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Museum of Pathology

CONGENITAL MELANOCYTIC NAEVUS

CONGENITAL MELANOCYTIC NAEVUS

Clinical History

None available other than that the lesion was an incidental necropsy finding in a young Greek woman who died of leukaemia.

Pathology

The specimen is an ellipse of skin bearing an irregular ovoid pigmented naevus 7 x 4.5 cm. in diameter. The naevus is light brown in colour with multiple small foci of dark brown and black pigmentation. Numerous fine hairs arise from the surface of the naevus. Although tissue has not been taken for histology, the appearance suggests that this is a compound naevus (i.e. nests of naevus cells present in both the junctional zone of the epidermis and in the dermis).

Note

Melanocytic naevi are benign skin tumours composed of naevus cells. Naevus cells may be regarded as being identical to melanocytes although they have a somewhat different histological appearance in that they are arranged in clusters (nests) and lack the dendritic processes of epidermal melanocytes. Naevi are classified according to the site and arrangement of the cells nests on histological examination. The main types are : (1) junctional (nests at dermo-epidermal junction). (2) compound (nests at dermo-epidermal junction and in dermis) (3) intra-dermal (nests in dermis only) Junctional and compound naevi may undergo transformation to malignant melanoma. The malignant change always arises in the junctional component of a compound naevus. Most melanocytic naevi appear during childhood and adolescence, and gradually regress in adulthood and old age. Compound naevi tend to mature into intra-dermal naevi. About 1% of newborn babies have melanocytic naevi. These are called congenital melanocytic naevi. They tend to be larger than the lesions acquired later in life. Congenital melanocytic naevi greater than 20 cm. in diameter are called giant congenital melanocytic naevi. Giant congenital naevi have a high incidence of malignant transformation. 0ther (non-giant) congenital naevi may have a slightly increased risk of malignant transformation, but this is controversial.