|
Deciduous teeth, otherwise known as baby teeth, temporary teeth, milk teeth, and now more commonly primary teeth,〔 are the first set of teeth in the growth development of humans and other diphyodont mammals. They develop during the embryonic stage of development and erupt—that is, they become visible in the mouth—during infancy. They are usually lost and replaced by permanent teeth, but in the absence of permanent replacements, they can remain functional for many years. ==Primary dentition stage== Primary teeth start to form during the embryo phase of pregnancy. The development of primary teeth starts at the sixth week of tooth development as the dental lamina. This process starts at the midline and then spreads back into the posterior region. By the time the embryo is eight weeks old, there are ten buds on the upper and lower arches that will eventually become the primary (deciduous) dentition. These teeth will continue to form until they erupt in the mouth. In the primary dentition there are a total of twenty teeth: five per quadrant and ten per arch. The eruption of these teeth ("teething") begins at the age of six months and continues until twenty-five to thirty-three months of age during the primary dentition period. Usually, the first teeth seen in the mouth are the mandibular centrals and the last are the maxillary second molars. The primary dentition is made up of central incisors, lateral incisors, canines, first molars, and secondary molars; there is one in each quadrant, making a total of four of each tooth. All of these are gradually replaced with a permanent counterpart except for the primary first and second molars; they are replaced by premolars. These are designated in the Universal Tooth Designation System by the capital letters ''A'' through ''T''.〔Illustrated Dental Embryology, Histology, and Anatomy, Bath-Balogh and Fehrenbach, Elsevier, 2011, page 255〕 The replacement of primary teeth begins around age six, when the permanent teeth start to appear in the mouth, resulting in mixed dentition. The erupting permanent teeth cause root resorption, where the permanent teeth push on the roots of the primary teeth, causing the roots to be dissolved by odontoclasts (as well as surrounding alveolar bone by osteoclasts) and become absorbed by the forming permanent teeth. The process of shedding primary teeth and their replacement by permanent teeth is called exfoliation. This may last from age six to age twelve. By age twelve there usually are only permanent teeth remaining. Teething age of primary teeth: * Central incisors : 6–12 months * Lateral incisors : 9–16 months * First molars : 13–19 months * Canine teeth : 16–23 months * Second molars : 22–33 months 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Deciduous teeth」の詳細全文を読む スポンサード リンク
|