Skin Structure and Epidermis Layers

Skin

• Consists of:
  • Epidermis (outermost layer)
  • Dermis (middle layer)
  • Subcutaneous tissue (innermost layer)

Epidermis

• A stratified squamous epithelium comprised of keratinocytes
• Keratinocytes undergo keratinization to form distinct epidermal strata (layers)
  • Stratum basale:
    • Single layer of mitotically active cuboidal cells
    • Attached to the basement membrane by hemidesmosomes and integrins
    • Attached to each other by desmosomes
  • Stratum spinosum:
    • Multiple layers of polyhedral cells
    • Joined by desmosomes at the tips of short projections containing bundled keratin (tonofibrils)
  • Stratum granulosum:
    • Thinner layer of flattened keratinocytes
    • Filled with keratohyaline granules containing filaggrin and proteins that bind tonofibrils
  • Stratum lucidum:
    • Thin, dense layer found in thick skin
    • Contains dispersed proteins throughout the tonofibril bundles, making it appear clear
  • Stratum corneum:
    • Most superficial layer
    • Cells have lost their nuclei and cytoplasm, leaving only flattened, keratinized structures called squames
    • Bound by hydrophobic, lipid-rich intercellular cement
    • Cells are worn away (thick skin) or flake off (thin skin)

Epidermis - Dermis Interface

• Interdigitating epidermal ridges (pegs) and dermal papillae strengthen the interface
• Microvasculature in these structures provides nutrients and oxygen to the epidermis

Melanocytes

• Located in the basal epidermis
• Synthesize melanin pigment in melanosomes, transporting it to adjacent keratinocytes for protection against UV damage

Langerhans Cells

• Antigen-presenting cells that form a network throughout the epidermis
• Intercept and sample microbial invaders
• Migrate to lymph nodes for an adaptive immune response

Skin Properties

• The stratum corneum provides resistance to water loss, friction, and microbial invasion.
• These properties are mostly determined by the lipid envelope and keratin-filled cells.