Sebaceous gland
A sebaceous gland is a small gland attached to a hair follicle that produces sebum, an oily substance released into the follicle and onto the skin surface. Together the gland and follicle form what is called the pilosebaceous unit.
These glands rarely work alone. Each is attached to a hair follicle, and the pair is treated as one structure, the pilosebaceous unit, because what affects one affects the other.
Sebum is the oily material they produce, released into the follicle and reaching the surface through its opening. It contributes to the skin’s surface film and to the environment that organisms living on skin encounter.
Where these glands are dense determines a good deal of dermatology. Face, scalp, chest and upper back carry them in quantity, and those are exactly the sites where acne and seborrhoeic dermatitis appear. Distribution is not a coincidence; it is the explanation.
Activity varies through life and responds to hormonal signals, which is why the pattern of conditions involving these glands shifts with age.
Two common conditions illustrate different failure modes. In acne the follicle blocks and inflammation follows within the unit. In seborrhoeic dermatitis the skin reacts to a yeast that thrives in the oily environment the glands create. Same structure, quite different problems, which is why the anatomy is worth having straight.
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