Accessory Structures of the Skin: Sebaceous Glands

Sebaceous glands are located in the dermis and are present over the entire body except the palms and soles. Most sebaceous glands are associated with hair follicles, while some large sebaceous glands called sebaceous follicles exist independently. These glands contain specialized cells called sebocytes that produce sebum.

Sebum is an oily secretion produced by sebaceous glands composed of triglycerides, cholesterol, fatty acids, squalene, and wax esters. It lubricates skin and hair while reducing water loss to retain moisture. Additionally, sebum contains antimicrobial peptides that inhibit microbial growth, creating an acidic environment that helps prevent bacterial and fungal colonization.

Sebaceous glands are holocrine glands because their mature sebocytes rupture to release sebum rather than using exocytosis. When cells burst, they release their accumulated secretory products directly into the epidermis through associated hair follicles or ducts. New sebocytes differentiate from surrounding tissue to replace those lost during secretion.

Excessive sebum production can block the gland's opening, trapping shed skin cells and sebum within hair follicles to form comedones. These clogged follicles become infected by Propionibacterium acnes bacteria, which consume sebum components and secrete enzymes that damage the follicle wall, causing inflammatory lesions including papules, pustules, and nodules.

Whiteheads and blackheads are both comedones formed when hair follicles become clogged by sebum and dead skin cells. Whiteheads remain covered by skin, keeping the clogged material hidden. Blackheads appear dark because the lipids in the exposed clogged follicle oxidize when exposed to air, creating the characteristic black discoloration.

Seborrheic dermatitis results from chronic inflammation of sebaceous glands, often triggered by stress or hormonal imbalance. In infants, it causes cradle cap with yellowish scales on the scalp. In adults, it produces poorly defined scaly, red patches on sebum-rich areas like the face, scalp, trunk, and upper chest, or dandruff limited to the scalp.

Most sebaceous glands associated with hair follicles release sebum through the follicle to the epidermis. Sebaceous follicles, which are large glands not connected to hair follicles, release sebum directly via their own ducts. Both pathways deliver the oily secretion to the skin surface for lubrication and protection.