Understanding the Epidermis: Structure, Function, and Layers

The epidermis is the outermost layer of the skin, a stratified squamous epithelium derived from ectoderm.

Key Features

• Thickness: 0.04–0.12 mm (4–5 cell layers thin skin; up to 30+ layers thick skin of palms/soles)
• Avascular: No blood vessels; nutrients diffuse from dermis.
• Cell types:
  • Keratinocytes (~90%) – produce keratin
  • Melanocytes (~8%) – produce melanin
  • Langerhans cells (~2%) – antigen-presenting immune cells
  • Merkel cells (~0.5%) – tactile receptors
• Layers (deep to superficial): Stratum basale → spinosum → granulosum → lucidum (thick skin only) → corneum
• Keratinization: As keratinocytes mature, they accumulate keratin, lose nuclei, and form the stratum corneum – a tough, waterproof barrier shed by friction.

Functions

• Barrier against pathogens, chemicals, UV radiation, water loss
• Immune surveillance (Langerhans cells)
• Sensation (Merkel cells, free nerve endings)
• Vitamin D synthesis (UVB conversion of 7-dehydrocholesterol)