9.13 : Accessory Structures of the Skin: Sebaceous Glands

A sebaceous gland is a type of oil gland found almost all over the skin ( except palms and soles) and helps lubricate and waterproof the skin and hair. Most sebaceous glands are associated with hair follicles. They generate and excrete sebum, a mixture of lipids, onto the skin surface, thereby naturally lubricating the dry and dead layer of keratinized cells of the stratum corneum, keeping it pliable.

These glands that produce the oils on the skin and hair are holocrine glands. The mature sebocytes accumulate their secretory products and release them only when they burst. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion.

Sebaceous glands also have some antimicrobial properties. The lipids in sebum create an acidic environment on the skin's surface, which helps inhibit the growth of certain bacteria and fungi. However, excessive sebum production or an imbalance in the composition of sebum can contribute to the growth of bacteria and the development of skin conditions like acne.

Acne occurs when hair follicles become clogged by shed skin cells and sebum, causing comedones and non-inflammatory lesions. Comedones can take the form of whitehead and blackhead pimples. Whiteheads are covered by skin, whereas blackhead pimples are not; the black color occurs when lipids in the clogged follicle become exposed to the air and oxidize. Often comedones lead to infection by Propionibacterium acnes, a gram-positive, non-spore-forming, aerotolerant anaerobic bacillus found on skin that consumes sebum components. P. acnes secretes enzymes that damage the hair follicle, causing inflammatory lesions that may include papules, pustules, nodules, or pseudocysts, depending on their size and severity.

Seborrheic dermatitis can occur on sebum-rich areas of the face, scalp, and trunk. In infants, it causes cradle cap, yellowish scales on the crown, and front of the scalp. In adults, it can cause 'dandruff,' which is a non-inflammatory condition limited to the scalp. However, chronic inflammation of the sebaceous gland causes poorly defined scaly erythematous patches to appear, often affecting areas like the face, hands, and upper chest back.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 5.2: Accessory Structures of the Skin,