Germinotropic Onychocytic Matricoma: A New Histopathologic Subtype of Onychocytic Matricoma in the Light of the Microanatomy of the Normal Nail Unit, With Special Reference to Nail Mesenchyme

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.

Onychocytic Matricoma: A Clinical, Dermoscopic, and Pathological Analysis of 14 Cases

ABSTRACT

Onychocytic matricoma (OCM) is a benign neoplasm of the nail matrix. Only 18 cases of this tumor have been reported in the literature to date. We retrospectively analyzed the clinical features of 14 patients with OCM. The most common clinical feature was longitudinal xanthopachyonychia (n = 9), followed by longitudinal leukopachyonychia (=3) and longitudinal pachymelanonychia (n = 2). The most common clinical findings identified following dermoscopy and analysis at high magnification of classical photographs were free-edge thickening of the nail plate without pitting (n = 14), longitudinal ridging (n = 7), round white clods (n = 7), white dots (n = 7), and filiform hemorrhages (n = 7), followed by oval and linear white clods (n = 5), fuzzy lateral border (n = 5), and red-purple blood clods (n = 3). Nail clipping histopathology showed a thickened nail plate with multiple, small, round-to-oval spaces. The tumor expressed immunopositivity for LEF-1. Dermoscopy of the nail plate and nail clipping histology provides useful information with regards to the differential diagnosis with subungual squamous cell carcinoma and nail melanoma. Ex vivo-in vivo correlation facilitates a better dermoscopic assessment of this unique underrecognized disease. However, the differential diagnosis between OCM and onychocytic carcinoma requires biopsy of the tumor. LEF-1 as an onychogenic marker can be used to resolve the differential diagnosis between OCM and subungual longitudinal acanthoma/seborrheic keratosis.

Basic Concepts in Nail Pathology

ABSTRACT

Understanding nail pathology is complex for general pathologists and even for those dermatopathologists who do not receive many nail samples in their laboratories. In this article, we attempt to review some of the primary entities in nail pathology with the aid of modern interpretations of nail histology and embryology. We also provide diagrams that can aid in comprehending this field of pathology.

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.

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.

Onychocytic matricoma

Onychocytic matricoma is a newly described tumor of the nail matrix. Clinically, it presents with localized thickening of the nail plate and melanonychia. Histologically, it represents a benign acanthoma of onychocytes. There are 8 cases reported in the literature. A 12-year-old girl presented with localized melanonychia and concurrent thickening of the nail plate restricted to the area of pigmentation affecting the right thumb, with no history of trauma or pain. We report a case of this rare tumor occurring in late childhood and provide a comprehensive review of its clinical presentation and differential diagnosis. Both clinicians and dermatopathologists should be aware of the presentation of onychocytic matricoma and include it in their scope of diagnosis of longitudinal nail bands.