Hair follicle

Folliculus pili

Also known as: pilosebaceous unit, hair bulb, dermal papilla, bulge region, root sheath

The hair follicle is a tubular invagination of epidermis into the dermis that produces a hair from its bulb, houses stem cells in its bulge and, with its sebaceous gland and arrector pili, forms the pilosebaceous unit.

Show on body map Skin — Layers and Appendages

Overview

Anagen (scalp)≈2–7 years; ≈85–90% of follicles

Catagen≈2–3 weeks; ≈1–2%

Telogen≈3 months; ≈10–15%

Each follicle extends obliquely from the epidermis into the deep dermis or hypodermis. Its expanded base, the hair bulb, encloses the dermal papilla — a mesenchymal cluster of specialised fibroblasts and capillaries that instructs growth. Over the papilla, the rapidly dividing hair matrix (with melanocytes) produces the cells that keratinise to form the hair shaft and the inner root sheath. The inner root sheath (layers of Henle and Huxley and its own cuticle) moulds the hair and disintegrates at the level of the sebaceous duct. It is surrounded by the outer root sheath, a downward continuation of the epidermis, then the glassy membrane (thickened basement membrane) and a connective tissue (dermal) sheath.

Regions from the surface: the infundibulum (to the sebaceous duct), the isthmus (to the arrector pili insertion) and the inferior segment (bulb). At the arrector pili insertion lies the bulge, a niche of multipotent epithelial stem cells that regenerate the follicle each cycle and contribute to epidermal repair after wounding; melanocyte stem cells also live here.

Follicles grow in cycles: anagen (active growth; ≈2–7 years on the scalp; ≈85–90% of scalp follicles), catagen (apoptotic regression; ≈2–3 weeks), telogen (rest; ≈3 months; ≈10–15%) and exogen (shedding of the club hair). The duration of anagen determines maximum hair length — years on the scalp, only weeks to months for eyebrows and body hair.

Location: Throughout thin skin; follicles slant obliquely, with the arrector pili on the obtuse-angled side. Absent from thick skin of palms and soles.

Parts & subdivisions

Hair bulb
Expanded terminal portion of the follicle containing the matrix and dermal papilla.
Dermal papilla
Mesenchymal core with capillaries; signals (Wnt, BMP, FGF) govern hair size and cycling; target of androgens.
Hair matrix
Highly mitotic epithelial cells (equivalent to stratum basale) with melanocytes; forms hair and inner root sheath — hence sensitivity to chemotherapy.
Inner root sheath
Henle layer, Huxley layer and cuticle (trichohyalin granules); disintegrates at the isthmus.
Outer root sheath
Continuous with the epidermis; contains the bulge stem cells.
Glassy membrane and connective tissue sheath
Thick basement membrane and dermal sheath surrounding the follicle.
Bulge
Stem-cell niche at the arrector pili insertion; destroyed in scarring alopecia.
Sebaceous gland and arrector pili
Complete the pilosebaceous unit.

Anatomy

Innervation: Lanceolate and circumferential (peritrichial) endings of Aβ, Aδ and C fibres surround the isthmus, detecting hair movement; sympathetic fibres to the arrector pili.
Arterial supply: Capillary network within the dermal papilla and a perifollicular plexus from the deep and superficial cutaneous plexuses.
Histology: Concentric layers from the hair outward: medulla, cortex, hair cuticle, inner root sheath (cuticle, Huxley, Henle), outer root sheath, glassy membrane, connective tissue sheath.
Development: Epidermal hair placode/peg grows into dermis over a condensation of mesoderm (future dermal papilla) from ≈week 9–12; sebaceous and apocrine glands bud from the follicle wall.

Function

Produces and anchors the hair
Stem-cell reservoir for hair regeneration and epidermal wound repair
Mechanosensation via peritrichial nerve endings
Channel for sebum and apocrine secretion to reach the surface

Physiology: Anagen → catagen → telogen → exogen; androgens (via DHT) enlarge follicles in beard and pubic regions but miniaturise genetically predisposed scalp follicles. Plucked anagen hairs show a pigmented bulb with sheaths; telogen "club" hairs show a keratinised, non-pigmented bulb.

Clinical significance

Folliculitis, furuncle and carbuncle

Infection of the follicle, usually Staphylococcus aureus: superficial folliculitis (pustule at the follicle) → furuncle (boil, deep follicular abscess) → carbuncle (coalescing furuncles with multiple sinuses, typically on the nape). Pseudomonas causes "hot-tub" folliculitis.

Telogen and anagen effluvium

Physiological stress (fever, childbirth, surgery) pushes many follicles into telogen simultaneously, so diffuse shedding begins ≈2–3 months later and recovers spontaneously. Cytotoxic chemotherapy halts dividing matrix cells, causing anagen effluvium within days to weeks.

Mnemonics

Hair cycle

Anagen = Active; Catagen = Collapse; Telogen = Time-out; eXogen = eXit.

Test yourself

Which statement about apocrine sweat glands is correct?

They are found over the whole body surface
They open directly onto the skin surface via a spiral duct
They open into hair follicles and become active at puberty
They are the main glands for thermoregulatory sweating

Answer: They open into hair follicles and become active at puberty

Apocrine glands are restricted to the axillae, areolae and anogenital region, empty into hair follicles above the sebaceous duct, become active at puberty under androgens and produce a protein-rich secretion that bacteria convert into body odour.