Epidermis

Epidermis

Also known as: cuticle (old term), epidermal layer

The epidermis is the avascular, keratinised stratified squamous epithelium forming the outer layer of the skin; it renews itself about every four weeks and provides the body's main barrier.

Show on body map Skin — Layers and Appendages

Overview

Layers4 (thin skin) or 5 (thick skin)

Renewal time≈4 weeks

Keratinocytes≈90% of epidermal cells

The epidermis is built from keratinocytes (≈90% of its cells) arranged in layers that represent successive stages of differentiation, from dividing basal cells to dead, flattened, keratin-filled corneocytes at the surface. Interspersed among them are melanocytes (pigment), Langerhans cells (antigen presentation) and Merkel cells (touch), plus occasional intraepithelial lymphocytes.

From deep to superficial the strata are basale, spinosum, granulosum, lucidum (thick skin only) and corneum. Thick skin (palms, soles) has all five layers, a very thick stratum corneum and an epidermis up to ≈1.5 mm thick; thin skin lacks a distinct stratum lucidum and its epidermis measures only ≈75–150 µm. Because the epidermis has no blood vessels, its deepest cells depend on diffusion from capillary loops in the dermal papillae, which is one reason the dermoepidermal junction is folded into rete ridges — to increase the area for exchange and attachment.

Keratinocytes are linked by desmosomes and anchored below to the basement membrane by hemidesmosomes. Disrupting either type of junction produces blistering diseases at characteristic levels (intraepidermal pemphigus versus subepidermal pemphigoid).

Location: Outermost layer of the skin, resting on the basement membrane of the dermoepidermal junction; continuous with the epithelium of hair follicles and sweat ducts.

Parts & subdivisions

Stratum basale (germinativum)
Single layer of cuboidal-to-columnar cells on the basement membrane: epidermal stem and transit-amplifying cells (mitotically active), with melanocytes and Merkel cells. Basophilic cytoplasm; keratins K5/K14; attached by hemidesmosomes.
Stratum spinosum
Several layers of polygonal keratinocytes joined by numerous desmosomes; shrinkage during processing pulls cells apart so the desmosomes look like "spines" (prickle cells). Keratin K1/K10 bundles (tonofilaments); Langerhans cells concentrated here.
Stratum granulosum
3–5 layers of flattened cells containing basophilic keratohyalin granules (profilaggrin, loricrin) and lamellar bodies (Odland bodies) that discharge lipids into the intercellular space; tight junctions; nuclei begin to degenerate — the key barrier-forming layer.
Stratum lucidum
Thin, clear, eosinophilic band of flattened anucleate cells containing eleidin; present only in thick skin of palms and soles.
Stratum corneum
Up to 15–20 layers in thin skin (and many more in thick skin) of flattened, anucleate corneocytes filled with keratin–filaggrin aggregates, enclosed by a cross-linked cornified envelope and embedded in lipid lamellae; continuously shed (desquamation).

Anatomy

Innervation: Free nerve endings (Aδ and C fibres) penetrate the epidermis up to the stratum granulosum (pain, temperature, itch); Merkel cell–neurite complexes in the stratum basale (sustained touch).
Arterial supply: None (avascular). Nutrients and oxygen diffuse from capillary loops of the superficial (subpapillary) plexus in the dermal papillae.
Histology: Keratinised stratified squamous epithelium. Thin skin ≈75–150 µm; thick skin ≈0.4–1.5 mm. Cell composition ≈90% keratinocytes; melanocytes ≈1 per 4–10 basal cells; Langerhans cells ≈2–5%; Merkel cells few, concentrated in fingertips and touch domes.
Development: Surface ectoderm: single layer covered by periderm (week 4–5) → basal layer + intermediate layers → all definitive strata by month 4. Shed periderm contributes to vernix caseosa.

Function

Principal permeability barrier against water loss and penetration of chemicals and microbes
Mechanical resilience through keratin filaments and desmosomes
UV protection by melanin transferred from melanocytes
Synthesis of vitamin D3 in basal and spinous keratinocytes
Immune surveillance by Langerhans cells; secretion of antimicrobial peptides and cytokines
Fine touch via Merkel cell–neurite complexes

Physiology: Continuous renewal from basal stem cells; differentiating keratinocytes reach the surface and are shed after ≈4 weeks (see epidermal renewal). Calcium gradient rising towards the surface, growth factors (EGF, KGF/FGF7) and retinoids regulate proliferation and differentiation.

Clinical significance

Calluses and corns

Repeated friction or pressure stimulates hyperkeratosis (thickening of the stratum corneum). A callus is diffuse; a corn has a central conical core pressing into the dermis and is painful.

Level of blistering

Subcorneal (pemphigus foliaceus, impetigo), suprabasal intraepidermal (pemphigus vulgaris) or subepidermal (bullous pemphigoid, dermatitis herpetiformis) — the level determines whether blisters are fragile and flaccid or tense.

Actinic keratosis

Rough, scaly patches on chronically sun-exposed skin caused by UV-induced dysplasia of keratinocytes; a premalignant lesion that can progress to squamous cell carcinoma.

Mnemonics

Strata, surface → deep

"Come, Let's Get Sun Burned" — Corneum, Lucidum, Granulosum, Spinosum, Basale.

Test yourself

Which epidermal layer is found only in thick (glabrous) skin?

Stratum spinosum
Stratum granulosum
Stratum lucidum
Stratum basale

Answer: Stratum lucidum

The stratum lucidum — a thin, clear band of flattened dead keratinocytes rich in eleidin — is present only in the thick skin of the palms and soles. Thin skin has four strata.