Taste Bud Regeneration Research Articles

Research progress in functional regeneration methods and mechanisms of taste buds

Gustation is one of the most important human senses. Taste dysfunctions, which may be due to aging, tongue cancer surgery, radiotherapy and chemotherapy, affect life quality. That is why the need for taste bud regeneration has received more attention. At present, research on development and renewal of taste cells provides a basis for taste bud regeneration; molecular mechanisms related to taste bud regeneration are being continuously uncoverd, aiding in the identification of more accurate targets for therapy. New methods such as nerve regeneration, tissue engineering, and cytokine therapy have emerged. The author reviews the mechanism and the latest methods of taste bud regeneration of lingual epithelium, aiming to open new horizions for the prevention and treatment of gustatory diseases, and provide theoretical references for its regeneration.

Regeneration of taste through sonic hedgehog upregulation by photobiomodulation.

This study investigated photobiomodulation (PBM) effects in Sonic hedgehog (Shh) signaling as a potential approach to taste preservation and regeneration. Primary taste cell (TC) cultures were treated with Shh antagonist vismodegib and irradiated using a continuous wave type 630 nm light-emitting diode (10 mW/cm2 ) array, with single or multiple doses of 30 J/cm2 to determine dose inducing significant upregulation effect. Shh, Ptch, Smo, and Gli1 were significantly upregulated at 120 J/cm2 , used as the minimum dose in vivo. Vismodegib was administered via daily oral gavage for 21 days (30 mg/kg) to induce Shh inhibition in the tongue of rat animal models resulting in taste bud damage and taste dysfunction. PBM treatment using a 630 nm laser (3 W/cm2 ) at a radiant exposure of 120 J/cm2 (24 J/cm2 × 5) successfully upregulated the Shh protein expression, regenerated taste buds, and recovered taste function.

Interleukin (IL)-1 Receptor Signaling Is Required for Complete Taste Bud Regeneration and the Recovery of Neural Taste Responses following Axotomy.

Experimental or traumatic nerve injury causes the degeneration of associated taste buds. Unlike most sensory systems, the sectioned nerve and associated taste buds can then regenerate, restoring neural responses to tastants. It was previously unknown whether injury-induced immune factors mediate this process. The proinflammatory cytokines, interleukin (IL)-1α and IL-1β, and their requisite receptor are strongly expressed by anterior taste buds innervated by the chorda tympani nerve. We tested taste bud regeneration and functional recovery in mice lacking the IL-1 receptor. After axotomy, the chorda tympani nerve regenerated but was initially unresponsive to tastants in both WT and Il1r KO mice. In the absence of Il1r signaling, however, neural taste responses remained minimal even >8 weeks after injury in both male and female mice, whereas normal taste function recovered by 3 weeks in WT mice. Failed recovery was because of a 57.8% decrease in regenerated taste buds in Il1r KO compared with WT axotomized mice. Il1a gene expression was chronically dysregulated, and the subset of regenerated taste buds were reinnervated more slowly and never reached full volume as progenitor cell proliferation lagged in KO mice. Il1r signaling is thus required for complete taste bud regeneration and the recovery of normal taste transmission, likely by impairing taste progenitor cell proliferation. This is the first identification of a cytokine response that promotes taste recovery. The remarkable plasticity of the taste system makes it ideal for identifying injury-induced mechanisms mediating successful regeneration and recovery.SIGNIFICANCE STATEMENT Taste plays a critical role in nutrition and quality of life. The adult taste system is highly plastic and able to regenerate following the disappearance of most taste buds after experimental nerve injury. Several growth factors needed for taste bud regeneration have been identified, but we demonstrate the first cytokine pathway required for the recovery of taste function. In the absence of IL-1 cytokine signaling, taste bud regeneration is incomplete, preventing the transmission of taste activity to the brain. These results open a new direction in revealing injury-specific mechanisms that could be harnessed to promote the recovery of taste perception after trauma or disease.

Epithelial plasticity enhances regeneration of committed taste receptor cells following nerve injury

Taste receptor cells are taste bud epithelial cells that are dependent upon the innervating nerve for continuous renewal and are maintained by resident tissue stem/progenitor cells. Transection of the innervating nerve causes degeneration of taste buds and taste receptor cells. However, a subset of the taste receptor cells is maintained without nerve contact after glossopharyngeal nerve transection in the circumvallate papilla in adult mice. Here, we revealed that injury caused by glossopharyngeal nerve transection triggers the remaining differentiated K8-positive taste receptor cells to dedifferentiate and acquire transient progenitor cell-like states during regeneration. Dedifferentiated taste receptor cells proliferate, express progenitor cell markers (K14, Sox2, PCNA) and form organoids in vitro. These data indicate that differentiated taste receptor cells can enter the cell cycle, acquire stemness, and participate in taste bud regeneration. We propose that dedifferentiated taste receptor cells in combination with stem/progenitor cells enhance the regeneration of taste buds following nerve injury.

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

SIS-ECM Laden with GMSC-Derived Exosomes Promote Taste Bud Regeneration

Oral cancer has a high annual incidence rate all over the world, and the tongue is the most frequently affected anatomic structure. The current standard care is ablative surgery of malignant neoplasm, followed by tongue reconstruction with free flap. However, such reconstructive modalities with postsurgery radiotherapy or chemotherapy can hardly support the functional recovery of the tongue—particularly, functional taste bud regeneration—in reconstructed areas, thus seriously affecting patients’ prognosis and life quality. Using a critical-sized tongue defect model in rats, we show that combinatory transplantation of small intestinal submucosa–extracellular matrix (SIS-ECM) with gingival mesenchymal stem cells (GMSCs) or their derivative exosomes promoted tongue lingual papillae recovery and taste bud regeneration as evidenced by increased expression of CK14, CK8, and markers for type I, II, and III taste bud cells (NTPdase 2, PLC-β2, and AADC, respectively). In addition, our results indicate that GMSCs or their derivative exosomes could increase BDNF expression, a growth factor that plays an important role in the proliferation and differentiation of epithelial basal progenitor cells into taste bud cells. Meanwhile, we showed an elevated expression level of Shh—which is essential for development, homeostasis, and maintenance of the taste bud organ—in wounded areas of the tongue among animals treated with GMSC/SIS-ECM or exosome/SIS-ECM as compared with SIS-ECM control. Moreover, our data show that GMSCs or their derivative exosomes promoted innervation of regenerated taste buds, as evidenced by elevated expressions of neurofilament and P2X3 at the injury areas. Together, our findings indicate that GMSC/SIS-ECM and exosome/SIS-ECM constructs can facilitate taste bud regeneration and reinnervation with promising potential application in postsurgery tongue reconstruction of patients with tongue cancer.

Long-term Follow-up Results of Regeneration Process of Fungiform Taste Buds After Severing the Chorda Tympani Nerve During Middle Ear Surgery

To elucidate the regeneration process of fungiform taste buds after severing the chorda tympani nerve (CTN) by confocal laser scanning microscopy in vivo. In 7 consecutive patients whose CTN was severed during tympanoplasty, an average of 10 fungiform papillae in the midlateral region of the tongue were periodically observed, and the number of taste buds was counted until 12 to 24 months after surgery. Gustatory function was assessed by EGM. EGM thresholds showed no response within 1 month after surgery in any patient. All taste buds had disappeared until 13 to 71 days after surgery. Regenerated taste buds were first detected 5 to 8 months after surgery in 5 of the 7 patients. EGM thresholds recovered to their preoperative values in 2 patients. In these 2 patients, the number of regenerated taste buds gradually increased in combination with a recovered taste function. However, a time lag existed between taste bud regeneration and taste function recovery. EGM thresholds did not recover in the other 3 patients with regenerated taste buds, suggesting that these taste buds were immature without gustatory function. The long-term regeneration process of fungiform taste buds could be clarified using confocal laser scanning microscopy.

Aging profoundly delays functional recovery from gustatory nerve injury

The peripheral taste system remains plastic during adulthood. Sectioning the chorda tympani (CT) nerve, which sends sensory information from the anterior tongue to the central nervous system, causes degeneration of distal fibers and target taste buds. However, taste function is restored after about 40 days in young adult rodents. We tested whether aging impacts the reappearance of neural responses after unilateral CT nerve injury. Taste bud regeneration was minimal at day 50–65 after denervation, and most aged animals died before functional recovery could be assessed. A subset (n=3/5) of old rats exhibited normal CT responses at day 85 postsectioning, suggesting the potential for efficient recovery. The aged taste system is fairly resilient to sensory receptor loss and major functional changes in normal aging. However, injury to the taste system reveals a surprising vulnerability in old rodents. The gustatory system provides an excellent model to study mechanisms underlying delayed recovery from peripheral nerve injury. Strategies to accelerate recovery and restore normal function will be of interest, as the elderly population continues to grow.

Light and Electron Microscopic Observation of Regenerated Fungiform Taste Buds in Patients with Recovered Taste Function after Severing Chorda Tympani Nerve

The aim of this study was to evaluate the mean number of regenerated fungiform taste buds per papilla and perform light and electron microscopic observation of taste buds in patients with recovered taste function after severing the chorda tympani nerve during middle ear surgery. We performed a biopsy on the fungiform papillae (FP) in the midlateral region of the dorsal surface of the tongue from 5 control volunteers (33 total FP) and from 7 and 5 patients with and without taste recovery (34 and 29 FP, respectively) 3 years 6 months to 18 years after surgery. The specimens were observed by light and transmission electron microscopy. The taste function was evaluated by electrogustometry. The mean number of taste buds in the FP of patients with completely recovered taste function was significantly smaller (1.9 +/- 1.4 per papilla; p < 0.01) than that of the control subjects (3.8 +/- 2.2 per papilla). By transmission electron microscopy, 4 distinct types of cell (type I, II, III, and basal cells) were identified in the regenerated taste buds. Nerve fibers and nerve terminals were also found in the taste buds. It was clarified that taste buds containing taste cells and nerve endings do regenerate in the FP of patients with recovered taste function.

Morphology of Human Fungiform Papillae after Severing Chorda Tympani Nerve

We aimed to clarify the postoperative morphology of the fungiform papillae (FP) of the tongue in patients who recovered gustatory function after the chorda tympani nerve was severed during middle ear surgery. Fifty-four patients with normal preoperative gustatory function measured by electrogustometry (EGM) were included. The proximal and distal stumps of the severed nerves were re-adapted or re-approximated during surgery to promote regeneration of the nerve. The EGM thresholds over 2 years after surgery were compared with preoperative values. At the same time, the morphological characteristics of the FP in the midlateral region of the tongue were recorded with a digital microscope. One month after surgery, EGM showed no response in any patients. At a time point of more than 2 years, the FP showed complete atrophy and no response to EGM on the surgical side in 21 of the 54 patients. In 16 patients who showed complete recovery of the EGM threshold (below 20 microA), the FP showed an almost normal appearance, and the mean number of FP was 77.5% (10 +/- 4.1 papillae per square centimeter) of that on the contralateral side (12.9 +/- 4.9 papillae per square centimeter; p > 0.05). The morphology of the FP was maintained in patients who recovered gustatory function after the chorda tympani nerve was severed. Because the results indicate regeneration of the taste buds, further observation is needed to detect regenerated taste buds in the FP.

Taste bud regeneration and the search for taste progenitor cells.

While the taste periphery has been studied for over a century, we are only beginning to understand how this important sensory system is maintained throughout adult life. With the advent of molecular genetics in rodent models, and the upswing in translational approaches that impact human patients, we expect the field will make significant advances in the near future.

Nerve-Target Interactions in the Gustatory System Following Unilateral Chorda Tympani Nerve Section

Although much effort has been directed at examining the morphological characteristics of denervated and regenerated taste buds, relatively little is known about the physiology of reinnervated taste cells (Vintschgau and Honigschmied, 1876; Guth, 1957; Fujimoto and Murray, 1970; Cheal and Oakley, 1977; Whitehead et al., 1987). Over 25 years ago, Oakley (Cheal et al., 1977) and colleagues found that some chorda tympani fibers reinnervated fungiform papillae on the gerbil’s anterior tongue in <2 weeks post-sectioning. Studies that examined recovery of taste function following nerve damage (Cain et al., 1996; Cheal et al., 1977; Ninomiya, 1998; Yasumatsu et al., 2003) indicate that regenerated taste nerves generally recover normal function after progressing through a transitional period. Only relatively recently has the impact of environment on the adult regenerating taste system been explored. Regenerating taste receptor cells in adult rats are susceptible to dietary influences (Hill and Phillips, 1994). Sectioning the chorda tympani unilaterally in rats fed a sodium-restricted diet at adulthood resulted in a regenerated nerve that had attenuated responses to sodium salts. Responses to other stimuli were unaffected. In addition, there were other, novel effects. In the same rat, the contralateral, uncut nerve was supersensitive to sodium salts. The supersensitivity developed progressively following an initial

Experimental study of the fungiform papilla and taste bud regeneration following microsurgical repair of lingual nerve in rat

To investigate the changes of taste buds following injury to lingual nerve and the regeneration of the fungiform papillae and taste buds following microsurgical epineurial anastomosis of transecting injured lingual nerves in rats. We observed the numbers and shapes of the fungiform papillae and taste buds with stereomicroscope, light microscope, and scanning electron microscope at 20 and 100 days after the clamp injury to lingual nerve, or the transecting injury to lingual nerve with/without immediate microsurgical epineurial anastomosis of the injured lingual nerve in rats. The fungiform papillae and taste buds degenerated, atrophied and their numbers diminished obviously at 20 days following either the clamp injury or transecting injury to the lingual nerve. The fungiform papillae and taste buds didn't regenerate spontaneously at 100 days following transection of the lingual nerve without microsurgery. The degenerated fungiform papillae and taste buds regenerated and recovered completely at 100 days following both clamp injury to the lingual nerve and transection of the lingual nerve with immediate microsurgical epineurial anastomosis. The degenerated fungiform papillae and taste buds have good ability to regenerate spontaneously following clamp injury to the lingual nerve; the degenerated fungiform papillae and taste buds can regenerate completely with immediate microsurgical epineurial anastomosis of the transected lingual nerve. The quantity and morphology of fungiform papillae and taste buds can be used as objective indicators in the function rehabilitation of injured lingual nerve.

The time course of taste bud regeneration after glossopharyngeal or greater superficial petrosal nerve transection in rats.

We previously have published data detailing the time course of taste bud regeneration in the anterior tongue following transection of the chorda tympani (CT) nerve in the rat. This study extends the prior work by determining the time course of taste bud regeneration in the vallate papilla, soft palate and nasoincisor ducts (NID) following transection of either the glossopharyngeal (GL) or greater superficial petrosal (GSP) nerve. Following GL transection in rats (n = 6 per time point), taste buds reappeared in the vallate papilla between 15 and 28 days after surgery, and returned to 80.3% of control levels (n = 12) of taste buds by 70 days postsurgery. The first appearance and the final percentage of the normal complement of regenerated vallate taste buds after GL transection resembled that seen previously in the anterior tongue after CT transection. However, in the latter case, regenerated taste buds reached asymptotic levels by 42 days after surgery, whereas within the time frame of the present study, a clear asymptotic return of vallate taste buds was not observed. In contrast to the posterior (and anterior) tongue, only 25% of the normal complement of palatal taste buds regenerated by 112 days and 224 days after GSP transection (n = 9). The difference in regenerative capacity might relate to the surgical approach used to transect the GSP. These experiments provide useful parametric data for investigators studying the functional consequences of gustatory nerve transection and regeneration.

放射線誘発性味覚障害

Taste disorders caused by radiation therapy for head and neck cancer are common, but the details are still unclear. This paper reviews the time course, degree and character of radiation-induced taste disorders retrospectively. Radiation-induced taste disorders appeared one to two weeks after the start of radiotherapy (10-20 Gy), and reached maximum 4 to 5 weeks later (40 Gy). In the 40-70 Gy irradiation period, taste disorders persisted subjectively, but objective recognition thresholds tended to become lower, suggesting taste bud regeneration. Radiation-induced taste disorders recovered within 3 months after the termination of radiotherapy subjectively, and within 6 months objectively. Bitter taste was affected most. A loss of sweet taste depended upon whether the tip of tongue was included within the radiation field. The slope of the taste intensity performance did not change during and after radiotherapy. The main cause of taste disorders during radiation is the damage to the taste cell surface structures, and the effects of damages to the peripheral nervous system governing gustation and the salivary gland may be limited.

Cellular expression of alpha-gustducin and the A blood group antigen in rat fungiform taste buds cross-reinnervated by the IXth nerve.

Although taste buds are trophically dependent on their innervation, cross-reinnervation experiments have shown that their gustatory sensitivities are determined by the local epithelium. Both the gustatory G-protein, alpha-gustducin, and the cell-surface carbohydrate, the A blood group antigen, are expressed by significantly fewer fungiform than vallate taste cells in the rat. In these experiments, one side of the anterior portion of the tongue was cross-reinnervated by the IXth nerve in order to determine whether the molecular expression of taste bud cells is determined by the epithelium from which they arise or by the nerve on which they are trophically dependent. The proximal portion of the IXth nerve was anastomosed to the distal portion of the chorda tympani (CT) nerve using fibrin glue (IX-CT rats). Control animals had the CT cut and reanastomosed using the same technique (CT-CT rats), or had the CT avulsed from the bulla and resected to prevent regeneration (CTX rats). The animals survived for 12 weeks postoperatively, and the tongues were removed, stained with methylene blue, and the fungiform taste pores counted on both sides. Tissue from the anterior 5 mm of the tongue was cut into 50-microm sections, which were incubated with antibodies against alpha-gustducin and the human blood group A antigen. In both CT-CT and IX-CT rats, there was regeneration of fungiform taste buds, although in both groups there were significantly fewer taste buds on the operated side of the tongue. The normal vallate papilla had a mean of 8.37 alpha-gustducin-expressing cells and 5.22 A-expressing cells per taste bud, whereas the fungiform papillae contained 3.06 and 0.23 cells per taste bud, respectively. In both CT-CT and IX-CT rats there was a normal number of cells expressing alpha-gustducin or the A antigen in regenerated taste buds; in the CTX animals there was a significant decrease in the expression of these markers. These results demonstrate that the molecular phenotype of taste bud cells is determined by the local epithelium from which they arise and not by properties of the innervating nerve.

Calbindin D28k-like immunoreactivity in the developing and regenerating circumvallate papilla of the rat

The distribution of calbindin D28k (CB)-like immunoreactivity (-LI) in the circumvallate papilla (CVP) was examined during development and regeneration following bilateral crush injury to the glossopharyngeal nerve in the rat. In the adult CVP, CB-like immunoreactive (-IR) nerve fibers were observed in the subgemmal region and some penetrated into the taste buds. CB-LI was also detected in the cytoplasm of the spindle-shaped gustatory cells in the lower half of the trench epithelium, which contained numerous synaptic vesicles and bundles of intermediate filaments. These CB-IR gustatory cells made synapse-like contacts with CB-IR nerve terminals. Some CB-IR nerve terminals made contacts with the gustatory cells negative for CB-LI. At least three developmental stages were defined with regard to the developmental changes in the distribution of CB-LI: (1) Stage I (embryonic day (E) 18-postnatal day (P)5): CB-IR nerve fibers appeared in the lamina propria just beneath the newly-formed CVP at E18, but the gustatory epithelium of the CVP contained no CB-IR structures. Taste buds with taste pores appeared at P1. (2) Stage II (P5-10): thin CB-IR nerve fibers began entering the trench epithelium, but no CB-IR cells were observed. (3) Stage III (P10-adult): in addition to the intragemmal and perigemmal CB-IR nerve fibers, very few CB-IR cells appeared in the taste buds around P10, and their numbers increased progressively. The changes in the distribution of taste buds and CB-LI following glossopharyngeal nerve injury were similar to those observed during development. On post-operative day (PO) 4, the taste buds and CB-IR cells decreased markedly in number. These CB-IR cells became round in shape, and the number of CB-IR nerve fibers decreased markedly. On PO8, both taste buds and CB-IR cells disappeared completely. The regenerated taste buds were first observed on PO12, increased rapidly in number by PO20, and increased slowly thereafter. CB-IR nerve fibers accumulated at the subgemmal region and began penetrating into the trench wall epithelium around PO16. CB-IR cells appeared between PO20 and PO24, and their numbers increased progressively and reached the normal level on PO40. The topographical localizations of the taste buds and CB-IR cells during development and regeneration were comparable to those of normal animals. The delay of the time courses for appearance of CB-IR nerve fibers and CB-IR cells compared to the appearance of taste buds during development and regeneration suggests that CB in the gustatory epithelium may participate in the survival of the taste bud cells rather than in the induction of the taste buds.

Salt discriminability is related to number of regenerated taste buds after chorda tympani nerve section in rats.

Transection of the chorda tympani nerve (CTX) impairs taste-guided discrimination of NaCl from KCl in rats. We wanted to determine whether this discrimination recovers after chorda tympani regeneration. Experiment 1 showed that few taste buds regenerated 14 days after CTX, whereas substantial regeneration occurred 42 days after surgery. Experiment 2 demonstrated that rats trained before CTX could clearly discriminate the two salts when tested starting 49 days after surgery, whereas rats tested starting 8 days after surgery were severely impaired in this task. Rats tested starting 28 days after CTX were unimpaired, moderately impaired, or severely impaired on the discrimination task. Overall, discrimination performance was significantly related to the number of regenerated taste buds. Unilaterally transected rats tested shortly after surgery were nearly as competent as controls. These results indicate that rats can recover the ability to discriminate NaCl from KCl after regeneration of anterior tongue taste buds.

味神経切断, 吻合後の茸状乳頭ならびに味蕾の変化に関する電子顕微鏡的研究

The fungiform taste buds and some mechanosensory endings in rabbits, which are supplied with chorda tympani and lingual nerves, were examined by means of light and electron microscopy. Three to seven taste buds were present in the dorsal epithelium of a fungiform papilla. The taste bud was composed of the type I, the type II, the type III cells and the basal cells. The type III cell was characterized by the presence of dense-core vesicles and synaptic vesicles, and showed afferent synaptic contact with nerve endings. Thus, it was suggested that the type III cell is involved in the chemoreceptive function. The presence of numerous free nerve endings in the extragemmal epithelium and mechanosensory endings, and of encapsulated corpuscles and Ruffini-like endings in the connective tissue of the fungiform papillae suggested that the fungiform papillae of the rabbit were highly adapted to both taste and mechanical sensation. The fungiform papillae were examined by light and electron microscopy from 1 week to 6 months after denervation followed by microsurgical reinnervation of both chorda tympani and lingual nerves. Further, these results were compared with those of experiments which were carried out in the same manner except for the microsurgical reinnervation. The first remarkable finding was the appearance of an ulcer in the dorsal surface of the tongue of the denervated side after the first week following denervation. The ulcer remained visible for about 10 weeks after the operation, and a severe defect of the tongue of the denervated side was observed occasionally. Taste buds and mechanosensory endings in fungiform papillae disappeared completely after the first week following denervation. The dorsal epithelium of the fungiform papillae showed filiform-like structures because of the hyperparakeratotic changes. However, 15 weeks after denervation followed by microsurgical reinnervation, the fungiform papillae regained their normal appearance. Encapsulated corpuscles and Ruffini-like endings regenerated in the connective tissue of reinnervated fungiform papillae in this experimental period, although these nerve endings were small in size and in number. 5 months after microsurgical reinnervation, regeneration of the taste buds was observed in the dorsal epithelium of the fungiform papillae. Ultrastructure of regenerated taste buds was essentially identical with that of normal taste buds, although the numbers of regenerated taste buds and their constituents were smaller than those of normal fungiform taste buds. In groups where no microsurgical reinnervation was carried out, regeneration of mechanosensory endings in the connective tissue was observed 5 months after denervation. Regeneration of taste buds, however, could not be observed during the experimental periods, even 6 months after denervation. The present study showed morphologically that the rapid and reliable regeneration of taste buds and mechanosensory endings in denervated fungiform papillae by chorda tympani-lingual nerves could be expected only when the proximal and distal cut ends of these nerves were repaired microsurgically.

A sensitive period for the neural induction of taste buds

Taste buds mature postnatally in the vallate papilla of the rat and reach a mean number of 610 by day 90. Although taste buds are neurotrophically dependent, the presence of widespread bilateral innervation permits more than 80% of the 610 vallate taste buds to survive after one IXth nerve is removed in adults. However, after a IXth nerve is removed at 0-3 d postpartum, about two-thirds of the vallate taste buds fail to develop. In the present investigation, the timing of the neural induction of taste buds was examined by unilaterally removing the IXth nerve at 12 different postnatal ages, from 0 to 75 d. Unilateral denervation revealed the existence of a sensitive period that is maximal from 0 to 10 d, when unilateral or bilateral interruption of the IXth nerve profoundly impairs the formation of taste buds. The number of taste buds that form is nonlinearly dependent upon the number of axons; at low levels of innervation, a doubling of the number of myelinated axons quintuples the number of taste buds. Thus, taste axons interact synergistically. In studying regeneration, we found that axons of both neonatal and adult IXth nerves elongate approximately 1.8 mm/d. Taste buds were re-formed more rapidly and a higher proportion were bilaterally innervated when regenerating axons and the sites of former taste buds were numerous. The proportion of bilaterally innervated taste buds could be approximated from the likelihood of random overlap of axons from the right and left IXth nerves. The greater ease with which taste buds are re-formed than developed suggests that taste bud regeneration does not recapitulate taste bud development.