Pilosebaceous Unit

The basic anatomical and developmental unit of the skin — a hair follicle plus the sebaceous gland(s) and small muscle that go with it. Most of the body’s surface is built from these units, and they explain why hair, oil, and skin sensation are so closely intertwined.

Components

• Hair follicle and shaft — the pit in the skin and the hair growing from it.
• Sebaceous gland(s) — one or two, opening into the follicle partway up its length.
• Arrector pili muscle — a tiny smooth muscle that erects the hair (“goosebumps”) and also helps squeeze sebum onto the surface.
• (Apocrine sweat gland) — in the armpits and groin, an apocrine gland joins the unit, which is then sometimes called the apopilosebaceous unit.

A single developmental origin

All of these parts grow from one embryonic skin bud and remain connected for life. This is why each hair on the body comes with its own built-in oil gland next to it, sharing the same duct.

Why it matters

• Skin physiology — sebum reaches the surface by flowing up the hair follicle; arrector contraction helps it along.
• Density and distribution — sebaceous-gland count tracks hair-follicle count fairly closely, but gland size and output varies dramatically by body region (face, chest, back), driven by hormones rather than by hair density.
• “Sebaceous follicle” — on the face, sebaceous glands are so much larger than the tiny vellus hairs they sit beside that the unit is sometimes called a sebaceous follicle. Acne-prone follicles are mostly of this type.
• Follicular skin disease — most occurs within the pilosebaceous unit, not in the surrounding skin: acne, folliculitis, and certain forms of hair loss all live here.

Related terminology

• Vellus hair — the short, fine, lightly pigmented body hair present nearly everywhere; sebaceous glands beside vellus hairs can be large.
• Terminal hair — the longer, thicker, pigmented hair of the scalp, beard, and (after puberty) armpit and pubic regions.
• Sebaceous follicle — a pilosebaceous unit where the sebaceous gland dominates over a small vellus hair.
• Apopilosebaceous unit — the version of the unit that includes an apocrine gland, found in the armpits, groin, and around the nipples.

Key idea

The pilosebaceous unit is the right unit of analysis for most of what happens at the skin surface. Treating hair, sebaceous glands, and the arrector pili muscle as one structure clarifies anatomy that would otherwise look like a coincidence.