Investigations On A Surgical Approach For Lingual Nerve And Chorda Tympani Nerve Injuries, And On Morphological Changes Of Taste Buds After Their Injuries

Objectives:To explore a convenient and effective surgical approach for taste nerve injury in rats,and to observe the morphological changes of taste buds on anterior two thirds on the tongue after injuries of chorda tympani nerve and lingual nerve in rats.Methods:In partⅠ,24 adult SD rats were placed in a supine position after sufficient anesthesia.Incision was made beside the midline of the neck,masseter muscle and mylohyoid muscle could be seen after separation of superficial fascia.Following further exposure of infratemporal fossa,lingual nerve could be seen among the fibers of the anterior belly of digastric muscle.Next we traced upward along this nerve and found the crotch where chorda tympani nerve joins in the lingual branch of trigeminal nerve.Transection was made either on the lingual nerve or on the chorda tympani nerve,followed by hemostasis and suture of the incision.Postsurgical observation was made to the incision and general condition of rats,rates of mortality and infection were also calculated.Finally we made drawings and descriptions to rats' neural pathway of taste sense and anatomy of surrounding tissues,according to surgical findings,autopsy and literature search.In partⅡ,24 adult SD rats were randomly assigned into 3 groups:chorda tympani nerver transection group,lingual nerve transection group and surgery control group.Animals in the first two groups were further divided into five different time points-5 days,10 days,14 days,28 days and 42 days after surgery,and those in the surgery control group were divided in two different time points-10 days and 28 days.There were two rats in each time point.We performed the taste nerve transection by the approach we described in partⅠ.Rats were allowed to live after the surgery until they were sacrificed at the time point according to their groupings.Tongues were removed and made into serial frozen sections.Taste buds and cell nucleus were visualized by anti-CK8 antibody and Propidium Iodide respectively,and observed under the fluorescent microscope and confocal microscope.Photos were taken and then used for calculation of taste bud density,volume and cell count by Photoshop software.Independent-sample T-test was performed to determine whether they were statistically different between groups.Results:Mean duration for the opertation was 51.7±8.2 minutes.Rats' survival rate was 100%and the infection rate was 0%.All of the animals recovered to normal activity and diet within 2 days after the surgeries.Drawings and descriptions were made to depict the pathway of taste-related nerves(including chorda tympani nerve, glossopharyngeal nerve and vagus nerve) and anatomy of their surrounding tissues. After transection of lingual nerve or chorda tympani nerve,ipsilateral taste buds on the anterior two thirds of the tongue appeared as atrophic.More specifically,both the staining intensity and staining extent decreased,and taste bud cells lost their polarities. The taste bud density,mean taste bud volume and mean taste bud cell count all reached minimum in 14 days after the surgery.Then the morphous of taste buds started to recover,proved by increased staining intensity and staining extent,and normal cell polarity.In 42 days after the surgery,taste bud density reached up to 70% of surgery control group level,while mean taste bud volume and mean taste bud cell count reached over 90%.On the contralateral side of the taste nerve injuries,however, mean volume and cell count of taste buds on the anterior two thirds of tongue increased after the transection of lingual nerve or chorda tympani nerve.Both of them reached maximum in 14 days after the surgery,and then decreased to around normal in 42 days.Taste bud density and cell polarities did not change significantly after the surgery.The mean volume and mean cell count of taste buds on the injury side of tongue were not statistically differernt at all five time points between lingual nerve transection group and chorda tympani transection group(P＞0.05),but the taste bud densities of the chorda tympani transection group were statistically higher than those of lingual nerve transection group at 10th day,14th day,28th day and 42nd day(P＜0.05).Conclusions:This surgical approach could achieve effective exposures and injury models of taste nerves that innervate anterior two thirds of the tongue in rats,without affecting the survival rate of animals.Taste buds on the anterior two thirds of tongue would become atrophic after transection of chorda tympani nerve or lingual nerve.Taste bud density,mean taste bud volume and mean taste bud cell count would reach minimum at around 14 days after the injury.Then taste buds would regenerate and reach around normal level in 42 days after the injury. Meanwhile,taste buds on the contralateral side would be compensatory hyperplasia mainly between 10 to 28 days after the injury of taste nerves,apprearing as increased mean taste bud volume and mean taste bud cell count.The lingual branch of trigeminal nerve may play a role in maintaining the morphous of taste buds. Regeneration of taste buds depends partly,but not only,on the repair of transected taste nerves.