What Are Fingernails Made Of?
Fingernails are made of keratin, the same tough structural protein found in hair, but with a much denser network of sulfur crosslinks that makes them hard and rigid. The visible nail plate is entirely dead keratinized cells, produced continuously by the living nail matrix under the skin at the nail base. This guide covers everything about fingernail composition: the three keratin layers, nail anatomy, growth rates, and what makes nails hard, soft, or brittle. Written by Nancy Davidson.
The Short Answer: Keratin
Fingernails are made of alpha-keratin, a fibrous structural protein. It is the same class of protein found in human hair and the outer layer of skin, but the keratin in nails is classified as hard alpha-keratin because of its unusually high cysteine content. Cysteine is a sulfur-containing amino acid that forms disulfide bonds (sulfur-to-sulfur covalent links) between adjacent keratin chains. The density of these crosslinks is what makes nails hard and rigid rather than soft and flexible like skin.
The visible nail plate, the hard surface you see and paint, is made entirely of dead cells. Living cells called onychocytes in the nail matrix (a small zone of tissue hidden under the skin fold at the nail base) divide and produce new cells continuously. As those cells migrate outward and upward, they fill with keratin and lose their nuclei and other living cellular components. By the time a cell reaches the visible nail plate, it is fully keratinized and dead, bound tightly to its neighbors in the plate structure.
This is the same basic process that produces hair, with the key difference that nail cells flatten into a plate structure while hair cells elongate into a filament.
"I always assumed nails were basically just really hard skin, and I was partially right. But once I learned about the sulfur crosslinks in keratin and how the nail matrix is the only part actually alive, it completely changed how I think about nail prep. You are literally applying products to a protein plate over living tissue, so protecting the matrix and the nail bed actually matters."
Nancy Davidson, Author
The Three Layers of the Nail Plate
The fingernail plate is not a single uniform slab of keratin. It is built from three distinct layers of keratinized cells that differ in thickness, orientation, and mechanical properties. Understanding these layers helps explain why nails delaminate (peel in horizontal layers) and why overfiling damages nails more than it appears.
| Layer | Position | Role |
|---|---|---|
| Dorsal layer | Thin outer surface layer | Provides a smooth surface and protective hardness; most exposed to mechanical wear and chemical contact (polish, remover) |
| Intermediate layer | Thick middle layer; makes up 70 to 80% of nail plate thickness | Primary strength layer; responsible for most of the nail's hardness and resistance to bending |
| Ventral layer | Thin inner layer in contact with the nail bed | Softer than the dorsal layer; contributes to adhesion between the nail plate and the nail bed |
When nails peel horizontally, it is usually the dorsal layer separating from the intermediate layer. This can be caused by dehydration, repeated acetone exposure, or aggressive buffing during nail prep. The dorsal layer is the thinnest and most exposed.
Fingernail Anatomy: All the Parts
The fingernail is more than just the hard plate. The nail unit includes several structures that work together to produce, protect, and support the plate. Here is what each part is and what it does.
| Structure | Location | Function |
|---|---|---|
| Nail plate | The visible hard surface you see and paint; the main structural component of the nail | Three layers of dead, keratinized cells; makes up most of the visible nail |
| Nail matrix | Located under the skin fold at the nail base, beneath the lunula | The only living tissue that produces nail plate cells; damage here stops or deforms growth |
| Lunula | The pale white half-moon at the nail base; visible on thumbs most clearly | The furthest visible edge of the nail matrix; less translucent than the rest of the plate |
| Nail bed | The skin directly under the nail plate from lunula to free edge | Rich in capillaries that give healthy nails their pink color through the translucent plate |
| Cuticle (eponychium) | The thin fold of skin at the base of the nail that seals the gap between plate and skin | Prevents bacteria and moisture from entering under the nail plate |
| Hyponychium | The skin under the free edge of the nail at the fingertip | Forms a waterproof seal between the nail plate and the skin of the fingertip |
| Free edge | The portion of the nail plate that extends past the fingertip and can be trimmed | Fully keratinized dead cells with no attachment to the nail bed underneath |
| Lateral nail folds | The strips of skin along each side of the nail plate | Anchor the nail plate in place and protect the edges from mechanical damage |
Why Are Fingernails Hard?
The hardness of fingernails comes from two things: the three-dimensional structure of the keratin protein chains and the density of disulfide crosslinks between them. Keratin proteins naturally fold into alpha-helix structures, and multiple helices coil around each other to form protofilaments, which bundle into the dense fibrous matrix of the nail plate.
The crosslinks are the key to hardness. When cysteine amino acids on adjacent keratin chains are positioned close to each other, their sulfur atoms bond covalently to form a disulfide bridge (R-S-S-R). Each of these bonds is a strong covalent link that locks the two chains together. Nail keratin has a much higher cysteine content than hair keratin, resulting in roughly three times more disulfide crosslinks per unit of protein. This is what makes the nail plate rigid while hair remains flexible.
Water content also matters. Healthy nails contain 7 to 12% water by weight. This moisture keeps the keratin chains slightly flexible, allowing the nail to bend under stress without cracking. Drop below 7% and the nail becomes brittle. Exceed about 30% (after a long soak) and the nail swells and softens temporarily.
Fingernail Growth Rate and What Affects It
The nail matrix produces new cells at a consistent rate of roughly 3 to 4 mm per month for most adults, meaning a full nail takes about 4 to 6 months to grow out from the matrix to the free edge. Several factors shift this baseline.
| Factor | Effect on growth |
|---|---|
| Age | Growth fastest in young adults (20s to 30s); slows gradually with age |
| Season | Growth is approximately 20 to 25% faster in summer than winter, linked to vitamin D and circulation |
| Dominant hand | Nails on the dominant hand grow slightly faster, possibly due to increased blood flow from more frequent use |
| Finger | Middle finger grows fastest; thumb grows slowest; index and ring fingers are intermediate |
| Health and nutrition | Iron deficiency, thyroid disorders, and severe protein deficiency can slow nail growth noticeably |
| Illness | Serious illness or surgery can cause temporary slowdown, sometimes visible as Beau's lines (horizontal ridges) on the growing nail |
| Biotin | Adequate biotin (vitamin B7) supports keratin synthesis; deficiency is rare in healthy adults eating a varied diet |
| Water content | Well-hydrated nails grow at a consistent rate; chronic dehydration does not significantly slow growth but does affect plate quality |
Fingernails vs Hair vs Bone: What Is the Difference?
Fingernails and hair are both made of alpha-keratin protein, but nails form a hard flat plate while hair forms a flexible filament. The difference comes from the higher cysteine content and disulfide crosslink density in nail keratin. Hair keratin has fewer crosslinks, which allows the protein chains to slide past each other slightly, giving hair its flexibility.
Bone is not related to keratin at all. Bone is a composite material made of collagen protein (which provides flexibility) reinforced with calcium phosphate mineral crystals called hydroxyapatite (which provide compressive strength). Nails have no mineral content. The apparent similarity in hardness between nails and teeth or bones is coincidental: nails are hard because of protein crosslinks, while bones and teeth are hard because of mineral deposits.
Nails are also not related to claws or hooves in other animals, though these structures are also keratin-based. Claws and hooves use a different arrangement of keratin with different cell types and curvature, but the core chemistry of alpha-keratin crosslinked by disulfide bonds is the same across all mammalian nail-type structures.
What Affects Fingernail Composition and Health?
Because the nail plate is dead keratin, you cannot repair or feed it directly once it has grown. Any intervention (strengtheners, oils, cuticle care) either slows damage or supports the living matrix in producing better-quality keratin in future growth. The factors with the most evidence are:
- Hydration: Keeping the nail plate moisturized prevents delamination and brittleness. Cuticle oil and hand cream applied regularly reduce water loss from the plate surface.
- Protein intake: The nail matrix uses dietary amino acids to synthesize keratin. Severe protein deficiency (rare in well-fed adults) can slow growth and reduce plate quality.
- Iron levels: Iron deficiency can cause spoon-shaped nails (koilonychia) and slow growth. Getting iron levels checked is reasonable if nails are consistently thin and soft.
- Acetone exposure: Acetone strips water and oils from the nail plate, temporarily increasing brittleness. Non-acetone removers are gentler but less effective at removing hard gel products.
- Nail strengtheners: Older formaldehyde-based hardeners create additional crosslinks in the keratin but can make nails too rigid if overused, causing breaking. Newer calcium or protein-based formulas are gentler.
- Artificial nail products: Any product applied over the nail plate eventually has to be removed. Incorrect removal (picking or peeling rather than soaking) strips the dorsal and intermediate keratin layers. This is the most common cause of thin, damaged nails after wearing acrylics or gel.
Frequently Asked Questions
What are fingernails made of?
Fingernails are made of keratin, a tough structural protein. The visible nail plate is formed from three tightly packed layers of dead, keratinized cells produced continuously by the nail matrix beneath the skin at the base of the nail. Keratin in fingernails is denser than hair keratin because of additional disulfide bonds between cysteine amino acids, which is what makes nails hard rather than flexible like hair.
What protein are fingernails made of?
Fingernails are made of alpha-keratin, a fibrous structural protein in the same family as hair and the outer layer of skin. Nail keratin specifically is classified as hard alpha-keratin because of its high cysteine content and the resulting density of disulfide crosslinks between protein chains. These sulfur bonds are what give nails their rigidity compared to the soft keratin of skin.
How fast do fingernails grow?
Fingernails grow an average of 3 to 4 mm per month. The middle finger tends to grow the fastest and the thumb the slowest among the five fingers. Growth is faster in summer than winter, faster on the dominant hand than the non-dominant hand, and faster in younger adults than older adults. A full regrowth of a fingernail from matrix to free edge takes approximately 4 to 6 months.
What is the nail matrix?
The nail matrix is the living tissue located under the skin fold at the base of the fingernail, beneath and around the lunula (the pale half-moon shape). It is the only part of the nail unit that produces new nail cells. Specialized matrix cells called onychocytes divide continuously and fill with keratin protein as they mature, eventually flattening and hardening into the dead keratinized cells that make up the nail plate. If the matrix is damaged, nail growth stops or deforms.
Why are fingernails hard?
Fingernails are hard because of the high density of disulfide bonds between cysteine amino acids in the keratin protein chains. Cysteine is a sulfur-containing amino acid; when two cysteine residues on adjacent keratin chains are close together, their sulfur atoms form a covalent disulfide bridge (S-S bond). These crosslinks lock the keratin chains into a rigid three-dimensional network. Nail keratin has roughly three times more cysteine crosslinks than hair keratin, which is why nails are rigid while hair is flexible.
What makes fingernails weak or brittle?
Fingernails become brittle mainly from dehydration of the keratin protein. When the nail plate loses too much water (below about 7% water content by weight), the keratin becomes inflexible and prone to splitting and breaking. Common causes include frequent hand washing, prolonged water exposure followed by rapid drying, nail polish remover use (especially acetone-based), iron deficiency, thyroid disorders, and aging. Biotin supplementation is often recommended for brittle nails but clinical evidence is limited to people with deficiencies.
Are fingernails made of bone?
No. Fingernails are not made of bone. Bone is composed primarily of collagen protein and calcium phosphate mineral (hydroxyapatite). Fingernails are made entirely of keratin protein with no mineral content. The hardness of nails comes from densely crosslinked keratin protein chains, not from mineral deposits. The confusion likely comes from the fact that nails and bones are both hard structures in the body.
Are fingernails made of dead cells?
Yes. The visible nail plate is made entirely of dead, keratinized cells. Living cells in the nail matrix produce new cells continuously, but as those cells migrate away from the matrix they fill with keratin protein and lose their nuclei and other cellular structures, becoming flat dead cells bound tightly together. The only living parts of the nail unit are the nail matrix (which produces new cells), the nail bed underneath the plate, and the surrounding skin folds.