The nail plate is made primarily of a tough, fibrous protein called alpha-keratin, the same protein family found in hair and the outer layer of skin. Keratin accounts for about 80 to 90% of the nail’s dry weight, with the remainder consisting of water, lipids, and trace minerals. What gives nails their distinctive hardness compared to skin or hair comes down to their internal architecture: tightly packed protein fibers bound together by sulfur-rich chemical bridges.
Keratin: The Core Building Block
The keratin in your nails exists as tiny microfibrils, roughly 7 to 10 nanometers in diameter, embedded in a surrounding matrix of smaller proteins. Think of it like rebar inside concrete. The microfibrils are made of low-sulfur keratin, while the surrounding matrix proteins are rich in sulfur and certain amino acids like glycine and tyrosine. This combination creates a material that is both strong and slightly flexible.
The sulfur content is especially important. Sulfur atoms in neighboring keratin strands form chemical connections called disulfide bonds, which act like molecular crosslinks holding everything together. Nails have a much higher concentration of these bonds than skin does, which is why your nail plate is rigid enough to protect the fingertip while skin remains soft and pliable. These same bonds are the reason nail-softening treatments work: they break disulfide links to make the keratin more flexible.
Three Layers Stacked Together
Under a microscope, the nail plate is not a single uniform sheet. It consists of three tightly bonded layers stacked on top of each other: the dorsal (top), intermediate (middle), and ventral (bottom) layers.
Each layer plays a slightly different role. The dorsal layer is the outermost surface you can see and touch. It acts as the main barrier against chemicals and outside substances trying to penetrate the nail. The intermediate layer is the thickest of the three, and its keratin fibers run sideways across the nail, giving it structural strength and resistance to bending. The ventral layer sits closest to the nail bed and interlocks with the tissue underneath, anchoring the plate in place.
This laminated design is part of why nails are so durable. Rather than relying on a single thick layer, the stacked structure distributes stress across multiple planes, similar to how plywood is stronger than a single board of the same thickness.
Water Content and Flexibility
Despite their hard appearance, nails are not completely dry. Water is one of the most important non-protein components of the nail plate, and its level directly controls how the nail feels and behaves. Hydration is considered the single most important factor influencing the nail’s physical properties.
A healthy nail typically maintains a water content in a functional middle range. When that water content drops below about 10%, nails become brittle and prone to splitting. When it rises above 20%, nails turn soft and tend to separate into layers, which is the peeling you sometimes notice after a long bath. This is also why nails feel noticeably softer and more flexible after hand washing or swimming. The nail plate absorbs water relatively easily because it behaves more like a porous, hydrophilic (water-attracting) material than like skin. Research on permeability has shown that the nail allows small water-soluble molecules to pass through more readily than larger, oil-based ones.
Lipids in the Nail Plate
Lipids, or fats, make up a small but meaningful fraction of the nail plate. The major types found in nails include cholesterol, ceramides, free fatty acids, and triglycerides. In adults, free fatty acids and ceramides together account for the largest share of nail lipids.
These fats sit between the flattened cells of the nail plate and contribute to the nail’s permeability barrier, helping regulate what gets in and out. The lipid composition changes with age. In infants, ceramides dominate, while in adults, free fatty acids become the most abundant lipid class. This shift may partly explain why infant nails tend to be thinner and more pliable than adult nails. There are also subtle differences between males and females, though the overall lipid profile follows the same general pattern across sexes.
How the Nail Plate Forms
The nail plate is produced by a region of specialized skin cells called the nail matrix, located just beneath and behind the cuticle. Cells in the matrix divide rapidly, and as new cells push forward, the older ones are compressed, flattened, and packed with keratin. By the time these cells reach the visible nail plate, they have lost their nuclei and are essentially dead, hardened sheets of protein stacked tightly together. These flattened cells are sometimes called onychocytes.
This process is continuous. Fingernails grow at an average rate of about 3 to 4 millimeters per month, meaning a completely new nail plate takes roughly four to six months to replace itself from base to tip. Toenails grow more slowly, typically taking 12 to 18 months for full replacement. The quality of the keratin produced, and therefore the health of your nails, depends heavily on the health and blood supply of the nail matrix.
Why Nails Differ From Hair and Skin
Hair, skin, and nails all contain keratin, but the specific mix and arrangement differ significantly. Nail keratin is classified as “hard” keratin, the same category as hair, which sets it apart from the “soft” keratin in skin. Hard keratin contains more sulfur and more disulfide crosslinks, making it tougher and more resistant to breakdown by enzymes.
The nail plate’s layered, laminated structure also sets it apart from hair, which is built as a cylindrical shaft with concentric rings. And while skin’s outer barrier relies heavily on lipids packed between cells to block water loss, the nail plate’s barrier depends more on its dense keratin network. This difference in structure is why topical medications designed for skin often can’t penetrate the nail effectively, and why treating nail infections typically requires longer treatment courses or oral medications that reach the nail from the inside.