Fingernails are a common part of human anatomy, serving various purposes from delicate manipulation to protecting sensitive fingertips. Their strength and flexibility allow for daily tasks and shield underlying tissues from injury. Understanding their composition provides insight into how these structures perform their functions.
Keratin: The Primary Building Block
Fingernails are primarily composed of keratin, a tough, fibrous protein also found in hair and the outer layer of skin. Keratin is a family of structural proteins, specifically alpha-keratins, characterized by their helical structure. These proteins are extremely insoluble in water and many chemical solvents, which contributes to their durability.
Keratin’s robust nature stems from its unique amino acid composition, particularly a high content of cysteine. Cysteine residues form strong covalent links called disulfide bonds. These bonds cross-link the protein chains, creating a stable and resilient structure that can withstand mechanical stress. Nails, being harder than skin, contain a higher concentration of these bonds.
Beyond Keratin: Structure and Composition
While keratin is the dominant protein, fingernails are more than just a block of this material; their properties also depend on how keratin is organized and the presence of other components. The nail plate, the visible part of the nail, consists of multiple layers of tightly packed, hardened cells called onychocytes. These cells originate from keratinocytes in the nail matrix, undergoing a process called cornification where they fill with keratin and lose their internal structures, including the nucleus. This loss of cellular contents contributes to the nail’s transparency.
These dead, keratin-filled cells are arranged in layers, with the dorsal (uppermost) layer containing older cells, and the intermediate layer making up about three-quarters of the nail’s material. The arrangement of keratin filaments within these onychocytes, along with their tight interconnections, provides the nail plate with its hardness and flexibility.
Fingernails typically contain water, ranging from about 7% to 18%, which contributes to their flexibility and can vary with humidity. Water molecules interact with keratin proteins, influencing their structural conformation and allowing the nail plate to swell and become more flexible when hydrated.
Fingernails contain a small amount of lipids, approximately 0.1% to 5%, which act as an intercellular “cement” between the keratinized cells and help maintain flexibility and protect against dehydration. Trace minerals like calcium, zinc, iron, and selenium are also present, incorporated into the keratin-rich proteins, and can reflect the body’s nutritional status. The combination of highly organized keratin, water content, and these minor components creates a robust and protective structure.