What Are Fingernails & Toenails Made Of? The Role of Keratin

Fingernails and toenails are not made of "hard skin." They are composed of keratin — a tough, fibrous structural protein also found in hair, the stratum corneum (outer epidermis), horns, hooves, claws, and feathers.

Keratin Properties

• Alpha-keratin: Coiled-coil helical structure; rich in cysteine → disulfide cross-links provide strength and rigidity.
• Insoluble, stable: Resists enzymatic degradation, water, solvents.
• Low water content: ~10–15% (vs. ~70% in living epidermis) — contributes to hardness.

Nail Anatomy & Growth

• Nail matrix (germinal matrix): Proximal, under proximal nail fold. Mitotically active basal cells → keratinize → form nail plate. Matrix length determines nail thickness.
• Nail plate: The visible hard structure — dead, fully keratinized cells (onychocytes), tightly packed, translucent.
• Nail bed: Vascularized epithelium under the plate; adheres via longitudinal ridges. Pink color from dermal capillaries.
• Lunula: Visible distal matrix — white, half-moon; cells not fully keratinized.
• Cuticle (eponychium): Seals proximal nail fold → protects matrix from pathogens.
• Hyponychium: Seal at distal free edge.

Growth Dynamics

• Fingernails: ~3 mm/month (complete renewal ~6 months).
• Toenails: ~1 mm/month (complete renewal ~12–18 months).
• Faster in summer, younger age, dominant hand, pregnancy; slower with age, illness, malnutrition.

Clinical Relevance

Nail changes reflect systemic disease: clubbing (cardiopulmonary), Beau's lines (acute illness), koilonychia (iron deficiency), pitting (psoriasis), onycholysis (thyroid, trauma), Terry's nails (liver), Mees' lines (arsenic, chemo).