Benign pigmented lesions

Simple melanocytic tumours

The melanocyte is generally believed to be derived from the neural crest. In normal skin, melanocytes appear as clear cells in the basal layer of the epidermis. They may become increased in number in the layers of the skin to form benign pigmented naevi (moles) which include:

  lentigo, which are present within the basal layer of the epidermis;

  junctional naevi, which occur as localised aggregations projecting into the dermis;

  dermal naevi, which occur entirely within the dermis;

  compound naevi, which show the features of both the junctional and dermal naevi.

These simple melanocytic turnouts may arise anywhere in the skin including the nail bed. They are seen occasionally in the conjunctiva of the eye, but are rare in other mucous membranes. Pigmented naevi may occur at any age but commonly appear in childhood and adolescence as small, brown, flat or slightly raised lesions in the skin. These are usually of junctional or compound type, but with increasing age they either atrophy or mature into dermal naevi. One or more melanocytic naevi are present in over 95 per cent of white adults. Naevi on the palms, soles and genitalia have an evil reputation as they were thought at one time to be especially prone to become malignant but, in fact, site is not a risk factor.

The blue naevus (Fig. 13.30) is a tumour of dermal melanocytes and occurs most commonly on the face, on the dorsum of the hands and feet, and over the sacrum in certain races (Mongolian blue spot). It is very darkly pigmented and because of its overlying layer of normal, although sometimes thinned, epidermis, looks shiny and blue or slate-grey in colour. Malignant change is exceptionally rare. Congenital naevi are rare. They are often darkly pigmented and may be hairy and papillary in appearance (Fig. 13.31). Occasionally, they cover extensive areas of skin, up to 25 per cent or more of the body surface. They may undergo malignant change even during childhood. With this exception, naevi are virtually always benign before puberty. Treatment may be indicated for cosmetic reasons, if by reason of its position the lesion is subject to the nuisance of repeated trauma, e.g. cut when shaving or rubbed by clothing. Although there is no evidence that trauma causes malignant change, if the history suggests that malignant change has occurred (see below under Malignant melanoma), surgical excision is the only acceptable treatment; all lesions must be sent for histological examination.

Cafe-au-lait macules are light-brown flat macules that are often apparent at birth. They are present as solitary lesions, or in increased numbers in syndromes such as neurofibromatosis and Albright’s syndrome.