The first evidence of the future teeth appears when the epithelial cells near the basement membrane begin to multiply (four to five cell layers) and invaginate into the underlying ectomesenchyme, giving rise to the dental lamina. The ectomesenchyme starts to change composition in response and becomes more condensed. Thus, this initial epithelial invagination clearly marks the apparition of the tooth crown area and will develop through several distinct stages (bud, cap, bell stage). Tooth development is a continuous process, so clear distinction between the transition stages is not possible.

Each dental lamina is at the origin of a tooth bud (Fig. 1.2). Tooth buds of the deciduous canines and incisors are apparent in the 8-week-old human embryo, and buds of the deciduous molars are formed during the ninth week. The bud stage is characterized by the progression of ectodermal invagination in the underlying ectomesenchyme, in which cells are packed closely around the epithelial bud. This will be followed by the changes in the shape of the dental bud and formation of the dental cap (Figs. 1.2 and 1.3). The cap stage is characterized by a concavity of the epithelium that partially envelops the underlying mesenchyme. During the cap stage, the epithelial outgrowth is referred widely as the enamel organ and is related to the differentiation of the outer dental epithelium, inner dental epithelium, and the appearance of the enamel knot. Also, there is a condensation of the ectomesenchyme in the concavity of the enamel organ forming the dental papilla, at the origin of the odontoblasts and the dental pulp [51, 52] (Figs. 1.4 and 1.5).

Starting from the late cap stage and through the transition from cap to bell stage of tooth development, many developmental changes are observed. All the elements of the enamel organ are well distinguished (histodifferentiation):

Besides the role of the enamel knots to regulate the size and shape of the teeth, the signals from the mesenchyme are also necessary for the formation and maintenance of epithelial compartments.

The cap stage is followed by the bell stage, during which the dental crown acquires its final shape (morphodifferentiation) and the formation of the cusp pattern is observed (Figs. 1.6 and 1.7).

The outer and inner enamel epithelia are continuous, and they meet at the rim of the enamel organ known as the zone of reflection or cervical loop. Extended in Hertwig’s epithelial root sheath, this cervical loop will control the formation of the root, including the root odontoblast differentiation (Figs. 1.8 and 1.9). The cervical loop progresses in apical direction, and many cell divisions sustain this preeruptive crown growth, thus delimiting increasingly the dental papilla area. In the pluricuspid teeth, the secondary enamel knots appear at the top of each cusp.

In the late bell stage, tooth morphogenesis is followed by a phase of cell differentiation of the inner enamel epithelium and of the ectomesenchymal cells at the epithelial-mesenchymal interface with the basement membrane (histodifferentiation). These cells will differentiate in pre-ameloblasts and pre-odontoblasts in order to become polarized and secreting ameloblasts and odontoblasts to form the enamel and dentin, respectively. The first layers of the enamel and dentine are visible at the end of the coronary morphogenesis. Thus, during embryogenesis, morphogenesis and differentiation are coupled. Cells acquire the competence to differentiate according to their position. Differentiation of pre-ameloblasts and odontoblasts is pre-coupled spatiotemporally to these morphogenetic movements that provide the pattern formation of the crown and the beginning of the root formation.

The condensed ectomesenchyme situated at the periphery of the enamel organ and dental papilla is referred as the dental follicle or dental sac and will give rise to the supporting dental tissues as the tooth cementum, periodontal ligament, and alveolar bone. Thus, the dental follicle is involved in the formation of the root and tooth eruption (Figs. 1.5 and 1.10).

The enamel organ, the dental papilla, and the dental follicle form the dental organ or tooth germ.

In humans, there are two dentitions, deciduous and permanent (primary, temporary). The development of the deciduous dentition begins around the sixth gestational week. Then quickly, there is coexistence of deciduous and permanent dental germs, and odontogenesis ends around the age of 18–25 years by formation of the dental root and the eruption of the third permanent molars.

The development of the permanent teeth begins during the odontogenesis of deciduous teeth. The deciduous teeth are formed from the primary dental lamina. In humans, the permanent teeth develop in two ways: (1) sequentially at the lingual region of the enamel organ of each temporary tooth (successional teeth) or (2) permanent molars grow from an extension of the initial dental lamina. These are non-successional teeth.

The spatiotemporal control of dental development is orchestrated by epithelial-mesenchymal interactions. At the molecular level, these interactions provide reciprocal and sequential exchanges of signals through the basement membrane. These signals/morphogens are associated with the “determination” (commitment) of the cells in different territories during the morphogenesis.