks after combined chorda tympani and lingual nerve resection. Fungiform papillae are difficult to find because of unusual keratinization. B: dorsal epithelium of fungiform papilla 15 weeks after neurectomy shows a filiform-like keratinization pattern with curved epithelial peak. Bars = 100/*m. Fig. 3. A: lingual surface of experimental side 18 months after neurectomy. No fungiform papilla can be found in the lingual surface. Bar = 400/~m. B: morphology of fungiform papilla (arrow) 18 months after neurectomy closely resembles that of adjacent filiform papillae. Bar = 100 ~m.
323 in accord with the result by Ganchrow and Ganchrow 1. A n o t h e r interesting point in the present study is the morphological changes which appeared in the covering epithelium of denervated fungiform papillae. These changes are characterized by unusual keratinization patterns which are reminiscent to those of filiform papillae. Similar findings were also described briefly in long-term denervated fungiform papillae of rats 1, in which dorsal surface of fungiform papillae was unusually keratinized and bore a filiform-like slight peak. Although nothing can as yet be said, a clue to understand these epithelial changes may be gained from a series of works by Mackenzie's group 5. They examined heterotypic recombination of epithelium and connective tissue from various sources and described that the epithelium acquired features of spatial organization or keratinization characteristic of the epithelium normally associated with the connective tissue component of the
recombinant. When buccal epithelium was recombined with tongue connective tissue, the epithelium developed a repeating unit pattern of organization with keratinized epithelial peaks corresponding to the normal distribution of tongue filiform papillae. Same was true in palatal epithelium recombined with tongue connective tissue. Thus, the possibility can be considered that the tongue connective tissue potentially has a t r o p h i c factor to develop and maintain filiform papillae as well as their characteristic keratinization pattern. It is not unreasonable to speculate that this potential ability of the tongue connective tissue may be regulated by sensory nerves or taste buds, or both. Denervation and following disappearance of taste buds may promote morphological changes of a keratinization pattern of fungiform papillae into filiform-like structures. Further studies into this problem present an intriguing field for research.
1 Ganchrow, J.R. and Ganchrow, D., Long-term effects of gustatory neurectomy on fungiform papillae in the young rat, Anat, Rec., 225 (1989) 224-231. 2 Guth, L., Histological changes following partial denervation of the circumvailate papilla of the rat, Exp. Neurol., 8 (1963) 336-349. 3 Karnovsky, M.J., A., formaldehyde-glutaraldehyde fixative of high osmolarity for use in electron microscopy, J. Cell Biol., 27 (1965) 137A. 4 Kimura, K. and Beidler, L.M., Microelectrode study of taste receptors of rat and hamster., J. Cell Comp. Physiol., 58 (1961) 131-139. 5 Mackenzie, I.C., Epithelial-connective tissue relationships and the development and maintenance of structure. In J. Meyer, C.A. Squier and S.J. Gerson (Eds.), The Structure and Function of Oral Mucosa, Pergamon, Oxford, 1984, pp. 119-139. 6 Murray, R.G., The ultrastructure of taste buds. In I. Friedmann (Ed.), The Ultrastructure of Sensory Organs, North-Holland/ American Elsevier, Amsterdam/New York, 1973, pp. 1-81.
7 Nishimoto, T., Akai, M., Inagaki, S., Shiokawa, S., Shimizu, Y., Yamamoto, K., Senba, E., Sakanaka, M., Takatsuki, K., Hara, Y., Takagi, H., Matsuzaki, T., Kawai, Y. and Tohyama, M., On the distribution and origins of substance P in the papillae of the rat tongue: an experimental and immunohistochemical study, Anat. Rec., 207 (1982) 85-92. 8 Whitehead, M.C., Beeman, C.S. and Kinsella, B.A., Distribution of taste and general sensory nerve endings in fungiform papillae of the hamster, Am. J. Anat., 173 (1985) 85-201. 9 Whitehead, M.C., Frank, M.E., Hettinger, T.P., Hou, L.-T. and Nah, H.-D., Persistence of taste buds in denervated fungiform papillae, Brain Research, 405 (1987) 192-195. 10 Zalewski, A.A., Changes in phosphatase enzymes following denervation of the circumvallate papilla of the rat, Exp. Neurol., 22 (1968) 40-51. 11 Zahm, D.S. and Munger, B.L., The innervation of the primate fungiform papilla - Development, distribution and changes following selective ablation, Brain Res. Rev., 9 (1985) 147-186.