Sebaceous glands are small, sac-like exocrine glands located in the dermis of the skin. They are almost always connected to hair follicles, forming a functional unit known as the pilosebaceous unit. The only places on the human body completely free of these glands are the palms of the hands and the soles of the feet, where our grip needs to be firm and friction-dependent.
Density is highest on the face, chest, and back, which explains why these regions are most prone to oiliness and breakouts. In certain unique areas of the body, sebaceous glands exist independently of hair follicles and open directly onto the skin surface. These include the margins of the eyelids, the labia minora, and the nipples.
Unlike sweat glands that secrete fluid while keeping their cells intact, sebaceous glands use holocrine secretion. In this process, cells within the gland accumulate oily droplets as they mature. Once full, the entire cell disintegrates, releasing its contents—along with cellular debris—into the duct. This mixture is called sebum. Because the secretory cells are destroyed during production, constant cell division in the basal layer of the gland is required to replace them and maintain a steady supply of sebum.
Sebum is a complex mixture of lipids, including triglycerides, wax esters, squalene, and cholesterol. This oily film plays vital roles in maintaining skin health:
Sebaceous gland activity is largely driven by androgens, male sex hormones present in both males and females. Although these glands are active during fetal life due to maternal hormones (often causing temporary neonatal oiliness or cradle cap), they become largely dormant during childhood.
As puberty approaches, rising adrenal and gonadal androgen levels stimulate the sebaceous glands to enlarge and increase sebum output. This surge in oil production is a primary biological trigger for adolescent skin changes and contributes directly to the development of acne vulgaris.
When sebaceous gland function is disrupted, several common dermatological conditions can arise: