The Epidermis

The epidermis is the outermost layer of the skin - a stratified squamous keratinized epithelium of ectodermal origin. It rests on the dermis below and forms the body's primary interface with the external world.

Overview

The skin is divided into three major layers: epidermis (barrier), dermis (structural support), and subcutaneous fat. The epidermis alone accounts for about 50 microns average thickness in most body sites. It contains no blood vessels - all nutrients reach it by diffusion from the dermis. - Dermatology 2-Volume Set 5e, p. 56

Skin cross-section - epidermis in context:

Schematic of normal skin layers including epidermis, dermis, and subcutaneous fat with adnexal structures

Thick vs. Thin Skin

Feature	Thin Skin	Thick Skin
Location	Most of the body	Palms, soles
Epidermal depth	75-150 µm	400-600 µm
Layers	4	5 (has stratum lucidum)
Hair follicles	Present	Absent

Junqueira's Basic Histology, p. 902

Layers of the Epidermis (from deep to superficial)

H&E micrograph and diagram of thick skin epidermis layers:

Micrograph and diagram showing all five epidermal layers from stratum basale to stratum corneum, with melanocytes, Langerhans cells, and Merkel cells labeled

1. Stratum Basale (Basal Layer)

A single layer of basophilic cuboidal or columnar cells sitting on the basement membrane
Contains the stem cell pool for keratinocytes; intense mitotic activity occurs here
Cells are attached to the basal lamina via hemidesmosomes and to each other via desmosomes
Also contains melanocytes (approximately 1 per 10 basal cells) and Merkel cells
Keratin intermediate filaments (~10 nm diameter) begin forming here
Junqueira's Basic Histology, p. 903

2. Stratum Spinosum (Spinous/Prickle Cell Layer)

A thick layer of polyhedral cells above the basal layer; some cells near the basal layer still divide
Cells actively synthesize keratins; keratin filaments bundle into tonofibrils
Tonofibrils terminate at the abundant desmosomes holding cells together
The "spines" or prickles seen microscopically are desmosome-associated cytoplasmic extensions
Thicker in areas subject to friction (palms, soles)
Junqueira's Basic Histology, p. 903

3. Stratum Granulosum (Granular Layer)

3-5 layers of flattened cells undergoing terminal differentiation (keratinization)
Cytoplasm fills with keratohyaline granules - dense masses of filaggrin and proteins that cross-link keratin filaments into tight bundles
Lamellar granules (Golgi-derived) undergo exocytosis here, secreting a lipid-rich material around cells - this creates the water-impermeable barrier of the epidermis
Cells begin to lose their nuclei
Clinical note: This layer is missing in psoriasis and thickened in lichen planus
Junqueira's Basic Histology, p. 904-905

4. Stratum Lucidum (Clear Layer) - thick skin only

A thin, optically refractile sheet of flattened acidophilic keratinocytes
Present only in palmar and plantar (thick) skin
Nuclei and organelles have disappeared; cytoplasm is almost entirely packed keratin filaments
Junqueira's Basic Histology, p. 906

5. Stratum Corneum (Horny Layer)

The outermost layer; a semipermeable, laminated aggregate of fully differentiated (keratinized) dead squamous cells called corneocytes
Acts as the primary physiologic barrier against chemical penetration, microbial invasion, and excessive water loss or uptake
Under normal conditions, keratinocytes take ~2 weeks to transit through the nucleated compartment and another ~2 weeks through the stratum corneum
Clinical note: Examine this layer for dermatophytes; thicker on palms/soles and in ichthyosis
Dermatology 2-Volume Set 5e, p. 56

Schematic of epidermis layers with clinical annotations:

Schematic cross-section of the epidermis showing stratum corneum, granular layer, stratum spinosum, basal layer, melanocytes, Langerhans cells, and basement membrane

Resident Cell Populations

The epidermis harbors four major resident populations: - Dermatology 2-Volume Set 5e, p. 56

Keratinocytes (majority)

Epithelial cells originating from stem cells in the basal layer
As they migrate upward they progressively differentiate, accumulating more and more keratin
Surface density ~50,000 nucleated cells/mm²
Respond to injury or inflammation by significantly increasing proliferation rate

Melanocytes

Found in the basal layer (~1 in every 10 cells)
Produce melanin pigment within organelles called melanosomes
Melanosomes are transferred to nearby keratinocytes, where they position above the nucleus - protecting DNA from UV radiation
Skin color is primarily determined by the number and size of melanosomes, not by the number of melanocytes

Langerhans Cells

Dendritic antigen-presenting cells found mainly in the stratum spinosum
Metabolize complex antigens into immunogenic peptides
After activation, they migrate from the epidermis to regional lymph nodes to drive immune responses
Express CD1a and S100 (used in immunohistochemistry)

Merkel Cells

Found in the basal layer, especially in fingertips, lips, and hair follicles
Contain neuroendocrine peptides and are associated with sensory nerve endings
Act as mechanoreceptors (tactile cells)
May not be apparent in routine H&E sections

Functions of the Epidermis

Function	Mechanism
Physical barrier	Stratum corneum - resists friction, chemicals, microbes
Water permeability barrier	Lipids secreted by lamellar granules in stratum granulosum
UV protection	Melanin in keratinocytes shields nuclear DNA
Immune surveillance	Langerhans cells present antigens; resident lymphocytes patrol
Sensory	Merkel cells for light touch; free nerve endings for pain/temperature
Vitamin D synthesis	UV light acts on precursors in keratinocytes to produce vitamin D3

Junqueira's Basic Histology, p. 901

Epidermal Turnover

The epidermis renews itself approximately every 15-30 days, depending on age, body region, and health status. This rapid turnover makes keratinocytes a major source of skin cancers - most arise from the basal or spinous layers, producing basal cell carcinomas and squamous cell carcinomas respectively. - Junqueira's Basic Histology, p. 903

Dermal-Epidermal Junction

Projections called dermal papillae interdigitate with invaginating epidermal ridges (rete pegs) to strengthen adhesion between the two layers. This junction is the site of pathology in conditions such as blistering diseases (pemphigoid attacks hemidesmosomes here). - Junqueira's Basic Histology, p. 900

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