Tumours of the Skin Appendages

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The skin appendages that most frequently give rise to tumours are the pilosebaceous apparatus and the eccrine ducts and glands. Tumours of apocrine glands are rare.

Two common tumours are derived from hair follicles: trichoepithelioma and pilomatrixoma

Trichoepithelioma commonly presents as a skin-covered nodule, less than one centimetre in diameter, on the faces of teenagers and young and middle-aged adults. Both clinically and histologically, these lesions resemble small nodular basal-cell carcinomas. The difference in age incidence is the main distinction between the two.

Pilomatrixomas occur mainly in children, arising on the face, neck and shoulders. They are raised, knobbly, dermal tumours, up to 2 cm in diameter, which are often very pale or white with thin epidermis stretched over them.
¶Tumours or tumour-like nodules derived from the sebaceous component of the hair follicle most frequently occur on the face. The most common is {\B sebaceous hyperplasia,} in which there are a number of yellow papules, usually on the forehead, cheeks and the area around the nose. They mainly affect elderly men and, less commonly, women; clinically they may be mistaken for small basal-cell carcinomas.

Tumours derived from eccrine sweat glands and their ducts are not common, but there are many different types

The main types are:

• Syringoma. Multiple, small, pale white nodules or papules on the lower eyelids and cheek, usually in women.
• Eccrine poroma. A raised, solitary, fleshy tumour, usually situated on the sole but occasionally seen in the palm.
• Hidroacanthoma simplex. Usually located on the lower leg in elderly people, often confused clinically with basal-cell carcinoma or seborrhoeic keratosis.
• Clear-cell acanthoma. A flat or slightly raised reddish lesion on the legs.
• Clear-cell or eccrine hidradenoma. Usually located on the head or neck of young adults. Arises as a solitary nodular tumour which may appear cystic. Solid lesions are usually flesh-coloured or slightly red; cystic lesions may have a bluish tinge.
• Cylindroma. One of the most common tumours, the majority occurring on the head, scalp and neck as slow-growing, pink, raised nodules. They are particularly common in women, and multiple cylindromas may occur with an autosomal dominant mode of inheritance. Multiple tumours on the scalp are sometimes called 'turban tumour'.

Most skin cysts are derived from skin appendages

The most common skin cyst derived from a skin appendage is the so-called pilar cyst, which occurs on the scalp of the elderly as a well-circumscribed, round tumour. Histologically, lesions are lined by squamous epithelium similar to that of hair follicles and contain thick, white, compacted keratin. They may be multiple.

Epidermal cysts are also believed to be derived from hair follicles and are very similar to pilar cysts. They are mainly seen on the face, neck and upper trunk in young and middle-aged adults, and may occur in an area previously damaged by severe acne. Similar epidermal cysts may be seen at the angle of the eye in children (external angular dermoid) and are regarded as congenital malformations. Epidermal cysts may also be seen anywhere on the body following penetrating injury and are believed to be derived from squamous epithelium from the epidermis, implanted during trauma (inclusion dermoid). The skin and subcutaneous tissue around some epidermal cysts may show reddening and swelling, and the lesion becomes tender. This is commonly called 'infected epidermoid cyst,' but the inflammation is caused by leakage of keratin into surrounding tissues following trauma and is not associated with infection.
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