Dermatology Online Journal

Osteo-nevus of Nanta (osseous metaplasia in a benign intradermal melanocytic nevus): an uncommon phenomenon
WaeI Al-Daraji
Dermatology Online Journal 13 (4): 16

Department of Histopathology, Darent Valley Hospital, Dartford, UK. waldaraji@aol.com

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Many interesting changes can uncommonly be seen in benign intradermal melanocytic nevi [1]. Cutaneous bone formation is an uncommon phenomenon in the skin that may be primary (excluding other pre-existing lesions such as osteoma cutis) and secondary (usually seen in relation to either inflammatory or neoplastic processes such as, basal cell carcinoma and acne) [2, 3].

We report a 46-year-old woman who presented to her general practitioner with a mole on right mandibular area of the face. An ellipse of skin (8 x 5 x 5 mm) bearing a benign looking nevus was seen macroscopically. Microscopic examination revealed a benign pigmented intradermal melanocytic nevus. No evidence of dysplastic changes or malignancy was seen. Interestingly, deep in the dermis, focal osseous metaplasia was present. Therefore, the diagnosis of osteonevus of Nanta (or osseous metaplasia in a benign intradermal melanocytic nevus) was made.

Figure 1	Figure 2a
Figure 1. Mole on the cheek below the ear
Figure 2. Hematoxylin and eosin stained sections showing a benign intradermal nevus with osseous metaplasia, osteo naevus of Nanta

Figure 2b	Figure 2c
Original magnification x 150 (2b) and x 250 (2c)

Overall, cases of osteo-nevus of Nanta are rarely reported in the English literature, but in reports of large series of skin osteomas, in the international literature, they represent approximately 20 percent of skin lesions with bone formation [2, 3, 4]. Considering the scarcity of published reports, this is somewhat surprising. Bone formation was usually present at the base of the melanocytic lesions and these lesions tend to be located in the upper part of the body, as in our case. In addition, there is a higher incidence of this phenomenon in females. This interesting observation remains largely unexplained. It is well known that osteoblasts have surface receptors for estrogens. The effect of such interaction releases cytokines, which results in down-regulation of bone resorption similar to that seen in osteoporosis.

Therefore, it can be speculated that the effects of estrogen on osteoblasts together with the lack of osteoclastic activity may explain why these lesions are seen more in female patients, although additional studies are required to confirm this attractive hypothesis. In our case no available history about estrogen therapy was available from her general practitioner.

Generally many hypotheses have been used to explain bone formation in skin such as increasing alkaline phosphatase and osteonectin activity. In addition the presence of fibroblast cells, deposits type one collagen and osteonectin around bone formation. Both are features of osteoblasts. However, differentiation of fibroblasts with subsequent bone formation, either with or without stimulatory factors, is likely to be the likely mechanism [3].

In conclusion, we report a unique case of osseous metaplasia within a benign intradermal melanocytic nevus, osteo-nevus of Nanta, in a female patient. There is also one reported case in the literature of a malignant melanoma arising within an osteo-nevus of Nanta by Culver et al. (1993) [5]. Therefore dermatopathologists should be aware of this association when they encounter osseous metaplasia in a melanocytic lesion, rather than just disregarding this interesting but unusual phenomenon as an unimportant finding. Different types of bone formation and the possible mechanisms underlying this uncommon even has been discussed and reviewed.

References

1. Sasaki S, Mitsuhashi Y, Ito Y. Osteo-nevus of Nanta: a case report and review of the Japanese literature. J Dermatol 1999; 26: 183-8.

2. Burgdorf W, Nasemann T. Cutaneous osteomas: a clinical and histopathologic review. Arch Dermatol Res 1977; 260: 121-35.

3. Conlin PA, Jimenez-Quintero LP, Rapini RP. Osteomas of the skin revisited: a clinicopathologic review of 74 cases. Am J Dermatopathol 2002; 24: 479-83.

4. Moulin G, Souquet D, Balme B. [Pigmented nevus and cutaneous ossifications. Apropos of 125 cases of osteonevi]. Ann Dermatol Venereol 1991; 118: 199-204.

5. Culver W, Burgdorf WH. Malignant melanoma arising in a nevus of Nanta. J Cutan Pathol 1993; 20: 375-7.

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