Hair and teeth are often considered among the hardest and most durable parts of the human body. This durability leads many to question whether these protective structures are built from the same fundamental materials. Although both originate from the skin’s outer layer, their final chemical composition and structural design are vastly different. Understanding their microscopic makeup reveals why they possess unique properties and functions.
The Primary Structure of Hair
The visible strand of hair, the hair shaft, is a non-living filament composed almost entirely of the structural protein called keratin. Keratin constitutes approximately 95% of the hair fiber’s dry weight and is the primary building block for the hair’s three main layers. This fibrous protein forms a highly resilient structure due to its high sulfur content, which allows for strong chemical bonds known as disulfide bridges.
The hair shaft is divided into an outer cuticle, a middle cortex, and sometimes a central medulla. The cortex makes up the bulk of the hair and contains the majority of the keratin, along with the melanin pigments that determine hair color. Cells forming the shaft undergo keratinization, filling with protein and dying, resulting in a strong, flexible, and chemically resistant material. Hard keratin is notably insoluble in water and highly resistant to biological enzymes.
The Layered Composition of Teeth
In sharp contrast to protein-rich hair, the structure of a tooth is a complex, layered system dominated by inorganic minerals. The outermost layer covering the crown is enamel, the hardest substance found in the human body. Enamel’s extreme durability comes from its composition, which is between 96% and 98% inorganic material.
This highly mineralized component is primarily crystalline calcium phosphate, known specifically as hydroxyapatite. These microscopic crystals are tightly packed and organized into enamel rods, giving the layer its dense, protective quality. Beneath the enamel lies the dentin, which forms the bulk of the tooth structure and is slightly less hard.
Dentin is a hydrated tissue, composed of about 50% hydroxyapatite crystals embedded within a supportive framework of organic matter. The organic matrix is almost exclusively made up of Type I collagen, a different structural protein than the keratin found in hair. Dentin also contains millions of tiny tubules that connect to the innermost pulp, a soft central tissue containing the tooth’s nerves and blood vessels. This layered arrangement provides a tough outer shell with a slightly more yielding interior to absorb chewing forces.
Biological Differences and Shared Components
The fundamental difference between hair and teeth lies in their primary building blocks: hair is a protein-based structure, while teeth are mineral-based. Hair’s strength is derived from the intricate cross-linking of keratin fibers, while the hardness of enamel comes from the dense packing of hydroxyapatite crystals. This compositional difference means the visible hair shaft is biologically inert, having no ability to heal or regenerate once damaged.
The dental pulp and the surrounding dentin contain living cells and blood vessels, allowing for sensation and a limited capacity for repair. Despite these major compositional differences, both hair and teeth share a common origin as ectodermal appendages. This means they develop from specialized epithelial cells during embryonic development and are governed by similar signaling pathways during initial formation.
Trace amounts of structural proteins like collagen are present in dentin, and hair contains small quantities of various minerals, but these are not the defining components. Hair is ultimately a complex arrangement of specialized protein, and teeth are a sophisticated composite of densely packed mineral crystals and a collagen matrix. Their similar appearance of durability is achieved through two entirely distinct biological engineering strategies.