The straightforward answer is that teeth and fingernails are fundamentally different, composed of distinct biological materials. While both structures originate from the ectoderm during embryonic development, their final compositions and resulting properties are vastly separated. Nails are primarily made of a specialized protein, whereas teeth are composed of a highly mineralized, bone-like substance. This difference dictates their functions, durability, and ability to repair themselves.
The Primary Material of Fingernails
Fingernails, technically known as the nail plate, are categorized as integumentary appendages, meaning they are extensions of the skin. The main component of the nail plate is keratin, a dense, fibrous structural protein. Nails are made mostly of hard alpha-keratins, which are rich in sulfur and provide rigidity.
The visible nail plate is composed of compacted, non-living epithelial cells that have undergone keratinization. New nail material is constantly produced in the nail matrix, an area hidden beneath the cuticle at the base of the nail. As new cells are generated, they push the older, hardened cells forward, resulting in continuous growth.
The Primary Material of Teeth
The visible portion of a tooth, the crown, is covered by enamel, the hardest substance found in the human body. Enamel is not protein-based like a nail; instead, it is almost entirely mineral. It consists of up to 96% inorganic material, predominantly a crystalline calcium phosphate known as hydroxyapatite.
Beneath the enamel lies dentin, which forms the bulk of the tooth structure and is slightly softer. Dentin is a calcified tissue containing hydroxyapatite crystals embedded within an organic matrix primarily made of collagen. Unlike the non-living cells of the nail plate, the inner dental pulp contains living tissue, including nerves and blood vessels, which provide nourishment.
Structural and Functional Divergence
The contrast in material composition leads to significant differences in the structure and biological behavior of nails and teeth. Keratin, the primary component of nails, offers relative flexibility and elasticity. This characteristic allows nails to bend slightly and prevents them from shattering under minor impact.
In contrast, the highly mineralized enamel of the tooth is rigid and exceptionally hard, ranking comparably to materials like steel on the Mohs hardness scale. This extreme hardness is necessary for the primary function of teeth: mastication, or the mechanical breakdown of food. However, this mineral structure is also more brittle and highly susceptible to erosion from acids.
Another divergence lies in their capacity for self-repair and growth. The nail matrix continuously generates new keratin cells, ensuring the nail plate is constantly growing and can replace itself following damage. This is possible because the generating cells remain active throughout life.
Enamel is acellular, meaning it contains no living cells. The cells that formed it are no longer present once the tooth erupts. Consequently, once the mineral structure of enamel is lost or damaged, the body cannot regenerate it naturally, unlike the constantly renewing nail plate.