The Epidermis

Thickness

Layers (Strata)

Junqueira's Basic Histology, 17e - Layers of thick skin epidermis (left: H&E ×100; right: annotated diagram)

1. Stratum Basale (Basal Layer)

• A single layer of basophilic cuboidal or columnar cells resting on the basement membrane
• Attached to the basal lamina via hemidesmosomes; connected to each other by desmosomes
• Site of intense mitotic activity - contains keratinocyte stem/progenitor cells
• Keratin intermediate filaments (~10 nm diameter) begin forming here
• Also contains melanocytes (~every 10th cell) and Merkel cells
• The human epidermis turns over approximately every 15-30 days

2. Stratum Spinosum (Prickle Cell / Spinous Layer)

• Multiple layers of polyhedral cells with central nuclei and nucleoli
• Cells continue synthesizing keratins
• Keratin filaments bundle into tonofibrils, converging on numerous desmosomes
• Cells appear to have short "spines" or prickles (artifact of cell shrinkage around desmosomes during processing)
• Thicker in glabrous skin (palms/soles) and areas of friction
• Langerhans cells reside predominantly in this layer
• Lower part of this layer still has some mitotic progenitor cells

3. Stratum Granulosum (Granular Layer)

• 3-5 layers of flattened cells undergoing terminal differentiation (keratinization)
• Cytoplasm filled with intensely basophilic keratohyaline granules - non-membrane-bound masses of filaggrin + proteins that cross-link keratin tonofibrils
• Also contains lamellar granules (Golgi-derived, 100 × 300 nm ovoid organelles) that undergo exocytosis, secreting a lipid-rich layer around cells - this creates the critical water-permeability barrier
• This is the last layer with living, nucleated cells

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

• A thin, optically refractile, translucent band of flattened, acidophilic keratinocytes
• Found only in thick palmar and plantar skin
• No nuclei or organelles; cytoplasm consists almost exclusively of packed keratin filaments in an electron-dense matrix
• Cells held together by desmosomes

5. Stratum Corneum (Horny Layer)

• The outermost, fully differentiated layer of dead, anucleate keratinocytes (called corneocytes or squames)
• Cells are flat, filled with keratin, and embedded in the lipid-rich lamellar granule secretions
• This layer is thicker on palms and soles; thickened in ichthyosis; and examined for dermatophytes in fungal infections
• Cells are continually shed (desquamation) from the surface

Dermatology 2-Volume Set, 5e - Clinical correlations by layer

Cell Populations

Keratinocytes (predominant)

• Form the bulk of the epidermis (vast majority of epidermal cells)
• Continuously renew: mitosis in basal/lower spinous layers → upward migration → progressive differentiation → cornification → shedding
• Progressively accumulate keratin intermediate filaments as they mature (up to 50% of total protein in superficial layers)
• Key products: keratin, filaggrin, involucrin (cross-linking proteins)

Melanocytes

• Neural crest-derived dendritic cells, located in the basal layer (~1 per 10 keratinocytes)
• Synthesize melanin (brown-black pigment) in organelles called melanosomes
• Transfer melanosomes via dendritic processes to keratinocytes in the spinous and granular layers
• Melanin caps over keratinocyte nuclei, protecting them from UV radiation
• Ethnic skin color differences are determined by amount, combination, and distribution of melanin - NOT by differences in melanocyte number

Langerhans Cells

• Bone marrow-derived antigen-presenting cells (APCs) - part of the innate and adaptive immune system
• Located predominantly in the stratum spinosum
• Contain characteristic Birbeck granules (tennis-racket-shaped on EM)
• Express CD1a, S100, and langerin
• Capture antigens, process them, and migrate to lymph nodes to present to T lymphocytes

Merkel Cells (Tactile Epithelial Cells)

• Located in the basal layer, associated with sensory nerve endings
• Function as slowly adapting mechanoreceptors (light touch, texture, spatial acuity)
• Form the Merkel cell-neurite complex with unmyelinated sensory fibers
• Express CK20 (cytokeratin 20) - clinically useful marker (Merkel cell carcinoma is CK20+)

The Dermal-Epidermal Junction (DEJ)

• Epidermal ridges (rete pegs) projecting down into dermis
• Dermal papillae projecting up between ridges
• This interdigitation greatly increases surface area for adhesion and nutrient diffusion

• Lamina lucida (beneath hemidesmosomes)
• Lamina densa (type IV collagen-rich)
• Sublamina densa (anchoring fibrils of type VII collagen)

Functions of the Epidermis

Thick vs. Thin Skin - Key Differences

Clinical Correlations

• Psoriasis: granular layer is absent/thinned; stratum corneum shows parakeratosis (retained nuclei)
• Lichen planus: granular layer is thickened (hypergranulosis)
• Dermatophyte infections: fungi colonize the stratum corneum (examined for hyphae in KOH prep)
• Ichthyosis: stratum corneum is abnormally thickened
• Friction blisters: lymph-filled spaces at the DEJ from shear forces; prolonged friction causes callus/corn formation (stratum corneum thickening)
• Basal cell carcinoma: arises from stratum basale
• Squamous cell carcinoma: arises from stratum spinosum
• Melanoma: arises from melanocytes

• Junqueira's Basic Histology: Text and Atlas, 17e, Ch. 18
• Fitzpatrick's Dermatology, 9e, Vol. 1
• Bailey and Love's Short Practice of Surgery, 28e
• Dermatology 2-Volume Set, 5e