Overview

Sebaceous glands lie in the dermis in the angle between the hair follicle and the arrector pili, emptying through a short duct into the follicular infundibulum; with the follicle and muscle they form the pilosebaceous unit. Each gland has several acini whose periphery is a layer of small, dividing basal cells. Their progeny move centrally, accumulating lipid droplets until the cells become large and foamy, their nuclei shrink and the cells disintegrate — holocrine secretion — so the entire cell content becomes sebum. Cell transit takes about one to three weeks.

They are found wherever there are hairs and are largest and most numerous on the face, scalp, upper chest and back (the acne-prone "seborrhoeic" areas), but are absent from the palms and soles. Free sebaceous glands opening directly onto the surface occur on the lips and buccal mucosa (Fordyce spots), eyelids (tarsal/Meibomian glands, whose oil stabilises the tear film), areola (Montgomery tubercles), labia minora and prepuce/glans (Tyson glands).

Sebum production is driven by androgens (dihydrotestosterone formed locally by 5α-reductase): glands are active at birth (maternal hormones — neonatal acne and cradle cap), quiescent in childhood, enlarge dramatically at puberty and decline in later life, especially in women after menopause.

Location: Dermis of thin skin in association with hair follicles; densest on face, scalp, upper trunk. Absent from palms and soles.

Anatomy

Histology: Branched acinar gland: peripheral flat basal cells, central large pale cells with foamy (lipid-vacuolated) cytoplasm and pyknotic nuclei; no lumen — the cells themselves are the secretion.
Development: Bud from the side of the hair follicle (epithelial origin) from ≈week 13–16; secrete during fetal life, contributing to vernix caseosa.

Function

Lubricate and waterproof hair and skin; reduce water loss
Antimicrobial and antifungal free fatty acids; transport antioxidants (vitamin E) to the surface
Contribute to the acid mantle and skin microbiome habitat
Specialised forms stabilise the tear film (Meibomian glands)

Secretions: Sebum: triglycerides and free fatty acids (≈55–60%), wax esters (≈25%), squalene (≈12%), cholesterol and its esters.
Regulation: Androgens (testosterone → DHT) stimulate gland size and output; oestrogens and isotretinoin (13-cis-retinoic acid) suppress it.

Clinical significance

Acne vulgaris

Androgen-driven sebum excess, follicular hyperkeratinisation (comedones), Cutibacterium acnes proliferation and inflammation within the pilosebaceous unit. Oral isotretinoin shrinks sebaceous glands but is highly teratogenic.

Seborrhoeic dermatitis

Greasy, scaly erythema of sebaceous-rich areas (scalp — dandruff, eyebrows, nasolabial folds, chest) linked to the lipophilic yeast Malassezia; infantile form is cradle cap; severe or extensive disease suggests HIV or Parkinson disease.

Chalazion and rhinophyma

A chalazion is a painless granuloma from a blocked Meibomian gland. Rhinophyma is sebaceous hyperplasia and fibrosis of the nose in advanced rosacea. (The so-called "sebaceous cyst" is usually an epidermoid cyst.)

Mnemonics

Holocrine

Sebaceous = Self-destructive: the whole cell (holo-) becomes the secretion.

Test yourself

Sebaceous glands release their secretion by which mechanism?

Merocrine (exocytosis)
Apocrine (pinching off of apical cytoplasm)
Holocrine (disintegration of the whole cell)
Endocrine (into the blood)

Answer: Holocrine (disintegration of the whole cell)

Sebaceous glands are holocrine: cells accumulate lipid, die and break down so that the entire cell content becomes sebum, which is replaced by division of basal cells at the periphery of the acinus.