Events that take place on the dentin reverberate to the pulp and vice versa [5].

The dentin–pulp complex is surrounded on the crown by dental enamel and on the root by cementum, periodontal ligament, and bone. The harmony of the complex is impaired if the surrounding tissues suffer some kind of injury that can reach the pulp by the root canal or through the dentinal tubules [5].

Although the tooth is a unique organ, the principles that guide its development are shared in common with other organs such as lung, kidney, heart, mammary glands, and hair follicles [6]. The most important developmental events are those guiding epithelial–mesenchymal interactions, which are characterized by a molecular cross talk between two tissues of different origins, the ectoderm and the mesenchyme [6].

Different stages of tooth development have been recognized at a microscopic level by their histologic appearance and were classically described as the dental lamina, bud, cap, and the early and late bell stages. In the modern literature, a functional terminology has been used to describe odontogenesis into four phases: initiation, morphogenesis, cell differentiation (cytodifferentiation), and matrix apposition [6]. The dental lamina is the first sign of tooth development. At the lamina stage, cells of the dental epithelium and those of the underlying ectomesenchyme divide at different rates and continue to grow and thicken to form a bud [6]. At the bud stage, cells of the ectomesenchyme proliferate and condense to give rise to the dental papilla. These cells have increased ability to proliferate, mobilize, and differentiate.

The morphogenetic phase involves the stages of bud, cap, and initial bell phase. During this period, a number of ectomesenchymal cells adjacent to the epithelium increase inside the ectomesenchyme, producing the site of origin of the dental papilla and of the dental follicle. These will develop into the dentin–pulp complex and into the support tissues of the tooth, respectively [6]. The formation of the enamel knot, during the transition from bud to cap, marks the beginning of crown formation. The cells of the enamel knot do not grow and serve as a signal for the cuspid formation pattern, influencing the form of the crown and the development of the dental papilla [6].

During the initial bell phase, the epithelium cells assume different morphologies, giving rise to the enamel organ, also called dental organ. This enamel organ is composed of the following four different stratums: internal enamel epithelium, stratum intermedium, stellate reticulum, and external enamel epithelium.

The internal epithelium of the enamel organ interacts with the undifferentiated superficial mesenchymal cells (also known as embryonic cells) of the dental papilla to form enamel, dentin, and pulp. Overall, cells of the internal dental epithelium elongate and become highly columnar, starting the late bell stage (phase of cytodifferentiation). This modification of the internal dental epithelium cells serves as a signal for the peripheral mesenchymal cells of the dental papilla, which after lining the basement membrane differentiate and assume an elongated morphology with odontoblast phenotype [6]. This phenomenon, characterized as differentiation of odontoblasts, has been intensely studied, leading to important advances in the knowledge of pulpal biology, particularly related with the mechanisms of healing of this specialized connective tissue against injuries and different pathological stimuli. More details about this issue will be discussed later in this chapter.

It is important to know the basic process of differentiation of the superficial undifferentiated mesenchymal cells from the dental papilla by stimuli expressed by cells of the internal dental epithelium. Growth factors, particularly those belonging to the superfamily TGF-β, are expressed by these epithelial cells. While the mesenchymal cells of the most superficial region of the dental papilla become competent, the last mitosis of the mother cell is positioned adjacent to the basal membrane. The daughter cells remain in the internal area of the papilla and will be part of the cell-rich zone that is clearly observed in the mature pulp. At this point both mother and daughter cells are referred as pre-odontoblasts because they assume the competency to differentiate into odontoblasts. Between the internal dental epithelium cells and the pre-odontoblasts is the basal membrane, which is composed of collagen, laminin, heparin sulfate, and other proteoglycans.

This basal membrane has an important role in the reciprocal activation of the epithelium/mesenchyme, resulting in a variety of epigenetic interactions, determining the phenotype of the odontoblasts. After the epithelial cells secrete the growth factors of the TGF-β superfamily, these bioactive proteins remain attached to the basal membrane. The components of the basal membrane activate these TGFs to interact with membrane receptors of the pre-odontoblasts. The translation of these signals results in the activation of the pre-odontoblasts, which start to secrete more growth factors and express the msxs genes. The final differentiation of the pre-odontoblasts into odontoblasts occurs only after the interaction of the fibronectin, which is deposited on the basal membrane and the 165 KDa pre-odontoblasts membrane receptors. The sequence of differentiation of the dental papilla mesenchymal cells into odontoblasts until the beginning of the synthesis and deposition of collagen-rich dentinal matrix is presented in Fig. 2.2.

The odontoblasts synthesize the dentinal organic matrix both adjacent to the cellular body and close to the mineralization fronts. During the period of upregulation of the TGFs, the pre-odontoblasts start to synthesize fibronectin and express the membrane protein 165 KDa, which is required to interact with the fibronectin. The msxs homeoproteins are probably involved in the reorganization of the pre-odontoblast cytoskeleton, which plays an important role in the process of differentiating into elongated cells, referred to as odontoblasts.

As dentin is formed, the most cervical cells of the internal epithelium of the enamel organ become pre-ameloblasts, an event that occurs from the incisal (or from the cuspids) to the cervical area. When the dentin formation approximates the cervical loop, the cells of internal and external epithelium of the enamel organ proliferate from the loop, forming a double layer of cells, also known as Hertwig’s epithelial root sheath. The expansion of the sheath is followed by the formation of the radicular dentin. The cells of the dental follicle closer to the external layer of the sheath differentiate into cementoblasts and start to produce the cementum organic matrix; the follicle also produces the periodontal ligament and alveolar bone. There is a free border on the epithelial sheath, the epithelial diaphragm, which closes slowly as the root is formed (Fig. 2.3). As long as the apex of the root is not totally formed, there will be dental papilla, composed of ectomesenchymal cells. This has clinical relevance, because part of these cells can remain vital even after pulp necrosis. In this specific condition, it is possible to have a continuous apexogenesis or the formation of the radicular apex with dentin. When there is a severe reduction in the number of ectomesenchymal cells in the dental papillae, the tooth can still have apexification induced by an intracanal dressing [5]. This issue will be dealt in detail in Chap. 8.