Epidermis: Plant and Animal Structure, Function, and Clinical Relevance

Epidermis (from Greek epi "upon" + derma "skin") is the outermost protective layer in both plants and vertebrates.

Plant Epidermis

• Single layer of tightly packed cells, usually lacking chloroplasts (except guard cells).
• Covered by cuticle (cutin + wax) → prevents desiccation.
• Functions: Barrier to pathogens, UV, mechanical damage; gas exchange (stomata); water regulation; secretion.
• Specialized cells: Guard cells (stomata), trichomes (hairs), root hairs (absorption).

Vertebrate (Animal) Epidermis

• Stratified squamous epithelium; keratinized (skin) or non-keratinized (mucous membranes).
• Layers (deep → superficial):
  1. Stratum basale (stem cells, melanocytes, Merkel cells)
  2. Stratum spinosum (desmosomes, Langerhans cells)
  3. Stratum granulosum (keratohyalin, lamellar bodies)
  4. Stratum lucidum (thick skin only: palms/soles)
  5. Stratum corneum (dead corneocytes, lipid matrix) — barrier
• Functions: Physical/chemical/immunologic barrier; UV protection (melanin); thermoregulation (sweat); sensation; vitamin D synthesis.
• Renewal: ~28–40 days (basal → corneal desquamation).

Clinical Relevance

• Barrier failure: eczema, burns, ichthyosis, Netherton syndrome
• Neoplasia: BCC, SCC, melanoma (melanocyte origin)
• Infection portal: S. aureus, HSV, HPV, fungi
• Drug delivery: topical, transdermal
• Cosmetic: aging (thinning, reduced turnover), hyperpigmentation

Key Difference

Plants: single layer + cuticle. Animals: multilayered, keratinized, dynamic renewal. Both = interface with environment.