Topics

Topic

design image
Epidermis

The epidermis is the outermost layer of the skin, forming a protective, avascular barrier composed primarily of stratified squamous keratinized epithelium. It serves as the first line of defense against environmental, microbial, and chemical insults, while also preventing water loss and contributing to cutaneous sensation and thermoregulation.

The epidermis varies in thickness according to location — being thickest on the palms and soles (0.8–1.5 mm) and thinnest on the eyelids (0.05 mm). It continuously renews itself through keratinocyte proliferation in the basal layer and desquamation at the surface.

Synonyms

  • Cutaneous epithelium

  • Outer layer of skin

  • Epidermal layer

Structure and Layers

The epidermis is organized into five histological layers (from deep to superficial):

  1. Stratum basale (basal layer):

    • Single row of cuboidal or columnar basal cells resting on the basement membrane

    • Contains keratinocyte stem cells, melanocytes, and Merkel cells

    • Responsible for cell regeneration and pigment production

  2. Stratum spinosum (prickle cell layer):

    • Several layers of polygonal keratinocytes connected by desmosomes

    • Provides mechanical strength and resistance to abrasion

  3. Stratum granulosum (granular layer):

    • 2–5 layers of flattened cells containing keratohyalin granules and lamellar bodies

    • Involved in keratinization and formation of water barrier

  4. Stratum lucidum (clear layer):

    • Present only in thick skin (palms and soles)

    • Thin translucent layer of anucleate keratinocytes filled with eleidin (a lipid-rich intermediate)

  5. Stratum corneum (horny layer):

    • Outermost layer of anucleate, keratinized cells (corneocytes)

    • Provides barrier protection, waterproofing, and resistance to friction

Relations

  • Deep (internal): Dermis (connected via the dermoepidermal junction)

  • Superficial (external): External environment and atmospheric interface

  • Lateral continuity: Continuous with epidermis of adjacent structures such as nails, hair follicles, and mucocutaneous junctions

Attachments

  • Dermoepidermal junction (basement membrane zone):

    • Anchors epidermis to dermis through hemidesmosomes and anchoring fibrils

    • Provides structural stability and selective permeability

    • Clinically important in blistering disorders (e.g., bullous pemphigoid, epidermolysis bullosa)

Nerve Supply

  • Free nerve endings penetrate into the epidermis, responsible for pain, temperature, and light touch

  • Merkel cells function as tactile mechanoreceptors at the dermoepidermal junction

Function

  • Protection: Physical and chemical barrier against trauma, pathogens, and UV radiation

  • Water retention: Prevents dehydration via keratinized stratum corneum

  • Sensation: Houses tactile and thermal receptors for sensory input

  • Thermoregulation: Supports sweat gland ducts and regulates evaporative cooling

  • Immune defense: Langerhans cells in stratum spinosum initiate immune responses

  • Pigmentation: Melanin from melanocytes absorbs UV radiation and protects deeper tissues

Clinical Significance

  • Burns: Depth of injury determined by epidermal involvement (superficial vs partial thickness)

  • Psoriasis: Accelerated keratinocyte turnover causing thickened, scaly plaques

  • Eczema and dermatitis: Inflammation with epidermal spongiosis and barrier disruption

  • Vitiligo and albinism: Disorders of melanocyte distribution or melanin production

  • Skin cancers: Basal cell carcinoma, squamous cell carcinoma, and melanoma originate within epidermal layers

  • Aging: Thinning and flattening of dermoepidermal junction reduce elasticity and healing capacity

MRI Appearance

  • T1-weighted images:

    • Epidermis: Low-to-intermediate signal intensity, appearing as a thin hypointense line on the surface of the dermis

    • Subcutaneous fat: Bright signal providing contrast for visualization

    • Thick skin (palms, soles): Slightly more visible due to thicker keratin layer

    • Pathology: Inflammation or tumor infiltration increases thickness or signal heterogeneity

  • T2-weighted images:

    • Normal epidermis: Intermediate-to-low signal, slightly darker than dermis

    • Thick keratinized layer: Appears hypointense due to compact keratin content

    • Edema, infection, or inflammation: Increased signal intensity and thickening

    • Ulceration or erosion: Focal discontinuity in the hypointense surface line

  • STIR:

    • Normal: Intermediate-to-dark signal

    • Pathologic: Bright hyperintensity representing edema, cellulitis, or inflammation extending from dermis

  • Proton Density Fat-Saturated (PD FS):

    • Normal epidermis: Intermediate-to-dark, thin uniform band

    • Inflammation or hyperkeratosis: Focal bright signal thickening

    • Useful for: Detecting subtle skin edema, superficial infection, or tumor infiltration

  • T1 Fat-Sat Post-Contrast:

    • Normal epidermis: Minimal enhancement

    • Inflammation or infection: Diffuse linear enhancement of dermoepidermal junction

    • Neoplasm or ulceration: Irregular or nodular enhancement pattern

CT Appearance

Non-Contrast CT:

  • Epidermis: Thin, low-attenuation layer overlying higher-density dermis

  • Thick skin regions: More apparent due to higher keratin density

  • Calcifications or hyperkeratosis: May appear as focal high-attenuation areas

  • Lesions or ulceration: Localized surface irregularity or soft-tissue discontinuity

Post-Contrast CT (standard):

  • Normal epidermis: Minimal to no enhancement

  • Inflamed or infected skin: Linear or patchy enhancement along skin surface

  • Neoplastic infiltration: Focal thickening with variable enhancement pattern

  • CT helps evaluate cutaneous tumors, abscesses, and ulcerative or infiltrative lesions

MRI image

Epidermis of skin of foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000