Onychomatricoma in the light of the microanatomy of the normal nail unit

Ultrasound of Nails: Why, How, When

Nail disorders are frequently a challenge to diagnose correctly only based on clinical findings. Moreover, biopsies may leave scars due to damage to the nail matrix. Currently, high-frequency ultrasound (HFUS) with probes >15 MHz is the first-choice imaging technique to study the nail. Compared with other imaging tests such as computed tomography and magnetic resonance imaging, HFUS has the advantage of high-resolution imaging of the nail apparatus without exposure to radiation, contrast media, and the risk of claustrophobia of these other expensive imaging methods. Notably, the axial spatial resolution of HFUS is higher than computed tomography and magnetic resonance imaging; therefore, it can better discriminate the anatomical alterations. Our objective is to review the main applications of HFUS of the nail through literature analysis and illustration of cases of the most frequent pathologies found in our experience, exemplifying why, how, and when HFUS could help diagnose nail diseases.

Nail Telocytes: Identification, Potential Physiological Function, and Role in Pathology: A Reappraisal of the So-Called Onychofibroblasts/Onychodermis

ABSTRACT

Some authors have suggested that the fibroblasts of the nail mesenchyme (onychofibroblasts) can be distinguished from skin fibroblasts by their high expression of CD10. My 2015 study documented the presence of a relatively sparse CD34 + /CD10 + dendritic subpopulation in the dermis and hypodermis of the matrix. For some time now, my hypothesis has been that these interstitial dendritic mesenchymal cells of the matrix correspond to telocytes. Telocytes have been described as peculiar interstitial dendritic cells present in the mesenchymal tissue of numerous organs, including the skin, but their presence and characteristics in the nail unit have not been explored. This study was undertaken to more comprehensively investigate the existence and characteristics of nail telocytes. A series of 20 normal adult nail units were examined with a combination of morphological and immunohistochemical analyses. The matrix dermis contained a sparse subpopulation of CD34 + /CD10 + elongated telocytes with a higher density in the lunular region and, at this distal level, a change in their immunohistochemical profile, resulting in a progressive loss of CD34 expression. The matrix hypodermis showed CD34 + /CD10 + telocytes in their classical elongated aspect, which acquired, especially in the distal fibromyxoid area of the thumb, an oval to round morphology with multiple intracytoplasmic vacuoles. The characteristic dynamic immunophenotypic profile of the dermal telocytes with a progressive distal loss of the defining molecule CD34 was equally observed in the distal hypodermis. The nail bed dermis was thick with a dense fibrous connective tissue. A reticular network of CD34 - /CD10 + telocytes was present in the superficial dermis of the proximal nail bed. The mesenchymal cells of the deep part of the proximal nail bed dermis and the entire distal nail bed dermis were CD34 - /CD10 - . The adult nail mesenchyme is composed of 3 microanatomically distinct regions. Only the thumb has a distal hypodermis rich in mucinous material. The population of telocytes is relatively sparse compared with the fibroblastic population of the entire nail mesenchyme. The concept of onychodermis/onychofibroblasts is not valid. Nail telocytes have a dynamic immunohistochemical profile depending on whether they are located proximally or distally. The CD34 + /CD10 + profile correlates with the onychogenic epithelial region, while the CD34 - /CD10 + profile correlates with a spatial rearrangement of the nail epidermal bed.

Onychomatricoma: a clinicopathological, immunohistochemical, and molecular study of 10 cases highlighting recurrent RB1 deletion and the potential diagnostic value of LEF-1

AIMS

Onychomatricoma (OM), an uncommon benign fibroepithelial neoplasm of the nail unit, is sometimes diagnostically challenging for clinicians and pathologists. OM consistently expresses CD34, but no specific immunohistohemical markers or recurrent genetic alterations have been identified to date. Recent studies have suggested that Wnt signalling is a key molecular characteristic of OM.

METHODS AND RESULTS

Ten cases were analysed: four classical OM including two with pleomorphic cells; two superficial acral fibromyxoma-like variants of OM; three micropapilliferum variants of OM including one with pleomorphic cells; and one proliferating variant of OM. Immunohistochemically, the spindle cells were positive with CD34 (n = 10) and CD99 (n = 1), with focal reactivity for CD10 (n = 5). The epithelial component of the tumours expressed immunopositivity for LEF-1. Using array comparative genomic hybridization (aCGH), we demonstrated that all OM, including its variants that were tested (n = 8), harboured a few copy number alterations with losses of whole or part of chromosome 13 including the RB1 gene (n = 8) and chromosome 16 (n = 6).

CONCLUSION

We report a recurrent loss of RB1 (13q) as a possible driver molecular event in OM indicating a relationship between OM and other lesions of the spectrum of the so-called '13q/RB1' family of tumours. We did not identify a role for the Wnt/beta-catenin signalling pathway, as has been proposed in a recent study. LEF-1 could be a potential sensitive and specific marker of OM and should be used in the differential diagnosis between OM, superficial acral fibromyxoma, and the CD34-positive fibrosing family of tumours.

Molecular characterization of onychomatricoma: Spatial profiling reveals the role of onychofibroblasts in its pathogenesis

Onychomatricoma (OM) is a rare nail unit tumour with a characteristic presentation of finger-like projections arising from the nail matrix. Due to the lack of transcriptome information, the mechanisms underlying its development are largely unknown. To characterize molecular features involved in the disease pathogenesis, we used digital spatial profiling (DSP) in 2 cases of OM and normal control nail units. Based on the histological evaluation, we selectively profiled 69 regions of interest covering epithelial and stromal compartments of each tissue section. Dermoscopic and histopathologic findings were reviewed in 6 cases. Single-cell RNA sequencing of nail units and DSP were combined to define cell type contributions of OM. We identified 173 genes upregulated in stromal compartments of OM compared to onychodermis, specialized nail mesenchyme. Gene ontology analysis of the upregulated genes suggested the role of Wnt pathway activation in OM pathogenesis. We also found PLA2G2A, a known modulator of Wnt signalling, is strongly and specifically expressed in the OM stroma. The potential role of Wnt pathway was further supported by strong nuclear localization of β-catenin in OM. Compared to the nail matrix epithelium, only a few genes were increased in OM epithelium. Deconvolution of nail unit cell types showed that onychofibroblasts are the dominant cell type in OM stroma. Altogether, integrated spatial and single-cell multi-omics concluded that OM is a tumour that derives a significant proportion of its origin from onychofibroblasts and is associated with upregulation of Wnt signals, which play a key role in the disease pathogenesis.

Comparative Analysis of Type I Keratin Expression By Nail Consistency: An Immunohistochemistry Study

The nail plate is one of the essential structures of the nail apparatus and is highly keratinized, making it difficult to handle this tissue experimentally. Different types of nail consistency were identified by applying distal pressure to the nail plate. To analyze the relationship between the keratins expressed in the nail plate and nail consistency, we chose a sample of 32 adult individuals (age 49.81±3.21 y) with the same number of each sex, who had a similar percentage of nail consistency types (56.25% hard consistency nails and 43.75% soft consistency nails). Immunohistochemical analyses showed that hard consistency nails contain more keratin 17 than soft consistency nails (P=0.026). These novel results allow nail consistency to be defined by the differential expression of keratins in the nail plate, and have potential clinical implications for the diagnosis of possible nail disorders and/or systemic disease.

Proliferating Onychomatricoma. Clinical, Dermoscopical, and Pathologic Features of Onychomatricoma New Variant Resembling Onycholemmal/Squamous Cell Carcinoma

Proliferating onychomatricoma is a new challenging variant of onychomatricoma that can clinically and histologically mimic squamous cell carcinoma/onycholemmal carcinoma. This is a retrospective case series study of the clinicopathologic and dermoscopic features of 6 patients with a pathologic diagnosis of proliferating onychomatricoma, which was conducted in the dermatology and dermatopathology departments of 2 university hospitals and a private nail's dermatology consultation. The clinical, histological, and immunohistochemical features and follow-up of 6 patients with proliferating onychomatricoma were analyzed; we compare our finding with 6 cases of conventional onychomatricoma. The female-to-male ratio was 1:1 with involvement of fingers in 4 and toe in 2. Among the symptoms were verrucous lesion simulating squamous cell carcinoma, nail thickening, periungual erythema, and pain; symptom duration ranged from 5 to 8 years. Clinical, dermoscopical en face free-margin view, and nail-clipping histologic findings reveal a nail wall-like pattern with pitting. Intraoperative, noncontact, polarizing, light dermoscopy was available in 1 case and showed the typical signs of onychomatricoma (OM). Histologically, all cases showed a well-differentiated, infiltrative, squamous, proliferative lesion exhibiting a lobulated and cystic pattern of growth in the dermis. Abrupt keratinization reminiscent of trichilemmal keratinization, but corresponding in fact to keratogenous spheres, was noted as well as a dysmaturative epithelial pattern. No atypical cytomorphological changes were found. Proliferating onychomatricoma is a new variant of onychomatricoma, which can be misdiagnosed as squamous cell carcinoma/onycholemmal carcinoma; its proper recognition may minimize morbidity associated with inappropriate treatment. Proliferating OM can be differentiated from conventional OM clinically by a free-edge wall-like pattern and on histology of nail clipping by the relatively small size of the cavities. Dermoscopic and nail clipping attributes as free-edge honeycomb-like cavities associated with conventional OM are well established and permit a diagnosis of OM without an invasive nail biopsy. The free-edge wall-like pattern is a distinct new dermoscopic and nail-clipping pattern that should raise for the others onychogenic neoplasms and prompt the clinician to obtain a biopsy specimen. In addition to proliferating OM, the differential diagnosis includes a micropapilliferum variant of OM, onychocytic matricoma, and onychocytic carcinoma.

Single-cell RNA sequencing of human nail unit defines RSPO4 onychofibroblasts and SPINK6 nail epithelium

Research on human nail tissue has been limited by the restricted access to fresh specimen. Here, we studied transcriptome profiles of human nail units using polydactyly specimens. Single-cell RNAseq with 11,541 cells from 4 extra digits revealed nail-specific mesenchymal and epithelial cell populations, characterized by RSPO4 (major gene in congenital anonychia) and SPINK6, respectively. In situ RNA hybridization demonstrated the localization of RSPO4, MSX1 and WIF1 in onychofibroblasts suggesting the activation of WNT signaling. BMP-5 was also expressed in onychofibroblasts implicating the contribution of BMP signaling. SPINK6 expression distinguished the nail-specific keratinocytes from epidermal keratinocytes. RSPO4⁺ onychofibroblasts were distributed at close proximity with LGR6⁺ nail matrix, leading to WNT/β-catenin activation. In addition, we demonstrated RSPO4 was overexpressed in the fibroblasts of onychomatricoma and LGR6 was highly expressed at the basal layer of the overlying epithelial component, suggesting that onychofibroblasts may play an important role in the pathogenesis of onychomatricoma.

Characterization of the Onychomatricodermis Containing Onychofibroblasts of the Nail Unit : Histology, Immunohistochemistry, and Electron Microscopic Study

BACKGROUND

We recently discovered the presence of specialized nail mesenchyme below the nail matrix and designated it as onychomatricodermis.

OBJECTIVE

We did further research to characterize the histologic, histochemical, immunohistochemical and ultrastructural features of the onychomatricodermis containing onychofibroblasts in the nail unit.

METHODS

Ten polydactyly nail unit specimens and 8 nail matrix biopsies were included. H&E-stained slides were reviewed. We did Alcian blue staining and Masson Trichrome staining, as well as immunohistochemical staining for type I collagen, CD10, CD13 and CD34. In addition, polydactyly nail units were examined by transmission electron microscopy.

RESULTS

In H&E staining, the specialized mesenchyme called onychomatricodermis was observed to be slightly distant from the undersurface of the nail matrix and be less eosinophilic area. Onychomatricodermal onychofibroblasts showed light purple abundant cytoplasm. Masson Trichrome staining revealed fewer collagen fibers within the onychomatricodermis. In Alcian blue staining the onychomatricodermis showed mucin deposition within the onychofibroblasts and around them. Immunohistochemically, type I collagen was expressed much less in the onychomatricodermis while it was strongly expressed elsewhere in the nail unit. In nail matrix biopsy specimens onychomatricodermal onychofibroblasts expressed CD10 and CD13 strongly, and expressed CD34 as well. Ultrastructurally, collagen fibrils were found sparsely within the onychomatricodermis, whereas collagen fibrils were densely distributed in the dermis of other parts of the nail unit.

CONCLUSION

We demonstrated that there was less collagen expression in the onychomatricodermis containing onychofibroblasts. In addition, we found morphological and immunohistochemical features of onychomatricodermal onychofibroblasts (onychofibroblasts of Dongyoun). These findings support the presence of onychomatricodermis containing onychofibroblasts in the nail unit.

Onychomatricoma Micropapilliferum, a New Variant of Onychomatricoma: Clinical, Dermoscopical, and Histological Correlations (Report of 4 Cases)

This is a report of a previously undescribed type of onychomatricoma (OM) with an unusual clinical presentation as a thickened free edge of the nail plate without discernible cavities and distinguished histologically from the ordinary OM by 3 features: (1) the lack of cavitation at the proximal border of the nail plate and the small sizes of the cavities at the free edge of the distal nail plate; (2) a papillated epithelial hyperplasia pattern very different from the digitate pattern of the ordinary OM; and (3) a special pattern of matrical keratinization with pseudohorn cysts that mirror closely those found in onychocytic matricoma (OCM). Furthermore, the sex ratio and sites of the lesion seem different than those of conventional OM with the caveat that the numbers in this series are small. A practical approach to the diagnosis of onychogenic tumor mainly involves consideration of tumors that clinically present as localized longitudinal pachyonychia including melanoma and Bowen disease. Whether pachyonychia is caused by a thickened nail plate or by a localized band of subungual hyperkeratosis may not be clinically and dermoscopically obvious, and leucoxanthonychia or melanonychia is observed in OCM, OM, and onychocytic carcinoma. Therefore, the definitive diagnosis of these 3 onychogenic tumors is made by histopathology on nail clipping specimen or nail biopsy. OM is easily diagnosed as a fibroepithelial tumor keeping in mind its micropapilliferum variant which can simulate trichoblastoma or basal cell carcinoma on biopsies without nail plate. In these biopsies, the fibroepithelial portion of OM micropapilleferum resembles trichoblastoma including trichoepithelioma, or keratotic basal cell carcinoma, whereas the pseudohorn cysts may be mistaken for seborrheic keratosis. As previously indicated in the seminal report of OCM and perfectly demonstrated in this series, the pseudohorn cysts of both OCM and OM micropapilleferum have 2 distinct layers with a ring pattern, the prekeratogenous and keratogenous zone, and the transitional eosinophilic onychocytes become progressively clear with shadow cells. By contrast, horn cysts with hyaline and trichilemmal keratinization have rounded or irregular shapes, a thin inner layer of eosinophilic cells with large, oval, pale, vesicular nuclei, and are filled with compact keratinous masses without transition to onychocytic shadow cells. The squamous eddies of irritated seborrheic keratosis are easily differentiated from the pseudohorn cysts of OM by their inner layer of eosinophilic flattened squamous cells, and their loose or compact eosinophilic keratinous masses without transition to onychocytic shadow cells. To avoid confusion with the pseudohorn cysts of seborrheic keratosis which present a thin granular layer and laminated cornified cells, we propose to designate the pseudohorn cysts of both OM and OCM as keratogenous spheres. The papillae of the latter end as a tip without keratogenous zone explaining the microcavities. The microcavities getting in touch with the surface of the nail plate are responsible for the white dots (the so-called milia cysts) observed by dermoscopy both in OCM and OM micropapilliferum. The low, projecting ridges separated by the irregular longitudinal furrows explain the clinically irregular white line. The evenly thickened free edge of the distal nail plate is explained either by the small size of the cavities or the presence of a keratogenous zone at the tip of the papillae which manufacture a homogeneous thick nail plate. This free edge nail wall-like pattern (with or without a pitted wall) is in stark contrast to the usual honeycomb-like cavity pattern seen in conventional OM. It is inferred that these dermatoscopic findings could be clinical clues to differentiate both OCM and OM micropapilliferum from conventional OM. In the initial description of OCM, this entity was clearly differentiated from seborrheic keratosis. From time to time, these 2 lesions continue to pose problems in the histological differential diagnosis, and OCM with its various clinical presentations as leucoxanthonychia or melanonychia has been described using different names as subungual seborrheic keratosis, nail unit acanthoma, or longitudinal subungual acanthoma. These new superfluous synonymies add confusion in nail tumors. In the estimation of the author, these so-called new entities are OCM, if the histologic criteria of keratogenous spheres defined in this article are used. In sum, there are 2 clinicopathological variants of OM: macropapilliferum and micropapillerum. As OM micropapillerum has small cavities, the main differential diagnosis on nail clipping is onychocytic carcinoma.

The concept of nail matrix onychodermis (onychomatricodermis) in the nail unit: Histology and elastin immunohistochemistry

BACKGROUNDS

We previously demonstrated the presence of onychodermis below nail matrix and nail bed. Because nail matrix is a producer of nail plate, we hypothesized that onychodermis below nail matrix could be the nail counterpart of follicular dermal papilla. In this study, we sought to further characterize histologic, histochemical, and immunohistochemical features of nail matrix onychodermis.

METHODS AND RESULTS

Hematoxylin and eosin slides of 10 polydactyly nail units and 10 nail matrix biopsies from children and adults were reviewed. In polydactyly nail units, the onychodermis beneath nail matrix was characterized by onychofibroblasts showing abundant cytoplasm, and this area was slightly separated from the undersurface of the nail matrix. Nail matrix biopsy specimens also showed similar histology in the nail matrix onychodermis. Alcian blue stain demonstrated mucin deposition in onychofibroblasts within the nail matrix onychodermis. Immunohistochemically, elastin was rarely expressed in the nail matrix onychodermis while it was strongly expressed in the dermis of other areas of polydactyly nail units. Elastin was not expressed in follicular dermal papilla of terminal hair follicles of the scalp.

CONCLUSION

Our findings demonstrate the presence and localization of nail matrix onychodermis (onychomatricodermis). Our study also demonstrates similar elastin expression patterns in the onychomatricodermis and follicular dermal papilla.

Nail neoplasms

Nail neoplasms include all tumors occurring in the nail or periungual apparatus tissue. While some nail tumors can be similar to tumors located on the skin, others are unique. Both benign and malignant lesions can affect the nail apparatus. In particular, early malignant tumors like melanoma and squamous cell carcinoma can present similarly to onychomycosis or benign melanonychia and frequently missed by clinicians. Therefore, physicians should be aware of nail structures and the characteristics of nail tumors. Our review covers the normal nail structure and the most common nail tumors from benign to malignant.

Giant Onychomatricoma of the Great Toenail: Case Report and Review Focusing on Less Common Variants

Onychomatricoma is a rare benign fibroepithelial filamentous tumor originating from the nail matrix. It typically presents with the clinical tetrad of xanthonychia, pachyonychia, proximal splinter hemorrhages and increased transverse overcurvature of the nail plate. The giant variant can easily confuse the clinician due to its extensive nail dystrophy that can mask the characteristic features of this tumor. Benign (fibrokeratoma, ungual fibroma, onycholytic matricoma) and malignant entities (Bowen's disease, squamous cell carcinoma, onycholytic carcinoma) are mimics of the disease. Nail surgery can facilitate the diagnosis, which should always be confirmed by histology, as rare variants do exist.

A tumor hidden beneath the nail plate: report of a rare case of onychomatricoma with three-dimensional histopathological analysis and immunohistochemical study

Onychomatricoma is a tumor of the nail matrix which often presents with alterations in the nail plate while the tumor itself is concealed beneath the nail plate. It is a benign, biphasic fibroepithelial tumor which has to be differentiated from other subungual and periungual tumors. We report a rare case of onychomatricoma and describe a three-dimensional histopathological analysis and immunohistochemical patterns of onychomatricoma.

Invasive onychocytic carcinoma

Neoplasms originating from nail matrix keratinocytes are very rare. Onychomatricoma and onychocytic matricoma are benign tumors arising from nail matrix keratinocytes. Only one case of onychocytic carcinoma, the malignant counterpart of onychocytic matricoma, has been reported in the literature. Herein, we describe a case of invasive onychocytic carcinoma. Two biopsy specimens of the tumor, obtained at early and invasive stages, were examined histopathologically. The first biopsy specimen showed a retiform proliferation of eosinophilic and basophilic cells in the nail matrix. The second biopsy specimen showed a retiform basophilic cell proliferation with focal keratinization. Similar to normal nail matrix keratinocytes, the proliferating basophilic cells failed to express cytokeratin (CK)1, CK6 and CK10. Focal expression of hair-specific keratins, including K31, K85 and K86, was observed. On the basis of these findings, the tumor was identified as an invasive malignant tumor originating from nail matrix keratinocytes.

Onychomatricoma has channel-like structures on in vivo reflectance confocal microscopy

BACKGROUND

Onychomatricoma is a benign fibroepithelial nail matrix tumor that infiltrates the nail plate leading to multiple tunneled cavities lined with matrix epithelium and filled with serum. Diagnostic features of onychomatricoma on reflectance confocal microscopy (RCM) have not been previously described.

OBJECTIVE

We sought to demonstrate the feasibility of using RCM to diagnose onychomatricoma.

METHODS

Reflectance confocal microscopy was used to evaluate four patients with onychomatricoma before tumor excision. We evaluated the affected nail and one unaffected nail of each patient with VivaScope 1500 (Lucid Inc., Rochester, NY, USA).

RESULTS

Reflectance confocal microscopy evaluation of onychomatricomas revealed longitudinal dark areas and bright/grey lines, forming channel like structures. The channels were outlined by bright circular lines with grey dot centers. These RCM features correlated with the pathology of the onychomatricomas within the nail plate.

LIMITATIONS

Proximal portion of onychomatricoma was not reach by RCM.

CONCLUSIONS

Reflectance confocal microscopy can assist in rapid and noninvasive diagnosis of onychomatricoma showing characteristic channel like structures within nail plates.