Hair follicles, sebaceous glands, eccrine and apocrine sweat glands, nails
Mnemonic
“Hair, nail, and three glands”:
- Hair follicles, with the arrector pili smooth muscle
- Nails
- Eccrine sweat glands - all over the body, opening directly onto the skin, sympathetic cholinergic supply, for thermoregulation
- Apocrine glands - axilla, groin, areola, opening into the hair follicle, active from puberty, and the source of body odour once bacteria act on the secretion
- Sebaceous glands - opening into the hair follicle, secreting sebum by holocrine secretion, androgen driven
“Eccrine everywhere and onto the skin; apocrine in the smelly places and into the follicle.”
Eccrine glands are sympathetic but cholinergic, which is the classic exception to the rule that sympathetic postganglionic fibres are noradrenergic, and it is why anticholinergics cause dry skin and heat intolerance.
Acne is disease of the pilosebaceous unit: androgen driven sebum, follicular hyperkeratinisation, Cutibacterium acnes and inflammation, which is exactly why treatment targets each in turn.
Expansion
| Appendage | Secretion | Distribution | Innervation |
|---|---|---|---|
| Eccrine sweat gland | Merocrine, watery | Everywhere, densest on palms and soles | Sympathetic cholinergic |
| Apocrine sweat gland | Apocrine, viscous, odourless until bacterial action | Axilla, groin, areola; opens into the hair follicle | Sympathetic adrenergic |
| Sebaceous gland | Holocrine, sebum | Everywhere except palms and soles | Androgen driven |
| Hair follicle | Arrector pili, sympathetic | ||
| Nail |
The sympathetic cholinergic innervation of eccrine glands is a notable exception to the general rule that sympathetic postganglionic fibres are adrenergic, and it explains why antimuscarinic drugs cause dry skin and why sweating persists in a patient on beta blockers who has hypoglycaemia.
Sebaceous glands are androgen dependent, which is why acne begins at puberty and why antiandrogens help. They open into the hair follicle to form the pilosebaceous unit, which is the structure obstructed in acne.
Hair growth cycles through anagen (growing, 85 per cent), catagen (regressing) and telogen (resting). Chemotherapy affects anagen hairs, and telogen effluvium follows severe illness or childbirth by about 3 months.