A process which starts at about the 6th week of intrauterine life from a thickening of the ectodermal epithelium, which forms the primary epithelial band (initiation stage); from this develops the dental lamina, on which swellings (enamel organs) differentiate through a series of stages (bud, cap, and bell) to form the tooth structure with the formation of both enamel (amelogenesis) and dentine (dentinogenesis). The bud stage occurs at about 8 weeks of intrauterine development. Clumps of mesenchymal cells induce the dental lamina to form swellings known as enamel organs or tooth buds. These swellings are subsequently responsible for the development of each tooth. Ten buds develop per arch representing the primary (deciduous) teeth; subsequently the permanent tooth buds develop lingually to the primary tooth buds. The cap stage starts at about 9–11 weeks of intrauterine life, when the cells on the inner aspect of the enamel organ change from cuboidal to columnar in shape. Unequal cell growth leads to the characteristic shape: the peripheral cells (outer enamel epithelium) are cuboidal and the cells in the concavity are tall and columnar (inner enamel epithelium); the cells in between differentiate to form a delicate cellular network (stellate reticulum). The bell stage takes place at about the 12–14th week of intrauterine life. The enamel organ is bell-shaped and consists of an inner enamel epithelium covered by the stratum intermedium, and these two layers are separated by the stellate reticulum. The internal enamel epithelium induces the adjacent cells of the dental papilla to differentiate into odontoblasts and form dentine; the dentine then induces the internal enamel epithelium to differentiate into ameloblasts and form enamel. The stellate reticulum expands by increasing the amount of intracellular fluid and then collapses before the formation of enamel. The enamel organ forms Hertwig's sheath, which determines the shape of the root and initiates the formation of root dentine.
During development, teeth may become ankylosed, dilacerated, or geminated. The number of teeth may be greater (hyperdontia) or fewer (hypodontia) than normal, resulting in supernumerary or supplemental teeth, or they may be altered in shape (peg-shaped, Moon's molars, Hutchinson's incisors). Abnormalities of tooth structure include enamel hypomineralization, amelogenesis imperfecta, and dentinogenesis imperfecta. See also dentition.
Katchburian E., Holt S. J. Studies on the development of ameloblasts. Journal of Cell Science 1972;11:415–47.
http://www.youtube.com/watch?v=LVC4_FIEeFs A video on YouTube describing the histological changes during tooth development.
Stages of tooth development