Odontogenesis, or tooth development, starts in the early embryonic developmental stages and continues for some time after birth. Teeth form in a corono-apical direction and are derived from ectoderm from the first arch, along with ectomesenchyme from neural crest cells. Size, shape, and location are genetically and independently determined for each tooth. By day 23 of gestation, the paired mandibular and maxillary, as well as the medial nasal process, have become distinct. The dental lamina first appears at 25 days of gestation in the dog embryo. By day 30, the laminae of the left and right arches fuse to make a continuous arch. The components of the tooth germ that later develop are the dental papilla, enamel (or dental) organ, and dental follicle. The enamel organ, oral epithelium, and dental lamina originate from the outer embryonic germ layer known as ectoderm. The dental papilla and sac appear in coordination with the enamel; however, they originate from the mesoderm.
Differentiation of the enamel organ for deciduous dentition begins at about day 30 and progresses through the following sequence of stages: bud, early cap, cap, advanced cap, early bell, and advanced bell. The enamel organ gives rise to or is responsible for root formation, primary epithelial attachment, enamel, and crown patterning. The dental papilla gives rise to or is responsible for crown patterning, dentin, and pulp. The dental follicle gives rise to or is responsible for the cementum, alveolar bone, and periodontal ligament.
Calcification of all deciduous teeth initiates at day 55 of gestation and is completed by day 20 after birth for the crowns and day 45 for the roots. The dental lamina buds that form the primary dentition develop lingual extensions referred to as successional lamina. The successional lamina also progresses through bud, cap, and bell stages to eventually form the successional permanent dentition. The only permanent tooth to calcify prenatally is the mandibular first molar, which can be noted in abdominal imaging studies at day 55 of gestation. All deciduous teeth also show calcification by this time. If a deciduous tooth is congenitally missing, the succeeding tooth usually will not form. The first premolar and the molars do not have deciduous counterparts. These nonsuccessional teeth develop directly from the dental lamina in the caudal portion of the arch.
Formation of the root begins after the general form of the crown has developed but before it completely calcifies. Tooth number and size may be altered when the induction phase is disturbed, and structural changes, such as enamel and dentinal hypoplasia or dysplasia, may develop if the insult occurs during apposition and maturation (bell and maturation stages). Genetic anomalies can affect all teeth of an individual, such as with dentinogenesis imperfecta and amelogenesis imperfecta. Environmental factors, such as trauma or metabolic, chemical, or infectious agents, may affect one or more teeth.
