Pigmented Fungiform Papillae (PFP) of the Tongue: A Systematic Review of Current Aetiopathogenesis and Pathophysiology

The pigmentation of the fungiform papillae of the tongue is a rare idiopathic condition in which only the fungiform papillae appear hyperpigmented. In the absence of any reviews on the subject, we conducted a systematic review of the aetiopathogenesis and pathophysiology of pigmented fungiform papillae (PFP) of the tongue, including its demographic and histopathological features, trying to outline a possible aetiology. The preferred reporting items for systematic reviews and meta-analyses (PRISMA) was performed using PubMed, Scopus, EMBASE databases and manual searches, for publications between January 1974 and July 2022. Inclusion criteria were case reports defining patients’ characteristics, their general medical and dental conditions, histopathological and/or immunohistochemical findings, all with a final definitive diagnosis of PFP. Overall, 51 studies comprising 69 cases of PFP which included histopathological descriptions were reviewed. Prominent features consisted of hyperpigmentation of melanocytes, melanophages, chromatophores, and a lymphocytic infiltrate in the subepidermal area of the fungiform papillae. On special staining, PFP contained melanin, not iron or hemosiderin. On immunohistochemistry, immune-reactive CD3+ T lymphocytes, S-100 and Sox10, but non-immune-reactive melan-A intraepithelial melanocytes were noted in some studies. The presence of hyperpigmented melanocytes and melanophages, with non-immune-reactive melan-A, suggests that PFP are a benign and physiological form of pigmentation. The inflammatory infiltrates described in some papillary lesions could possibly be due to traumatic events during mastication. Nevertheless, the true reasons for the hyperpigmentation of the fungiform papillae are as of yet elusive, and remain to be determined.

Fungiform papillae are located on the tip and lateral portions of the tongue; infrequently, they may become hyperpigmented solely, appearing as mushroom-like structures with a brown or dark colour [31] (Figure 1).
Fungiform papillae are located on the tip and lateral portions of the tongue; inf quently, they may become hyperpigmented solely, appearing as mushroom-like str tures with a brown or dark colour [31] (Figure 1).
Clinically, PFP can be easily diagnosed by careful naked eye examination, with without dermoscopy [32]. The latter provides a much clearer visualisation of the lesio which usually have a cobblestone or rose-petal appearance [32]. Some authors have us tissue biopsy as a diagnostic tool to rule out underlying malignancies.
In the absence of any reviews on the subject in the literature to our knowledge, conducted the current systematic review on PFP, its possible aetiopathogenesis, and pa ophysiology.

Materials and Methods
A systematic search of the literature, including publications from January 1974 to Ju 2022, was conducted using the databases PubMed, Scopus, and EMBASE, as well a manual search utilizing the MESH terms and keywords "pigmented fungiform papilla "oral", and "tongue".
The PFP have been classified into three types by Holzwanger [33] (Figure 1). In ty 1, the pigmentation affects a well circumscribed area on the anterolateral sides or towar the tip of the tongue; in type 2, the pigmentation affects a few fungiform papillae of Clinically, PFP can be easily diagnosed by careful naked eye examination, with or without dermoscopy [32]. The latter provides a much clearer visualisation of the lesions, which usually have a cobblestone or rose-petal appearance [32]. Some authors have used tissue biopsy as a diagnostic tool to rule out underlying malignancies.
In the absence of any reviews on the subject in the literature to our knowledge, we conducted the current systematic review on PFP, its possible aetiopathogenesis, and pathophysiology.

Materials and Methods
A systematic search of the literature, including publications from January 1974 to July 2022, was conducted using the databases PubMed, Scopus, and EMBASE, as well as a manual search utilizing the MESH terms and keywords "pigmented fungiform papillae", "oral", and "tongue".
The PFP have been classified into three types by Holzwanger [33] (Figure 1). In type 1, the pigmentation affects a well circumscribed area on the anterolateral sides or towards the tip of the tongue; in type 2, the pigmentation affects a few fungiform papillae of the dorsum of the tongue; while in type 3, the pigmentation involves all of the dorsum fungiform papillae.
From all databases, a total of 82 articles were identified, and those with duplicate titles/resources were excluded. The remaining total of 51 articles was then screened based on the availability of the abstracts and full texts. Further filtering of these yielded 51 articles for final review. The 51 articles were divided into two categories, 21 articles, encompassing 36 cases that presented or discussed a PFP case by including histopathological findings accompanied by another examination. The other 30 articles encompassed 33 cases that presented or discussed a PFP case.

Liver and Kidney Function Tests
Eleven studies reported liver and renal function tests, or urinalysis [36,37,40,51,68-71, 79,82]. The liver function test was reported normal as well as renal function test, creatinine levels, and urinalysis in nine studies. One study reported a high level of aspartate aminotransferase, alanine aminotransferase, and bilirubin [81], and in other study high levels of anti-smooth muscle antibodies, anti-liver antibodies, and anti-kidney microsomal antibodies were detected [82].

Histopathology
The histopathological descriptions of PFP revealed the following general characteristics: (i) hyperpigmentation of melanocytes with brown melanin of the basal cell layers of epithelium; (ii) melanophages in the lamina propria, sub-epithelial or sub-mucosal connective tissue; (iii) scant lymphocytic infiltrate in the lamina propria; (iv) chromatophores in the sub-epithelial area and surrounding the blood vessels of the fungiform papillae; and finally, (v) dilated vascular spaces (Table 2).  NR: not reported.

Immunohistochemistry
Immunohistochemical studies showed an immune-reactive for CD3+ T lymphocytes, S-100 and Sox10 in the intraepithelial melanocytes and non-immuno-reactive melan-A (Table 3).

Other Features
Using specific staining, two studies showed the presence of melanin with Fontana-Masson staining. PFP were not linked with the presence of iron or hemosiderin with Berlin blue and Perl's staining (Table 3).

Discussion
The tongue is the largest organ of the oral cavity, and its surface is studded with four types of papillae: filiform, fungiform, foliate, and vallate papillae. Lingual papillae are thought to increase the surface area of the tongue, serve as bearers of taste buds, and also to increase the area of contact and friction between the tongue and food [45]. The fungiform papillae are club shaped projections generally found scattered on the tip and sides of the tongue. They are, in health, pink to red in colour as the rest of the lingual mucosa, and usually do not stand out as discrete organelles. However, in some people, the fungiform papillae are pigmented and can be readily seen as discrete projections, even by naked eye examination. In most of the reports of the current review PFP appeared as dark-brown, blue-grey, or bluish-black papillae.
A number of hypotheses have been proposed for the origin of PFP. Some contend that it is due to the transfer of darker melanocytes in the basal layer of the epithelium to the superficial keratinocytes via membrane-bound organelles called melanosomes [83]. The melanin is likely to be the darker variant eumelanin found in black ethnicity, as opposed to the lighter variant pheomelanin seen in Asian ethnicity [84]. The melanin was later confirmed in PFP, and not related to the presence of iron and hemosiderin [43,44].
Pathophysiology 2022, 29, FOR PEER REVIEW 9 subepithelial tissue [54,72], adjacent filiform papillae [75], and the submucosa [65,73] of the fungiform papillae ( Figure 2). Melanophages-macrophages that contain melanin-are a common feature in inflammatory and non-inflammatory pigmentation [85]. Melanophages are large cells with indistinct cytoplasmic boundaries, usually located around or near superficial dermal vessels [73]. Another histological feature described by some authors in PFP is the presence of so-called 'chromatophores', a disputed, and confusing term used by Holzwanger [33,71] and Koplon [71] in the older literature, referring to melanocytes, or perhaps more proba- Melanophages-macrophages that contain melanin-are a common feature in inflammatory and non-inflammatory pigmentation [85]. Melanophages are large cells with indistinct cytoplasmic boundaries, usually located around or near superficial dermal vessels [73]. Another histological feature described by some authors in PFP is the presence of so-called 'chromatophores', a disputed, and confusing term used by Holzwanger [33,71] and Koplon [71] in the older literature, referring to melanocytes, or perhaps more probably to melanophages (pigment carrying cells) as mentioned by Steigmann [86]. The presence of melanin was later confirmed with Fontana-Masson staining [44].
The presence of a lymphocytic infiltrate reported in some studies, could be considered as a sign of an inflammatory response associated with the genesis of PFP. In an immunohistochemical study, Ghigliotti et al. revealed that the lymphocytes in PFP were mainly CD3 + T lymphocytes [72]. The localised traumatic event or mechanical injury provokes inflammatory responses. The inflammatory responses cause inflammatory cytokine production from keratinocytes as well as from fibroblasts, which in turn stimulate melanocytes, often resulting in pigmentation. In some research, it has been mentioned that several cytokines such as interleukin, tumor necrosis factor (TNF), and prostaglandin E (PGE) modulate the proliferation and differentiation of human melanocytes to pigmentation [87]. The presence of the latter lymphocytic infiltrate, together with the melanophages, implies the possibility that the pigmentation is due to an inflammatory process, provoked most likely by a localised traumatic event ( Figure 2).
Not surprisingly, some authors have investigated whether PFP is a precursor of melanoma. To our knowledge, no malignant transformation has been reported from PFP. Only in two studies were immunohistochemical staining for melanotic markers done, detecting Sox10 [43] and S-100 [75] expressions, but they were non-reactive for melan-A [75].
On further detailed review of the reports, we noted that a number of investigators have used dermoscopy to evaluate the nature of PFP. This technique increases the sensitivity and specificity of the clinical examination, and almost eliminates biopsy procedures and by extension reduces patient discomfort. Nevertheless, dermoscopy is infrequently used in dentistry [88]. A majority of clinicians in our review used dermoscopy, but in combination with biopsy examination, which seems to be a waste of resources [34,35,64,65,67,72,80].
Based on the current study findings and interpretation, it may be speculated that PFP could arise due to injury and disruption of the vascular architecture of the fungiform papillae during masticatory traumatic events. This would cause the extravasation of blood and a subsequent influx of inflammatory infiltrate of macrophages and lymphocytes into the papillary tissue [64]. Sugiyama et al. have postulated that the inflammatory process then causes an increase in melanin synthesis in the lamina propria of the fungiform papillae leading to the hyperpigmentation [64]. However, this fact does not explain why only the fungiform papillae-and not the other papillary variants of the lingual mucosa-are affected by such events. Moreover, if traumatic events were the main cause acting in the origin of PFP, it would be expected to occur more frequently and at any age. Its description, affecting several and different ethnicities as we observed in our study, does not add further information about a possible predisposition in that sense, although an apparent predominance among Asian versus White people was noted. These findings contrast with those of Holzwanger [33], who stated that PFP is a relatively common variant of oral In order to ascertain the aetiology of PFP and to rule out systemic diseases, numerous laboratory analyses have been conducted, including complete blood cell count [37,40,46,47,49,62,64,[69][70][71]79,81], blood glucose [69,71,82], or fasting blood glucose level [51], liver function tests [36,37,51,69,79], renal or kidney function tests [36,68,79,82], urinalysis [40,70,71,79], adrenal cortex function (synacthen) tests [64], electrolyte levels [37,51,69,70,81], vitamin B12 [49,51,52,69], vitamin D [36], folate [82], trace element [36,46,47,62,68,82], ferritin [49,51,52], and urea [37,69,71]. The infection markers, as hepatitis markers [51], fungal [52], VDRL [79], the endocrinological [36,40,47,49,64,79,82], and anti-nuclear antibody levels [46,62,82] were assessed. The results of the foregoing tests as well as ophthalmological and neurological tests conducted by some [40,79], were negative, and did not reveal an oral-systemic connection with the origin of PFP. Only three cases showed low hemoglobin and leucocytes counts [41] and high MCV counts [82], heterozygocity of HBs [36] was noted amongst the 69 studies reviewed. These data point to the fact that PFP is a normal physiological condition with an aberration of the focal melanocyte function of the fungiform papillae.
Based on the current study findings and interpretation, it may be speculated that PFP could arise due to injury and disruption of the vascular architecture of the fungiform papillae during masticatory traumatic events. This would cause the extravasation of blood and a subsequent influx of inflammatory infiltrate of macrophages and lymphocytes into the papillary tissue [64]. Sugiyama et al. have postulated that the inflammatory process then causes an increase in melanin synthesis in the lamina propria of the fungiform papillae leading to the hyperpigmentation [64]. However, this fact does not explain why only the fungiform papillae-and not the other papillary variants of the lingual mucosa-are affected by such events. Moreover, if traumatic events were the main cause acting in the origin of PFP, it would be expected to occur more frequently and at any age. Its description, affecting several and different ethnicities as we observed in our study, does not add further information about a possible predisposition in that sense, although an apparent predominance among Asian versus White people was noted. These findings contrast with those of Holzwanger [33], who stated that PFP is a relatively common variant of oral pigmentation which, although more prevalent in Negroes, it is seen in other heavily pigmented races as well. Therefore, the reason/s for the focal pigmentation of fungiform papillae is/are yet speculative, and remain to be clarified ( Figure 4). 2022, 29, FOR PEER REVIEW 12 pigmentation which, although more prevalent in Negroes, it is seen in other heavily pigmented races as well. Therefore, the reason/s for the focal pigmentation of fungiform papillae is/are yet speculative, and remain to be clarified ( Figure 4). The major limitation of this study is the absence of systematic evaluations of PFP, despite the plethora of case reports which are mainly anecdotal in nature. This leads to difficulties in systematic data collection. Furthermore, none of the case reports were followed up, and it is unclear whether the PFP is a transient phenomenon or lasts for prolonged periods. Hence prospective studies are needed.

Conclusions
Our findings, taken together, clearly indicate that PFP is a normal physiological condition with an aberration of the focal melanocyte function of the fungiform papillae of the tongue. Some reports indicate that the condition could be associated with several systemic conditions and syndromes but, clearly, not caused by them. The inflammatory infiltrate seen in some papillary lesions are likely to be due to masticatory trauma. Yet, the true reasons for the discolouration of fungiform papillae are as of yet elusive.  The major limitation of this study is the absence of systematic evaluations of PFP, despite the plethora of case reports which are mainly anecdotal in nature. This leads to difficulties in systematic data collection. Furthermore, none of the case reports were followed up, and it is unclear whether the PFP is a transient phenomenon or lasts for prolonged periods. Hence prospective studies are needed.

Conclusions
Our findings, taken together, clearly indicate that PFP is a normal physiological condition with an aberration of the focal melanocyte function of the fungiform papillae of the tongue. Some reports indicate that the condition could be associated with several