Mummified ossified melanocytic naevus

ARTICLE

Ossification (secondary osteoma formation) within the skin may occur in a variety of conditions, including pilomatricomas, basal cell carcinomas, appendageal and fibrous proliferations, inflammation, trauma and calcification [1]. Even though melanocytic naevi (MN) represent the most frequent cause of secondary ossification of the skin [2] (referred to as "osteo-naevi of Nanta"), the occurrence of ossification within a MN is an unusual event, occurring in 1.4% of all lesions [2, 3]. Mummification, defined as the presence of keratinised epithelial cells with a characteristic appearance (shadow cells) is typical of pilomatricoma, a benign tumour originating from hair matrix cells that has recently been found to be caused by mutations in the beta-catenin gene [4]. To the best of our knowledge, mummification has never been described in association with an MN. We present herein a benign MN that combined both these unusual histological features, i.e. ossification and mummification.

Case report

A 31-year old female consulted for a long-standing lesion of the chin, diagnosed clinically as MN and excised for cosmetic reasons. Histologically, the lesion was dome-shaped and overlaid by a normal-looking epidermis. The upper and mid-dermis contained a dense proliferation of benign nevus cells, some of which were multinucleated; they were arranged in nests surrounding hair follicles. In the deep dermis, underneath the nevus cell nests, bony nodules were seen (Fig. 1); some of them were compact while others contained mature fatty tissue. Close to the bony nodules, a hair follicle was present that showed eosinophilic keratinization of its matrix, resulting in the formation of shadow cells (Fig. 2). In the vicinity of this follicle, round, well-demarcated masses consisting of shadow cells were seen in the dermis, surrounded by foreign-body type giant cells (Fig. 3). Shadow cells strongly expressed high MW keratin polypeptides, recognised by AE3 monoclonal antibody, but did not react with AE1 antibody, recognizing low MW keratin polypeptides. The adjacent dermis also contained keratin masses, surrounded by a foreign-body type reaction (Fig. 3).

Discussion

Mummification corresponds to a peculiar type of keratinisation of epithelial cells, resulting in the characteristic appearance of "shadow cells". These are keratinocytes with a dense, homogeneous, eosinophilic cytoplasm and a central unstained area representing the shadow of the disintegrated nucleus. Mummification characteristically occurs in pilomatricomas, the shadow cells being the end-result of keratinisation of the basophilic hair follicle matrix cells. Rarely, mummification can be observed in other lesions, including basal cell carcinomas, epidermoid cysts, keratoacanthomas, chondroid syringomas and inflamed hair follicles [5]; to the best of our knowledge, shadow cells have never been described in association with MN. Ossified MN may contain keratin masses within the dermis, but mummification has never been reported within them [2, 6]. In the present case, the occurrence of mummification within hair follicles could be secondary to preceding inflamation (folliculitis), although this was inconspicuous on the sections examined. Had the lesion not been excised, it can also be speculated that the changes observed within hair follicles would have resulted in the development of a true pilomatricoma. This association could be fortuitous, since we are not aware of any published cases associating these two lesions (despite the fact that both frequently occur on the face). However, it is tempting to speculate that mummification could be somehow related to osteoma formation. Indeed, the origin of ossification within an MN is not precisely known. It has been claimed that ossification may represent a disordered embryological process, the osteoma regarded as a hamartoma as the MN itself [7]; however, the prevailing theory is that osteoma formation is metaplastic, resulting from inflammatory changes involving the surrounding hair follicles [2, 6]. Since mummification may result in ossification (as happens in pilomatricoma), the present case suggests that ossification in MN may also occur as a consequence of mummification occurring within surrounding hair follicles.

Article accepted on 27/4/00

REFERENCES

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