Keratin: The Structural Protein That Prevents Skin Water Loss

Keratin is the key structural protein of the epidermis (outer skin layer) and its appendages (hair, nails).

Role in Skin Barrier Function

• Keratinocytes (epidermal cells) synthesize keratin as they differentiate and migrate from basal layer to stratum corneum.
• In the stratum corneum, dead, flattened corneocytes are filled with keratin filaments embedded in a lipid matrix (ceramides, cholesterol, free fatty acids).
• This "brick-and-mortar" structure provides:
  • Impermeability: Prevents excessive transepidermal water loss (TEWL).
  • Mechanical strength: Resists abrasion, shear.
  • Chemical resistance: Limits penetration of irritants, allergens, toxins.
  • UV protection: Keratin absorbs/scatters UVB.

Keratin Types

• Soft (type I/II) keratins: Epidermis (KRT1, KRT10), hair/nails (KRT31–KRT86).
• Hard keratins: Higher cysteine content → disulfide cross-links → rigidity (hair, nails).

Related Proteins

• Filaggrin: Aggregates keratin filaments; breakdown products = natural moisturizing factor (NMF). Mutations → ichthyosis vulgaris, atopic dermatitis risk.
• Loricrin, involucrin: Cornified envelope proteins cross-linked to keratin.
• Collagen/elastin: Dermal structural proteins (not epidermal).

Clinical Relevance

• Ichthyoses, epidermolysis bullosa, palmoplantar keratodermas = keratin/associated protein mutations.
• Barrier repair: Ceramide-dominant moisturizers, urea, lactic acid support keratinocyte differentiation.

Keratin is the cornerstone of the epidermal barrier—without it, terrestrial life would be impossible.