Early Tooth Development

Teeth are formed from oral epithelium, in the form of a dental lamina, and neural crest ectomesenchyme of the maxillary and mandibular processes (Fig 1-1). The oral epithelium contributes the enamel component, and the ectomesenchyme contributes the dentin and cementum components of the fully formed tooth. Although the initiating events that trigger downgrowth of the oral epithelium to form a dental lamina are incompletely understood, it is known that neural crest ectomesenchyme is necessary.1,2 Early reciprocal inductive interactions between the oral epithelium and the underlying ectomesenchyme, and subsequent interactions between the enamel organ and dental papilla, coordinate the sequential events of tooth development.3–5

Efforts to understand the instructional signals that originate in each of these interacting tissues have been ongoing for more than 50 years.6,7 Most investigations have been performed with dental tissues obtained from embryonic mice and rats or with the continuously growing incisor teeth of adult mice and rats. Organ culture techniques have been perfected to study the growth of dental tissues in chemically defined media, to observe the results of various epithelial-mesenchymal combinations, and to examine the effects of various growth factors on odontogenesis. Thus, nearly all current insight into the regulatory mechanisms of tooth development has come from studies of animal models, often from tooth buds grown in organ culture.

This chapter contains a discussion of the initiation of tooth formation and the histodifferentiation of the enamel organ and dental papilla. Subsequent chapters will examine the cytodifferentiation of dentin-and enamel-forming cells and the secretion and mineralization of their respective matrices.

Role of the Neural Crest

Early in embryogenesis, soon after the neural tube forms by invagination of the overlying ectoderm, migratory pluripotent neuroepithelial cells, the neural crest cells, migrate from the dorsal midline region of the neural tube.8 In exiting from the neural tube, neural crest cells lose their epithelioid characteristics and assume a mesenchymal phenotype capable of directed cell migration. Cranial neural crest cells invade the developing branchial arches