any protuberance on the skin resulting from the overdevelopment of the horny outermost covering of the skin, or epidermis, the main constituent of which is the protein keratin, which is synthesized in special cells of the skin, the keratinocytes. More generally, keratosis is any skin disorder attended by horny growths. The causes and lesions characteristic of keratotic skin disorders are varied, giving rise to some 20 different forms of keratosis; the more common ones are as follows:

1. Palmar and plantar keratosis is a congenital, often hereditary, thickening of the horny layer of the skin of the palms and soles, sometimes with painful lesions resulting from the formation of fissures.

2. Keratosis pilaris, also called ichthyosis follicularis, lichen pilaris, or follicular xeroderma, is a condition in which abnormal keratinization is limited to the hair follicles, manifesting itself as discrete, tiny follicular papules (solid, usually conical elevations); they are most commonly seen on the outer surface of the arms and thighs.

3. Seborrheic keratosis is a benign skin tumour, ordinarily developing as a small yellow or brown, sharply marginated, slightly raised protuberance, covered by a thin greasy scale; these lesions result from an abnormal increase in the number of keratinocytes and seldom either undergo malignant changes or disappear spontaneously.

4. Senile keratosis is a condition marked by sharply outlined, gray to grayish black, slightly elevated flat papules, which are generally associated with aging and exposure to sunlight. They are more frequent in sunny climates and occur on exposed areas of the body—unless ingestion of arsenic compounds has taken place in the past, in which case the lesions may appear on the palms, soles, and trunk. In some cases the lesions become cancerous, especially those on the lips and other mucous surfaces. See also wart; callus.