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
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.