Sebaceous Glands
Oil glands of the skin. On most of the body, each one is paired with a hair follicle as part of a pilosebaceous unit; a few specialized glands open directly onto a surface instead.
What they produce
Section titled “What they produce”Sebum — an oily, waxy mixture of lipids. It also carries fat-soluble vitamin E to the skin surface and contains fatty acids (including sapienic acid, almost unique to humans) with mild antimicrobial activity.
Where they are
Section titled “Where they are”Nearly everywhere there is hair, but very unevenly distributed:
| Region | Density |
|---|---|
| Forehead, cheeks, chin, scalp | 400–900 per cm² — by far the densest |
| Upper chest and upper back | ~200–400 per cm² |
| Most of the rest of the body | ~50–100 per cm² |
| Palms and soles | None |
Across the whole body, estimates range from a few hundred thousand to around one million glands. Most open into a hair follicle; a few open directly onto skin or mucous membrane — see the specialized glands below.
What they do
Section titled “What they do”- Lubricates and waterproofs skin and hair.
- Slows water loss from the skin.
- Antimicrobial — sebum’s fatty acids suppress many bacteria and fungi, and together with sweat it maintains the skin’s slightly acidic “acid mantle” (pH ~4.5–5.5), hostile to most pathogens but friendly to the normal skin microbiome.
- Forms vernix caseosa — the waxy white coating newborns are born with is largely sebum; it protects the fetus’s skin in amniotic fluid and eases birth.
How they secrete
Section titled “How they secrete”By holocrine secretion — the gland cell fills with sebum and then ruptures completely, releasing its contents. The cell is constantly replaced from below. This is unusual: most glands release their contents without destroying the cell.
Control
Section titled “Control”Activity is driven by hormones, especially androgens — which is why output surges at puberty.
Specialized sebaceous glands
Section titled “Specialized sebaceous glands”A few sebaceous glands have distinct roles and do not follow the usual hair-follicle pattern:
- Meibomian glands — line the eyelid margins; produce the oily layer of the tear film.
- Glands of Zeis — at the base of each eyelash; lubricate the lashes.
- Montgomery’s tubercles — around each nipple; lubricate the areola and signal during nursing.
- Fordyce spots — on the lips and genitals; sebaceous glands opening directly onto the surface, with no hair.
Compared with sweat glands
Section titled “Compared with sweat glands”Sebaceous glands produce oil; sweat glands produce water. Sebaceous output is continuous and hormonally controlled; sweat output is triggered by nerves in response to heat or stress.
Key idea
Section titled “Key idea”The hormonal control of sebum is why blocked, overactive sebaceous glands underlie acne — and why acne tracks puberty.