that the periodontal ligaments play an important role in tooth eruption. primary teeth erupt into the mouth from around six months until two years of age. these teeth are the only ones in the mouth until a person is about six years old. at that time, the first permanent tooth erupts. this stage, during which a person has a combination of primary and permanent teeth, is known as the mixed stage. the mixed stage lasts until the last primary tooth is lost and the remaining permanent teeth erupt into the mouth. there have been many theories about the cause of tooth eruption. one theory proposes that the developing root of a tooth pushes it into the mouth. another, known as the cushioned hammock theory, resulted from microscopic study of teeth, which was thought to show a ligament around the root. it was later discovered that the "ligament" was merely an artifact created in the process of preparing the slide. currently, the most widely held belief is that the periodontal ligaments provide the main impetus for the process. the onset of primary tooth loss has been found to correlate strongly with somatic and psychological criteria of school readiness. supporting structures histologic slide of tooth erupting into the mouth a: tooth b: gingiva c: bone d: periodontal ligaments the periodontium is the supporting structure of a tooth, helping to attach the tooth to surrounding tissues and to allow sensations of touch and pressure. it consists of the cementum, periodontal ligaments, alveolar bone, and gingiva. of these, cementum is the only one that is a part of a tooth. periodontal ligaments connect the alveolar bone to the cementum. alveolar bone surrounds the roots of teeth to provide support and creates what is commonly called an alveolus, or "socket". lying over the bone is the gingiva or gum, which is readily visible in the mouth. periodontal ligaments the periodontal ligament is a specialized connective tissue that attaches the cementum of a tooth to the alveolar bone. this tissue covers the root of the tooth within the bone. each ligament has a width of 0.15–0.38mm, but this size decreases over time. the functions of the periodontal ligaments include attachment of the tooth to the bone, support for the tooth, formation and resorption of bone during tooth movement, sensation, and eruption. the cells of the periodontal ligaments include osteoblasts, osteoclasts, fibroblasts, macrophages, cementoblasts, and epithelial cell rests of malassez. consisting of mostly type i and iii collagen, the fibers are grouped in bundles and named according to their location. the groups of fibers are named alveolar crest, horizontal, oblique, periapical, and interradicular fibers. the nerve supply generally enters from the bone apical to the tooth and forms a network around the tooth toward the crest of the gingiva. when pressure is exerted on a tooth, such as during chewing or biting, the tooth moves slightly in its socket and puts tension on the periodontal ligaments. the nerve fibers can then send the information to the central nervous system for interpretation. alveolar bone the alveolar bone is the bone of the jaw which forms the alveolus around teeth. like any other bone in the human body, alveolar bone is modified throughout life. osteoblasts create bone and osteoclasts destroy it, especially if force is placed on a tooth. as is the case when movement of teeth is attempted through orthodontics, an area of bone under compressive force from a tooth moving toward it has a high osteoclast level, resulting in bone resorption. an area of bone receiving tension from periodontal ligaments attached to a tooth moving away from it has a high number of osteoblasts, resulting in bone formation. gingiva the gingiva ("gums") is the mucosal tissue that overlays the jaws. there are three different types of epithelium associated with the gingiva: gingival, junctional, and sulcular epithelium. these three types form from a mass of epithelial cells known as the epithelial cuff between the tooth and the mouth. the gingival epithelium is not associated directly with tooth attachment and is visible in the mouth. the junctional epithelium, composed of the basal lamina and hemidesmosomes, forms an attachment to the tooth. the sulcular epithelium is nonkeratinized stratified squamous tissue on the gingiva which touches but is not attached to the tooth. tooth decay