Spiradenoma and cylindroma originate from the hair follicle bulge and not from the eccrine sweat gland: an immunohistochemical study with CD200 and other stem cell markers.

BACKGROUND: Spiradenoma and cylindroma have historically been described as sweat gland tumors and have often been considered to be of eccrine lineage. However, (a) associations with trichoepitheliomas in Brooke-Spiegler syndrome or with trichoepitheliomas and milia in Rasmussen syndrome, (b) neoplastic combinations with hair follicle tumors in solitary cases, and (c) anatomical considerations support a folliculosebaceous-apocrine lineage. Follicular stem cell markers may allow for further characterization of these neoplasms. METHODS: A total of 97 tumors were examined for the expression pattern of follicular stem cell markers cytokeratin 15 (CK15), cytokeratin 19 (CK19), pleckstrin homology-like domain, family A, member 1 (PHLDA1), and CD200. The tumors were comprised of 27 spiradenomas, 30 cylindromas, 16 hidradenomas, 19 poromas, 4 dermal duct tumors and 1 hidroacanthoma simplex. RESULTS: All spiradenomas and cylindromas were CD200-positive whereas the other tumors classified as eccrine in lineage were all CD200-negative. CK15 also discriminated between spiradenomas and cylindromas and the remaining neoplasms but not to the degree of CD200. PHLDA1 and CK19 were noncontributory. CONCLUSIONS: It is concluded that both spiradenoma and cylindroma are not eccrine but follicular tumors. More specifically, it is proposed that both adnexal neoplasms are derived from the hair follicle bulge and as such represent one of the least differentiated follicular tumors.

The concept of the onychodermis (specialized nail mesenchyme): an embryological assessment and a comparative analysis with the hair follicle.

BACKGROUND: Recently, an intriguing concept was introduced into the literature that defines the area underlying the nail bed as a specific mesenchymal substructure unique to the nail organ. It has been termed onychodermis. The onychodermis expresses CD10 with remarkable specificity. Herein, we compare adult and fetal human hair follicles with fetal nail organs in an attempt to draw analogies for the mesenchyme associated with both adnexal structures. METHODS: We examined immunohistochemically samples from adult and fetal hair follicles for the expression of CD10, CD34 and the mesenchymal stem cell marker nestin and compared the antigen profile with that of the fetal nail organ. RESULTS: The CD10-positive/CD34-negative onychodermis is prominently visible at the end of the second trimester. A corresponding follicular structure was not identified, either in the adult or in the developing hair follicle. Nestin staining does not define the onychodermis. CONCLUSIONS: The concept of the onychodermis is equally valid in the developing nail organ where it is also defined by its expression for CD10. Its function may be related to the anchorage of the overlying nail bed but may also involve a more dynamic role in the induction of hard keratins in the latter, contributing to the formation of the nail plate.

The immunohistochemical differential diagnosis of microcystic adnexal carcinoma, desmoplastic trichoepithelioma and morpheaform basal cell carcinoma using BerEP4 and stem cell markers.

BACKGROUND: Microcystic adnexal carcinoma (MAC), desmoplastic trichoepithelioma (DTE) and morpheaform basal cell carcinoma (BCC) frequently impose a considerable differential diagnostic challenge and immunohistochemistry is often used as a differentiating diagnostic adjunct. METHODS: Using standard immunohistochemical techniques, we examined 21 examples of DTE, 17 examples of morpheaform BCC and 10 examples of MAC for the expression of BerEP4, a marker of epithelial cells, and of three stem cell markers, pleckstrin homology-like domain, family A, member 1 (PHLDA1) [T cell death-associated gene 51 (TDAG51)], cytokeratin 15 (CK15) and cytokeratin (CK19). RESULTS: All but one MAC was negative for BerEP4 and all morpheaform BCC expressed BerEP4. Sixteen out of 21 DTE were immunoreactive for BerEP4. All 21 DTE were PHLDA1 positive and all 17 morpheaform BCC were PHLDA1 negative. MAC showed a mixed staining pattern for PHLDA1. CK15 was expressed in 20/21 DTE, whereas the majority of cases of MAC and morpheaform BCC were CK15 negative. CK19 stained more MAC than DTE and morpheaform BCC. CONCLUSIONS: BerEP4 differentiates between MAC and morpheaform BCC but not between MAC and DTE whereas PHLDA1 differentiates between DTE and morpheaform BCC but shows variable staining in MAC. CK15 and CK19 are helpful adjuncts in the differential diagnosis of sclerosing adnexal neoplasms but are second in line to BerEP4 and PHLDA1. We propose an algorithm for the immunohistochemical work-up of sclerosing adnexal neoplasms.

Basaloid tumors in nevus sebaceus revisited: the follicular stem cell marker PHLDA1 (TDAG51) indicates that most are basal cell carcinomas and not trichoblastomas.

BACKGROUND: Until the 1990s, basal cell carcinoma (BCC) was viewed as the most common epithelial neoplasm developing in association with nevus sebaceus (NS). Currently, trichoblastoma is thought of as the most prevalent basaloid neoplasm in NS. The follicular stem cell marker pleckstrin homology-like domain, family A, member 1 (PHLDA1) also known as T-cell death-associated gene 51 (TDAG51) labels trichoepithelioma (TE) but not BCC. Therefore, we explored its usefulness in basaloid neoplasms developing in NS. METHODS: We studied immunohistochemically PHLDA1 in 10 nodular BCCs, 11 TEs, 11 trichoblastomas and 25 NS with basaloid tumors. Additionally, we examined the expression of BCC marker BerEP4 and the distribution of Merkel cells that function as surrogate markers for benign follicular neoplasms. RESULTS: Nineteen of the 25 basaloid tumors in NS were PHLDA1-negative comparable to BCC arising de novo and six tumors were PHLDA1-positive comparable to solitary trichoblastomas and TEs. Fewer Merkel cells were seen in BCCs associated with NS when compared with trichoblastoma. BerEP4 did not discriminate between the neoplasms. CONCLUSIONS: We raise concern that the unquestioned assessment that basaloid tumors developing in association with NS represent mostly trichoblastomas and not BCC may not be true. This influences clinical care, as it is paramount in the decision of whether to excise these lesions or not.

Nail stem cells.

Our knowledge on stem cells of the hair follicle has increased exponentially after the bulge was characterized as the stem cell niche two decades ago. In contrast, little is known about stem cells in the nail unit. Whereas hair follicles are plentiful and easy to access, the human body has only twenty nails and they are rarely biopsied. Therefore, examining fetal material offers unique advantages. In the following mini-review, our current knowledge on nail stem cells is summarized and analogies to the hair follicle stem cells are drawn.

The ventral proximal nail fold: stem cell niche of the nail and equivalent to the follicular bulge--a study on developing human skin.

BACKGROUND: Although the bulge is well characterized as a stem cell niche of the hair follicle, comparatively little is known about the location of stem cells in the nail. Herein, we describe the spatiotemporal expression pattern of six stem cell markers in the developing human nail and compared it with the embryonic and fetal human hair follicle. The areas of proliferative activity were additionally examined using labeling with Ki-67. METHODS: We examined immunohistochemically samples from embryonic and fetal human nail, hair and skin for the expression of cytokeratin 15 (CK15, two clones), cytokeratin 19 (CK19), PHLDA1, CD200, nestin and Ki-67 using standard techniques. RESULTS: CK15 (clone LHK15), CK19 and PHLDA1 are negative in the nail and hair matrix but positive in the ventral proximal nail fold and in the follicular bulge. Over the course of embryogenesis they display a highly specific spatiotemporal expression pattern both in the nail and in the hair follicle. CONCLUSIONS: We propose that at least during embryogenesis the proximal ventral nail fold represents the niche for the nail stem cells. In contrast to animal experiments, autoradiographic pulse-chasing studies cannot be performed in human, and immunohistochemical studies are a valid alternative although they have their limitations. Further studies on adult human nail units are suggested.

Stem cell markers can help identify adnexal tumor differentiation when evaluated in the context of morphology: methodology matters.

Modern medicine is increasingly being influenced by the concepts of stem cell biology. In dermatopathology, one of the most intriguing yet unresolved questions is the lineage of adnexal neoplasms. With an expanding arsenal of stem cell markers, we have the tools available to address this question. The application of stem cell markers in dermatopathology requires intimate familiarity with the underlying basic science in order to avoid misinterpretation of their staining pattern. At the same time, the basic science itself can be misleading when immunohistochemical staining patterns for adnexal tumors are under consideration. The markers critically reviewed in this manuscript include B-lymphocyte-induced maturation protein 1 (Blimp-1), nestin, CD34, p63, cytokeratins 15 and 19 and the most recent ones appearing on the horizon: pleckstrin homology-like domain, family A, member 1 (PHLDA1) (TDAG51) and CD200. Of those, cytokeratin 15, PHLDA1 (TDAG51) and CD200 offer the most promise. A combined approach of modern morphology with basic science holds potential to advance one important aspect of our field further with the final goal of logically classifying adnexal tumors based on stem cell biology.

Follicular stem cell marker PHLDA1 (TDAG51) is superior to cytokeratin-20 in differentiating between trichoepithelioma and basal cell carcinoma in small biopsy specimens.

BACKGROUND: Biopsies submitted to dermatopathologists are becoming increasingly smaller in size and thus the available diagnostic material is reduced. The distinction between trichoepithelioma and basal cell carcinoma remains challenging, particularly if tissue is limited. Merkel cells, which can be highlighted by means of cytokeratin-20 (CK20) immunostaining, are used as a surrogate marker for the diagnosis of trichoepithelioma, as Merkel cells commonly colonize trichoepithelioma but are generally lacking in basal cell carcinomas. In the current study, we examined the expression of a recently characterized follicular stem cell marker, PHLDA1 (pleckstrin homology-like domain, family A, member 1), also known as TDAG51 (T-cell death-associated gene 51). METHODS: Using standard immunohistochemical techniques, we examined 19 trichoepitheliomas and 11 basal cell carcinomas for the expression of PHLDA1 and compared it with CK20 expression. RESULTS: All 19 trichoepitheliomas were immunoreactive for PHLDA1 and all 11 basal cell carcinomas lacked PHLDA1 expression. Two of eleven basal cell carcinomas harbored CK20-positive Merkel cells. Three trichoepitheliomas lacked secondary CK20-positive cells. CONCLUSIONS: Our results suggest that PHLDA1 represents a practical and easily used tool that can be applied to the differentiation of trichoepithelioma and basal cell carcinoma in small biopsy specimens. Rather than searching for CK20-positive Merkel cells, assessing PHLDA1 expression allows the differential diagnosis between trichoepithelioma and basal cell carcinoma to be solved at scanning magnification.

Does the peritumoral stroma of basal cell carcinoma recapitulate the follicular connective tissue sheath?

BACKGROUND: There are compelling embryologic and anatomic relationships within adnexal tumors. However, these are mostly perceived within the epithelial component while the stromal component of the tumors is frequently overlooked. In postnatal skin, nestin is almost exclusively expressed in the perifollicular mesenchyme. This study examines the expression of this neuroepithelial stem cell protein in trichoblastoma/trichoepithelioma and in basal cell carcinoma (BCC), which is increasingly being viewed as follicular in nature. METHODS: We employed standard immunohistochemical methods with three different antibodies to examine the expression of nestin in 25 BCCs and compared the staining pattern with that of 7 trichoblastomas and 11 trichoepitheliomas. RESULTS: Nestin is expressed in the peritumoral stroma of all tumors examined and is limited to the immediate layer of mesenchymal cells surrounding the tumor epithelium. In BCC, nestin-immunoreactive cells are found as a sheath of thin, spindled fibroblasts, while reactive cells are plump in trichoepitheliomas/trichoblastomas. CONCLUSIONS: We postulate that the peritumoral stroma of BCC imitates the perifollicular connective tissue sheath, while in contrast that of trichoepithelioma/trichoblastoma is similar to the papillary and immediate peripapillary follicular mesenchyme. Further functional and animal experimental studies are needed to test this hypothesis.

Expression of embryonic stem cell markers SOX2 and nestin in dermatofibrosarcoma protuberans and dermatofibroma.

BACKGROUND: Dermatofibrosarcoma protuberans (DFSP) was recently proposed to originate from a nestin-positive stem cell. In postnatal skin, nestin and another embryonic stem cell marker, SOX2, display a similar expression pattern with immunoreactivity in the hair follicle papilla and scattered cells of the perifollicular connective tissue sheath. The distribution pattern differs only in early embryogenesis, when nestin but not SOX2 is also expressed throughout the entire interfollicular dermis. We speculated that DFSP would not only be nestin-positive but also SOX2-positive. METHODS: With appropriately reacting external and internal controls, we examined 24 examples of DFSP for SOX2 and nestin. For comparison, we included 10 dermatofibromas (DFs). RESULTS: All 24 cases of DFSP were immunoreactive for nestin but negative for SOX2. The DFs were both nestin-negative and SOX2-negative. CONCLUSIONS: The observed staining pattern may indicate that DFSP derives from a subtype of nestin-immunoreactive mesenchymal stem cell that is different from the nestin- and SOX2-positive cell population of the perifollicular mesenchyme. Alternatively, nestin expression in DFSP may represent a recapitulation of the staining pattern in early embryogenesis without necessarily indicating that the nestin-positive cells represent stem cells. Also, DFSP may derive from hair follicle-associated mesenchymal stem cells that have lost their SOX2 expression.

Spatiotemporal expression pattern of neuroepithelial stem cell marker nestin suggests a role in dermal homeostasis, neovasculogenesis, and tumor stroma development: a study on embryonic and adult human skin.

BACKGROUND: Whereas keratinocytic bulge stem cells are well characterized, comparably little is known about cutaneous mesenchymal stem cells. The follicular connective tissue sheath is proposed as a niche for dermal stem cells. OBJECTIVE: Because the neuroepithelial stem cell marker nestin represents a marker for mesenchymal stem cells in various tissues, our aim was to characterize its spatiotemporal expression pattern in the skin with special reference to the follicular mesenchyme. METHODS: We studied immunohistochemically nestin expression over the course of human cutaneous embryogenesis, in postnatal skin, in scalp wounds, and in the peritumoral stroma of basal cell carcinomas and compared its expression with that of other known mesenchymal markers. RESULTS: Nestin is expressed throughout the entire early embryonic dermis but confined later during development to the follicular connective tissue sheath, where it can also be found in postnatal human hair follicles. Its expression is up-regulated in scalp wounds and the nestin-positive cells seem to originate from the follicular mesenchyme. Nestin is also expressed in a thin layer of fibroblasts in the immediate vicinity of basal cell carcinomas. LIMITATIONS: The examination for nestin expression of scalp wounds is considered preliminary, because we examined scalp wounds representing re-excisions of previously diagnosed neoplasms from which we had no exact time table available as to when the original excision took place. CONCLUSION: We propose that nestin functions as a stem cell marker of the follicular mesenchyme and has a major regulatory role in dermal homeostasis, cutaneous neovasculogenesis, and tumor stroma development.

Skin mesenchymal stem cells: prospects for clinical dermatology.

Stem cell-based therapies are expected to have a great impact on the medicine of the 21st century. The focus of dermatologic stem cell research is on the epidermis and the hair follicle. In contrast, the characterization of stem cells in the mesenchymal compartments of the skin has largely escaped the attention of the dermatologic community. This is surprising because the dermis may represent a larger reservoir for adult stem cells than the epidermis and the hair follicle together. In 2001, mesenchymal stem cells residing within the dermis were first isolated. They have the capacity to differentiate into adipocytes, smooth muscle cells, osteocytes, chondrocytes, and even neurons and glia as well as hematopoietic cells of myeloid and erythroid lineage. The perifollicular connective tissue sheath and the papilla crystallize as the likely anatomic niche for these multipotent dermal cells. These previously unidentified mesenchymal stem cells have the potential to function as an easily accessible, autologous source for future stem cell transplantation. Potential therapeutic applications include the treatment of acute and steroid-refractory graft-versus-host disease, systemic lupus erythematosus resistant to currently available therapies, or idiopathic pulmonary fibrosis. The neuronal differentiation potential of cutaneous mesenchymal stem cells may also be exploited in the treatment of neurodegenerative disorders. The most immediate impact can be expected in the field of wound healing. In line with the cancer stem cell hypothesis, the potential contributions to dermatopathology include a conceptual understanding of mesenchymal skin-based neoplasms as evolving from a genetically altered dermal stem cell clone.

Cutaneous mesenchymal stem cells: status of current knowledge, implications for dermatopathology.

Stem cell biology is currently making its impact on medicine, which will probably increase over the next decades. It not only influences our therapeutic thinking caused by the enormous plasticity of stem cells but also affects diagnostic and conceptual aspects of dermatopathology. Although our knowledge of the keratinocytic stem cells located within the follicular bulge has exploded exponentially since their discovery in 1990, the concept of cutaneous mesenchymal stem cells (MSCs) is new. Described initially in 2001 in mice, MSCs later were also found in the human dermis. The connective tissue sheath and the papilla of the hair follicle probably represent the anatomical niche for cutaneous MSCs. In line with the cancer stem cell hypothesis, mutations of these cells may be the underlying basis of mesenchymal skin neoplasms, such as dermatofibrosarcoma protuberans. Furthermore, research on cutaneous MSCs may impact our thinking on the interaction of the epithelial component of skin neoplasms with their surrounding stroma. We are only in the early stages to recognize the importance of the potential contributions of cutaneous MSC research to dermatopathology, but it is not inconceivable to assume that they could be tremendous, paralleling the early discovery of the follicular bulge as a stem cell niche.

Blimp-1: a marker of terminal differentiation but not of sebocytic progenitor cells.

BACKGROUND: The role of stem cells in maintaining the sebaceous gland throughout the various stages of life is not satisfactorily resolved. In a recent article, the transcription factor B lymphocyte-induced maturation protein 1 (Blimp-1) was proposed as a marker of a population of unipotent progenitor cells that reside in the sebaceous gland, regulating its size and activity. METHODS: We used standard immunohistochemical methods to examine Blimp-1 expression in samples from embryonic, fetal and adult human skin and in 119 sebaceous lesions comprising all major categories of sebocytic lineage, including hamartomas, cysts and benign and malignant neoplasms. RESULTS: Blimp-1 is expressed late in embryonic development and is restricted to the evolving sebaceous gland, the terminally differentiating components of the hair follicle and nail organ and the granular layer. This pattern is preserved into adult life. In all sebaceous lesions, Blimp-1 labels only the most mature cellular constituents. CONCLUSIONS: The reported expression pattern is difficult to reconcile with a function of Blimp-1 as a marker for sebocytic progenitor cells but indicates a major role in terminal differentiation. Within the interfollicular epidermis, its exclusive localization to the granular layer suggests a central function in skin barrier homeostasis in the human.

Sox9, more than a marker of the outer root sheath: spatiotemporal expression pattern during human cutaneous embryogenesis.

BACKGROUND: The sex-determining gene Sox9 was recently unexpectedly found to have an essential role in outer root sheath differentiation. It was also characterized as a general marker of basal cell carcinoma. Herein, we describe its spatiotemporal expression pattern outside the hair follicle during human cutaneous embryogenesis. METHODS: We examined immunohistochemically samples from embryonic and fetal human skin for the expression of SOX9 using standard techniques. For comparison reasons, we also included scalp skin from adults. RESULTS: SOX9 is expressed in the developing nail organ, eccrine glands, blood vessels and melanocytes/melanoblasts. In the nail organ, the nail bed but not the nail matrix was immunoreactive for SOX9. In plantar skin, SOX9 specifically labels the evolving eccrine glands but not the interfollicular keratinocytes. CONCLUSIONS: The distinctive expression pattern of SOX9 during human cutaneous embryogenesis indicates a key role in skin homeostasis that includes but goes beyond its role in outer root sheath differentiation. Studying immunohistochemical markers in developing human skin has the potential to further our understanding of adult skin physiology and to deepen our concepts especially of the histogenesis of adnexal tumors (including those of the nail unit) and the relationship of the various adnexal structures to each other.

Expression pattern of GATA-3 in embryonic and fetal human skin suggests a role in epidermal and follicular morphogenesis.

BACKGROUND: The transcription factor GATA-3 was recently identified as a master regulator in the specification of the inner root sheath. Additionally, it seems to play a role in skin barrier physiology. p63 binds and transactivates the GATA-3 promoter. While the expression profile of GATA-3 is delineated for the mouse, little is known about its expression in the adult human hair follicle and no studies are published about its distribution during human cutaneous embryogenesis. METHODS: We examined samples from embryonic, fetal and adult human skin for the expression of GATA-3 using immunohistochemistry. RESULTS: GATA-3 is expressed late during human skin development. Its expression pattern is comparable to the mouse and confined to the Huxley layer and inner root sheath cuticle but sparing the Henle layer. In addition, GATA-3 localizes to the spinous cell layer of the interfollicular epidermis. CONCLUSIONS: From the described expression pattern, it is highly probable that GATA-3 plays a role in follicular and epidermal morphogenesis. What the anatomically confined expression of GATA-3 to the spinous layer means biologically for the physiology of the skin is still unclear. Likewise, it still needs to be shown if GATA-3 could be exploited in the diagnosis of adnexal neoplasms.

Disseminated Fusarium infection originating from paronychia in a neutropenic patient: a case report and review of the literature.

Fusarium is a saprophytic organism that is widely found distributed in soil, subterranean and aerial plants, plant debris, and other organic substrates. It can cause local tissue infections in immunocompetent patients, such as onychomycosis, bone and joint infections, or sinusitis. The incidence of disseminated disease has notably increased since the initial cases of disseminated Fusarium were described, particularly affecting immunocompromised patients with hematologic malignancies. We report a 39-year-old man hospitalized with newly diagnosed acute myelocytic leukemia who developed disseminated Fusarium infection originating from toenail paronychia in the setting of neutropenia. Pathologic diagnosis of Fusarium is difficult because the septate hyphae of Fusarium are difficult to distinguish from Aspergillus, which has a more favorable outcome. Cultures of potential sources of infection as well as tissue cultures are essential in identifying the organism and initiating early aggressive therapy.

A case of recurrent pseudolymphomatous folliculitis: A mimic of cutaneous lymphoma.

Pseudolymphomatous folliculitis is a rare entity. We present a 62-year-old man with a recurrent solitary nodule on his nose requiring multiple excisions. Microscopic examination of the excisions showed a dense lymphocytic infiltrate containing numerous histiocytes and S100+, CD1a+ dendritic cells that surrounded and infiltrated hypertrophic hair follicles. Diffuse sheets of CD3+ T cells and nodular clusters of CD20+ B cells were also seen. There was normal reactive pattern of follicular centers. Light chain restriction was not detected. T-cell receptor and immunoglobulin heavy chain gene rearrangements by polymerase chain reaction revealed negative findings. A diagnosis of pseudolymphomatous folliculitis was made based on the hypertrophic hair follicles, periadnexal S100+ and CD1a+ dendritic cells, and negative clonal gene rearrangement study findings. This case of recurrent pseudolymphomatous folliculitis is instructive because of the resemblance to cutaneous lymphomas and cutaneous lymphoid hyperplasias, and the need for correct diagnosis to avoid overtreatment of this indolent condition.

A scanning microscopic clue to the diagnosis of arthropod assault reaction: alteration of interstitial tissue is more common than a wedge-shaped inflammatory infiltrate.

BACKGROUND: A scanning microscopic clue to the diagnosis of arthropod assault reactions is a wedge-shaped inflammatory infiltrate. However, to describe an inflammatory infiltrate as wedge-shaped or not involves a high degree of subjectivity. METHODS: We studied hematoxylin and eosin-stained sections of 137 biopsies of arthropod assault reactions for epidermal and dermal changes and for the composition, density and depth of the inflammatory infiltrate. RESULTS: We found a wedge-shaped inflammatory component in only 10.2% of the cases. A much more common feature is an alteration of the interstitial tissue present in 85.4% of the biopsies. It consisted of a narrowing of the spaces between the collagen bundles, which was readily observable on scanning magnification. On higher magnification, a loosely textured basophilic material was often noted within the dermis. CONCLUSIONS: The hitherto often emphasized wedge-shaped configuration of the inflammatory component of arthropod assault reactions is not of great diagnostic value. The altered interstitial tissue is easily recognizable by its diminished interstitial spaces at low power magnification and can serve as a scanning magnification clue to the diagnosis of arthropod assault reactions.

p16 expression differentiates between desmoplastic Spitz nevus and desmoplastic melanoma.

BACKGROUND: Loss of p16 in melanomas reflects worse outcomes for patients. It is associated with depth of invasion, ulceration, vascular invasion, lymph node metastases, metastases, recurrence of melanoma and decreased 5-year survival. Desmoplastic melanoma is an insidious malignant melanoma subtype that commonly occurs on sun-damaged skin of the head and neck area in elderly patients. The diagnostic conundrum occurs with confusion of desmoplastic melanoma with scars, hyalinizing blue nevi, desmoplastic Spitz nevi and diffuse neurofibromas. METHODS: The present study uses immunohistochemistry with a p16 antibody to differentiate desmoplastic Spitz nevi (n = 15 cases) from desmoplastic melanomas (n = 11). To date, no other studies have been published defining the expression pattern of p16 in desmoplastic melanoma. RESULTS: 81.8% of desmoplastic melanomas were negative for p16 and 18.2% were only weakly stained. In contrast, all desmoplastic Spitz nevi were moderately to strongly positive for p16. CONCLUSIONS: The staining pattern for p16 in desmoplastic melanomas and Spitz nevi in conjunction with the histopathologic features, S100 staining, Ki67 proliferation index and clinical scenario may aid in the difficult differential diagnosis between these two entities. Further confirmatory studies are indicated.