"Think of the scalp as a densely planted garden: each hair is a flower, the follicle is its root system, the sebaceous gland is the built-in irrigation nozzle, and the arrector pili is the muscle that makes the plant stand up in the cold."
The scalp is home to approximately 100,000 follicular units, with density highest in the frontal and vertex regions. Each follicle is a self-contained mini-organ capable of cycling through growth, regression, and rest phases throughout a person's lifetime.
Understanding the anatomy underpins your ability to interpret hair loss patterns, counsel patients accurately, and recognise which histological compartment a given drug or disease is most likely targeting.
Infections in the loose areolar (L) layer can spread widely — it is the "danger space" of the scalp. Lacerations of the galea (A) must be closed separately from skin to prevent gaping wounds.
The pilosebaceous unit (PSU) is the functional building block of hair-bearing skin. It consists of three intimately connected structures that act together as a single physiological unit.
A tubular invagination of the epidermis extending down into the dermis and subcutaneous fat. It contains the machinery for hair production and is the primary target in alopecia areata, lichen planopilaris, and androgenetic alopecia.
A holocrine gland — the entire cell disintegrates to release its product — attached to the upper third of the follicle. It secretes sebum (triglycerides, wax esters, squalene, fatty acids) into the infundibulum, lubricating the hair and providing weak antimicrobial activity.
"The sebaceous gland is the follicle's built-in oil dispenser — it squeezes an entire cell's worth of lipid into the hair canal every time it fires."
A small bundle of smooth muscle running obliquely from the dermal papillary layer to the follicle bulge. Innervated by sympathetic adrenergic fibres, it contracts in response to cold or emotional stimuli — producing piloerection (goosebumps). Though functionally vestigial in humans, it anchors the follicle; its destruction disrupts follicular architecture in scarring alopecias.
| Component | Type | Function | Clinical relevance |
|---|---|---|---|
| Hair follicle | Epithelial invagination | Produces hair shaft | Target in AA, AGA, LPP |
| Sebaceous gland | Holocrine gland | Sebum secretion | Hyperactive in seborrhoeic dermatitis; absent in FFA |
| Arrector pili | Smooth muscle | Piloerection, anchoring | Destroyed in scarring alopecias |
The follicle is divided into three zones. Each has distinct regenerative capacity, immunological privilege, and disease susceptibility.
If you feel scarring/fibrosis at the follicular openings, or see absent follicular ostia on dermoscopy → refer urgently. Scarring alopecia = stem cell loss = irreversibility. Time matters.
| Zone | Boundaries | Keratinisation | Permanent loss if destroyed? |
|---|---|---|---|
| Infundibulum | Follicular pore → sebaceous duct | Epidermoid (surface-type) | No stem cells here |
| Isthmus | Sebaceous duct → arrector pili | Trichilemmal | YES — contains bulge stem cells |
| Lower follicle/Bulb | Arrector insertion → matrix/papilla | Matrix | Potentially reversible — bulge spared |
The hair shaft is a dead, keratinised filament produced by the matrix cells of the bulb. It has three concentric layers — analogous to a reinforced concrete column.
"Think of the hair shaft as a pencil: the outer varnish coating is the cuticle, the wooden barrel providing strength is the cortex, and the soft graphite core — often absent — is the medulla."
6–10 flattened, overlapping dead cells arranged like roof tiles pointing toward the tip. Rich in disulphide bonds and surface lipids (18-MEA). Healthy cuticle = smooth feel and light reflection. Hair cosmetics principally target the cuticle.
| Property | Structural basis |
|---|---|
| Mechanical strength | Coiled α-keratin macrofibrils cross-linked by disulphide bonds |
| Elasticity | Disulphide bonds (targeted by perms and relaxers) |
| Colour | Eumelanin (brown-black) and phaeomelanin (red-yellow) synthesised by melanocytes in the bulb |
| Texture/curl | Asymmetric keratin distribution and follicle shape |
A discontinuous central column of loosely packed cells and air spaces. Absent in vellus hair. Function debated — may contribute to thermal insulation and light reflection.
Trichorrhexis nodosa (the most common hair shaft defect) presents as white nodes on the shaft — fracture points where the cortex has splayed open. Ask about heat styling, bleaching, or physical trauma.
Each follicle independently cycles through distinct phases. On a normal scalp, approximately 85–90% of follicles are in anagen at any time, 1% in catagen, and 10–15% in telogen.
"Imagine each follicle is a worker on a 3-year contract. A factory crisis triggers mass early retirements. Three months later all those workers' replacements arrive — but you notice the factory floor was empty for a while. That 3-month lag explains why patients notice shedding after a fever, not during it."
| Phase | Duration | % scalp | Key event | Relevant condition |
|---|---|---|---|---|
| Anagen | 2–6 years | 85–90% | Active keratinocyte proliferation | Anagen effluvium (chemo); AGA (shortened) |
| Catagen | 2–3 weeks | <1% | Apoptosis, club hair formation | Rarely isolated pathology |
| Telogen | 3–4 months | 10–15% | Club hair resting; new anagen initiates | Telogen effluvium |
| Exogen | Days–weeks | Variable | Active club hair release | Chronic telogen effluvium |
| Finding | Anatomy targeted | Action |
|---|---|---|
| Scarring / absent follicular openings | Isthmus/bulge destruction | Urgent referral — irreversible |
| Diffuse shedding 2–3 months post-illness | Telogen effluvium — lower follicle | Reassure, investigate triggers |
| Pattern thinning (temporal recession/vertex) | AGA — bulb miniaturisation | Discuss treatment early |
| Patch of non-scarring loss, exclamation hairs | AA — peribulbar inflammation | Confirm, treat or refer |
| Broken hairs + scaling | Infundibular infection (tinea) or shaft defect | Fungal swab/microscopy |
18 cards covering all key concepts from this chapter. Click a card to reveal the answer.
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APA 7th edition format.