Hair Follicles and the Pilosebaceous Unit
Hair Follicles and the Pilosebaceous Unit — the combined structure of a hair follicle and its attached sebaceous gland, which together produce hair and sebum through a shared opening at the skin's surface. This unit is central to skin lubrication and is the site where acne typically develops.
Key points
- The pilosebaceous unit consists of the hair follicle, sebaceous gland, and an associated small muscle called the arrector pili.
- Sebum produced by the gland travels up the follicle to lubricate and help protect the skin and hair.
- Acne forms when the follicle becomes clogged with excess sebum and dead skin cells, often followed by bacterial activity and inflammation.
- The density and activity of these units vary across the body, being highest on the face, scalp, and upper chest.
The unit, part by part
| Component | What it is | What it does |
|---|---|---|
| Hair follicle | A tube of epithelium running from the surface into the dermis | Builds the hair shaft and channels sebum upwards |
| Bulb and dermal papilla | The follicle base, cupped around dermal cells and capillaries | Supplies the dividing cells and blood supply that grow hair |
| Sebaceous gland | A lobed gland opening into the upper follicle | Makes sebum by holocrine secretion |
| Sebaceous duct | Short channel from gland to follicular canal | Delivers sebum into the shared opening |
| Infundibulum | The top of the canal, above the sebaceous duct | Where keratin and sebum accumulate; where comedones form |
| Arrector pili muscle | Smooth muscle from follicle to upper dermis | Contracts to raise the hair, producing goosebumps |
How the unit is built
The follicle is an inward fold of epidermis, so its lining is continuous with the surface. At its base sits the bulb, cupped around the dermal papilla, where dividing cells keratinise and are pushed up as the hair shaft. Above the point where the sebaceous duct joins lies the infundibulum — the widest segment, and the part that blocks.
Follicles are not all alike. Terminal follicles are large and deep and produce thick, pigmented hair on the scalp, brows and lashes. Vellus follicles are small and shallow and produce the fine, pale hair covering most of the body. On the face, vellus follicles carry disproportionately large sebaceous glands, which is why facial skin is oily without being hairy.
The growth cycle in outline
Each follicle cycles rather than growing continuously. Anagen is the active growth phase, lasting years on the scalp but only weeks to months on the brows, arms and legs — which is why body hair reaches a short fixed length. Catagen is a brief regression of a few weeks; telogen is a rest of about three months, ending as the old hair sheds and a new one replaces it.
Follicles cycle independently rather than in waves, so shedding is continuous; losing on the order of 50 to 100 scalp hairs a day is usual. Illness, childbirth or marked weight loss can push many follicles into telogen at once, with visible shedding appearing a couple of months later. Persistent or patchy loss is for a clinician to assess.
Pores, and what pore size really reflects
What is called a pore is the follicular opening, the mouth of the infundibulum; sweat glands have their own, far smaller openings. Follicular openings have no muscle, no valve and no mechanism for opening or closing. Steam softens the sebum and keratin inside, and cold thickens sebum, so an opening can look different — but nothing has opened or shut.
Visible pore size mostly reflects three things: the follicle's own diameter, largely inherited and naturally larger on the nose and central face; how much sebum is distending it; and how much elastic support the dermis still gives, which falls with age and sun exposure. A blackhead is not trapped dirt — the dark colour is sebum and keratin oxidising in air.
Frequently asked
What is the difference between a pore and a hair follicle?
In everyday language a pore usually refers to the visible opening of a hair follicle at the skin's surface, through which hair emerges and sebum is released.
Why do acne breakouts happen in the same areas?
Areas with a high density of active pilosebaceous units, such as the face, chest, and back, produce more sebum and are therefore more prone to clogged follicles and breakouts.
Can pores open and close?
No. A follicular opening has no muscle or valve, so it cannot be opened by steam or closed by cold water. Heat softens the sebum inside it, which changes how it looks without changing the structure.
What is the difference between terminal and vellus hair?
Terminal hairs are long, thick and pigmented and come from large, deep follicles. Vellus hairs are the fine, pale hairs covering most of the body. Androgens can convert vellus follicles to terminal ones at some sites after puberty.
Are blackheads trapped dirt?
No. A blackhead is a follicular opening filled with sebum and shed keratin; the dark colour comes from that material oxidising in air, not from dirt.
Do sebaceous glands exist where there is no hair?
Almost always they are attached to a follicle, even a tiny vellus one. A few open directly onto surfaces such as the eyelid margin and lips, and the palms and soles have neither.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
