The hair follicle from which the hair shaft emerges serves as a duct for conducting sebum, the oily product of the sebaceous gland, to the skin surface. The hair follicle and sebaceous gland together form the pilosebaceous unit. Sebum secretion is under hormonal control and increases substantially at puberty, then continues at an increased rate into advanced adult life. An important disease of the pilosebaceous unit is acne vulgaris, which occurs on the face, shoulders, and upper torso of adolescents. The hallmark of acne vulgaris is the comedo, commonly known as the blackhead. The comedo consists of a mixture of keratinous debris and sebum that lodges in the mouth of the pilosebaceous duct. Affected areas of the skin also exhibit inflamed papules, pustules, and cysts. The inflammation is caused by leakage of sebum and keratin debris outside the distended pilosebaceous duct. The bacillus Propionibacterium acnes, which populates the lesions, may also contribute indirectly to the inflammation by metabolizing the sebum to produce irritant fatty acids.

Eruptions can be brought on by contact with dirty oils or tars and halogenated hydrocarbons. Chloracne, caused by exposure to chlorine compounds, may develop in workers involved in the manufacture of cables, insecticides, fungicides, and herbicides. Industrial accidents involving the release of tetrachlorodibenzodioxin vapour have caused severe community outbreaks of chloracne. Acne rosacea, an acne of middle age that exhibits nodular swelling of tissues in the face, is a disease that occurs rarely among blacks.

Sweat glands

Most sweat glands in humans are eccrine (i.e., they secrete outwardly) and are under the control of the autonomic nervous system. Eccrine sweat glands are distributed uniformly over the body and function mainly in controlling temperature, although on the hands and feet they assist grip. Inability to secrete adequate sweat (anhidrosis) is usually a congenital abnormality. Hyperhidrosis is the abnormal excessive secretion of sweat, usually on the palms and soles and in the axillae. Emotional stress normally stimulates sweating in these areas, but in hyperhidrosis excessive sweating may follow minor stimuli or no obvious stimulus at all. Hyperhidrosis may be an occupational problem for those who handle books, papers, or metallic objects that can rust.

Most serious disorders of the eccrine sweat glands result from the retention of sweat. Prickly heat (miliaria) occurs in persons living in a hot, humid environment and is due to sweat retention brought on by the combination of increased sweat production and blockage of the duct from hydration of the surrounding horny layer of skin. The gland continues to secrete, and the pressure may rupture the sweat duct at its junction with the epidermis. In heatstroke, another disorder of the eccrine sweat glands, the glands are exhausted and unable to secrete sweat. Hyperpyrexia, an excessive rise in body temperature, may ensue and is sometimes fatal. The most common disorder of the eccrine glands, however, is excessive sweating of the palms, soles, armpits, and face. Emotional stress normally stimulates sweating in these areas, but emotional hyperhidrosis (increased sweating) occurs when excessive sweating follows minor or no obvious stimuli.

Apocrine sweat glands, which are normally limited to the armpits, perineum, and genitalia, differ anatomically and physiologically from eccrine sweat glands. In the apocrine glands the secretions are concentrated at the free end of the cell, which is cast off along with the secretory products. Apocrine secretion is oily, opalescent, and odourless when secreted, but acquires an offensive smell when it mingles with the bacteria of the skin. The actions of deodorants and antiperspirants are based mainly on an antibacterial effect, on mechanical blockade of the apocrine gland pores, or both.

Nails

The most common causes of diseased nails are chronic paronychia—infection of the soft tissue of the fingers between the lateral edges of the nail plates and the lateral nail folds—fungus infections, trauma, psoriasis, impaired circulation, and dermatitis. Chronic paronychia is an occupational hazard of dishwashers, bar workers, hairdressers, and vegetable preparers. When the cuticle is lost, water, bacteria, and yeasts seep under the nail fold and cause inflammation. The lateral nail folds extrude beads of pus when they are squeezed. The infection is commonly caused by the yeast Candida albicans, and when this organism is identified, infection elsewhere in the body should be sought.

Like the hair, the nails may be useful indicators of skin diseases and of internal disorders. Psoriasis causes pitting on the surface of the nails and an increase in their growth rate. Lichen planus causes a characteristic scarring of the posterior nail fold, leading to fusion of the posterior nail fold with the nail plate. Club-shaped deformity of the nail plate is associated with chronic chest disease and cyanotic congenital heart disease. A reduction in the rate of nail growth, a yellow discoloration of the nail plate, and a smooth curvature to the nails are all features of the yellow nails syndrome, a rare hereditary condition in which the lymphatic drainage system of the body is defective. In chronic anemia the nail plate may become concave (koilonychia). Transverse depressions of the nail plates called Beau’s lines result from temporary interruption of nail growth and are seen in a number of acute illnesses, including coronary thrombosis, pneumonia, and severe injuries. Various forms of interference with the blood supply may affect the nail matrix and produce irregular growth of the nails. Drug treatment may cause shedding of the nails. Photo-onycholysis—destruction of the nails from exposure to sunlight—occurs in patients taking photosensitizing drugs such as certain broad-spectrum antibiotics.