Effect of ageing process on fungiform papillae density

© Slovak University of Agriculture in Nitra Faculty of Agrobiology and Food Resources


Introduction
The dorsal anterior tongue in humans is rich in fungiform papillae (FP), which play an important role in taste functioning and oral sensation.Most of the FP contains at least one taste bud comprising of the taste cells (Zhang et al., 2008;Fischer et al., 2013).Each taste bud has single apical pore through which the taste cells project microvilli into the oral cavity.A cell membrane of each microvillus possesses multiple taste receptors providing a surface for the reception of taste stimuli and transduction of chemical into electrical signals (Chandrashekar et al., 2006;Cvijanovic et al., 2015).
The FP density commonly varies across the anterior tongue.In general, the highest values for healthy participants were recorded near the tip.Since 99% of papillae in humans contain at least 1 taste bud, papillae density is related to taste sensitivity.The subjects who have higher numbers of papillae are being more sensitive to taste stimuli (Correa et al., 2013;Fischer et al., 2013;Zhang et al., 2008;Just et al., 2006).
Up to now, it is not clear what factors are associated with the FP number or with possible changes in FP density over time (Fischer et al., 2013).In the past, it was suggested that higher density of FP may explain biological background of PROP super-tasting ability (Shahbake et al., 2005).However, the newest evaluating models revealed that predictors of taste sensitivity to PROP are age, sex, and haplotype rather than FP density (Garneau et al., 2014).
By contrast, some conditions such as degeneration or loss of taste following neural damage are accompanied by reduced density of FP (Zhang et al., 2008).The patients with transected chorda tympani nerve, during middle ear surgery because of cholesteatoma, exhibited a significant decrease of taste function and FP density on the respective side of the tongue (Just et al., 2006).Keratinisation of FP, decreased capillary vessels and distortion of filiform papillae has been reported for heavy smokers, but there was no significant difference in taste sensitivity and FP density, compared to non-smokers (Konstantinidis et al., 2010).Moreover, the FP density is related to alcohol consuming.Heavy alcohol drinkers exhibit lower densities, compared to non-heavy ones Fischer et al., 2013).Ageing is accompanied by physiological changes in the human oral cavity.These include potential reduction of the fungiform papillae (FP) density which has been associated with impaired taste acuity.Because the studies have demonstrated either contrary or non-significant evidences, we decided to verify the hypothesis about FP reduction and ageing.Total of 150 human living subjects aged 19-87 years participated in the study.The tongues were stained with blue food dye, quantification area with 10 mm diameter was defined, and images of the tongues were captured with digital camera.The number of FP has been scored on each photo by five individual scorers according to a decision tree.The scoring was accomplished in ImageJ open source program using a cell counter plugin.The mean FP density value within all subjects was 18.02 ±15.95 FP per cm -2 , the lowest and the highest values were 0.25 ±0.57FP per cm -2 and 65.00 ±1.66 FP per cm -2 of the tongue tip, respectively.Based on age, the subjects were divided into categories with 6.0 increasing step, in order access effect of ageing on FP densities.Age categories were significantly different in mean FP densities as determined by Welch's F (8, 22.04) = 28.81,P <0.01).The Games-Howell post hoc test revealed that participants aged 18-24 years had significantly higher FP densities than subjects aged more than 48 years.High degree of intragroups variance was observed, which could reflect in non-significant differences between the other age groups.Nevertheless, the results were sufficient to support the hypothesis that FP quantity diminish with age.

Original Paper
Finally, a relationship between FP reduction and ageing process has also been examined.Some studies have not reported either significant association or gender differences (Zuniga et al. 1993;Just et al., 2006;Zhang et al. 2008), but others have done (Segovia et al. 2002;Pavlidis et al. 2012;Correa et al. 2013;Fischer et al. 2013).
The mechanisms that cause age-associated declines in the peripheral taste structures remain mostly unclear (Feng, Huang and Wang, 2013).This issue is the serious one accounting for taste disorders in elderly, which may alter appetite, lead to malnutrition and be concerned with morbidity and mortality of elderly persons (Imoscopi et al., 2012;Ikeda et al., 2007).
Since contrary evidences have been published, we decided to examine the hypothesis whether ageing is associated with reduction of FP density.

Subjects
The number of participants was 150 (44 men and 106 women), aged from 19 to 87 years (mean age: 50.27 ±22.40 years).Totally, 21 participants were smokers.Besides, 80 of them were being treated on some diseases (the most frequently on cardiovascular diseases and diabetes) when the study was conducted.Written consent was obtained from each participant once he had read an information sheet describing the purpose of the study.

Quantification of fungiform papillae
Acquisition of FP data was done following a protocol of Nuessle et al. (2015).Using a macro setting, the photos were captured with digital camera Canon EOS 550D attached to a tripod.The number of FP has been scored on each photo by five individual scorers according to a decision tree.The scoring was accomplished in ImageJ open source program using a cell counter plugin.Thus, 5 different values for each subject were obtained, each of which presented the number of FP per 10 mm diameter stained section of the tongue tip.The values were recalculated in order that final mean FP density value corresponded to 1 cm 2 area of the tongue tip.
There was statistically significant difference between the means of groups as determined by Welch's F (8, 22.04) = 28.81,P <0.01).The Games-Howell post hoc test revealed that participants aged 18-24 years had significantly higher FP densities than subjects aged 48 years and more.These are the only significant differences observed.However, some p-values suggest there is almost provable significant difference between particular categories ( ±3.06 FP per cm -2 in the second study.Thus, quantity of subjects and inter-individual variability seems to be relevant factors. Several studies have investigated the age-and lifestylerelated changes in FP density and consequent impact on taste acuity.For instance, Segovia et al. ( 2002) found out children have significantly higher FP densities, much higher taste pore densities and rounder FP than adults, despite of the fact they have smaller FP and taste pore diameters.
In addition, Correa et al. (2013) conducted study with 30 adults and 85 children (7-12 year olds).Statistical analysis revealed significant differences in the total number of FP in the anterior tongue among age groups, but not between sex ones.Post hoc tests have shown that only 7-8 year old children had significantly more papillae than adults.Because there were no significant differences between other groups, the finding suggested that number of FP stabilised by 11-12 years of age, which is very close to the age that cessation of growth of the anterior tongue occurs.
Moreover, in the study with 156 non-smokers (74 males, 82 females), aged 10-80 years, the density of FP at the tip of the tongue decreased significantly in men aged >50 years and in women aged >60 years compared with younger individuals.The mean FP density value reported for elderly individuals was similar to that one in our study.Moreover, ageing was also significantly associated A significant dose-response relationship between level of consumed alcohol and papillary density was observed among those who consumed alcohol in the past year.Both smoking and alcohol consumption were termed as modifiable factors contributing to overall taste quality.

Conclusions
The results from this study supported the hypothesis FP densities decrease during ageing.Due to the fact that individual FP densities extremely varied in particular age categories, it was not possible to demonstrate statistically significant difference between such groups.Nevertheless, the data were sufficient for the purpose of the study.In addition to high intra-groups variance, there was also lack of participants in some age groups.All these factors should be considered in designs of future studies.

Table 2
The mean age of participants in these studies was about equal whereas the number of them was different.The first study reported for 30 adults (aged 20-24 years) an average of 156 ±5.80 FP per cm -2 .On the contrary, 182 subjects (aged 18-23 years) had an average of 96.96 http://www.acta.fapz.uniag.sk © Slovak University of Agriculture in Nitra Faculty of Agrobiology and Food Resources Center lines show the median, red crosses show mean, box limits indicate the 1 st and 3 rd quartiles, whiskers (terminated by blue rhombuses) extend 1.5 times the interquartile range from the 1 st and 3 rd quartilesn Faculty of Agrobiology and Food Resources