Teeth are often grouped with bones due to their hardness and white appearance, and both are mineralized tissues containing calcium and phosphorus. However, teeth are distinct biological structures that differ significantly from bones in their fundamental makeup, formation, maintenance, and specialized roles. These differences explain why teeth are not classified as part of the skeletal system.
Distinctive Composition
Teeth are composed of four unique tissues, each with a specific role. Enamel, the outermost layer covering the visible crown, is the hardest substance in the human body, consisting of 96% mineral, primarily crystalline hydroxyapatite. This tissue is acellular, meaning it contains no living cells, blood vessels, or nerves once fully formed. Beneath the enamel lies dentin, which makes up the bulk of the tooth. Dentin is less mineralized than enamel but harder than bone, composed of about 70% mineral, and contains living cellular components called odontoblasts that maintain it throughout life.
The root of the tooth is covered by cementum, a bone-like tissue that provides attachment but is generally avascular, unlike bone. At the center of the tooth is the pulp, a soft tissue containing nerves, blood vessels, and connective tissue, which supplies nutrients and sensation to the tooth. In contrast, bone tissue is a dynamic, living tissue comprising a collagen protein matrix (about 30%) interwoven with minerals like calcium phosphate (about 70%). Bones contain various living cells, including osteoblasts for bone formation, osteocytes for maintenance, and osteoclasts for bone resorption, and feature a central marrow cavity responsible for blood cell production.
Unique Development and Renewal
Tooth formation (odontogenesis) begins during embryonic development. Primary teeth start forming between the sixth and eighth week of prenatal development, with permanent teeth following around the twentieth week. Once tooth enamel is fully formed and the tooth erupts, it is not living tissue and cannot regenerate or repair itself if significantly damaged, such as from decay or trauma. Dentin, however, can undergo limited repair through the activity of odontoblasts, which can produce reparative dentin in response to stimuli, but this process does not fully restore the tooth like bone healing.
Bone development (osteogenesis) involves a continuous, lifelong process of remodeling. Old or damaged bone is constantly removed by osteoclasts, and new bone tissue is formed by osteoblasts. This dynamic remodeling allows bones to adapt to stress, repair themselves after fractures, and continuously renew their structure. The extensive cellular activity and blood supply within bones enable this remarkable capacity for self-repair, a fundamental difference from the limited regenerative abilities of teeth.
Specialized Functions and Attachment
Teeth perform specialized functions primarily related to digestion and communication. Their main role is mastication, or chewing, which mechanically breaks down food, and they also contribute to speech articulation. Bones, conversely, serve broader skeletal system functions, including providing structural support for the body, protecting internal organs, facilitating movement, and acting as a reservoir for minerals like calcium and phosphorus. Bones also play a role in hematopoiesis, the production of blood cells, which occurs within the bone marrow.
The way teeth are anchored in the jaw also distinguishes them from bones. Teeth are embedded in the jawbone via the periodontal ligament, a flexible connective tissue that allows for slight movement and acts as a shock absorber during chewing. This flexible attachment contrasts with the more rigid articulations or direct fusions seen between most bones in the skeletal system. Unlike bones, teeth do not contain bone marrow.