The bud stage is the first stage of tooth development. It represents the first epithelial intrusion into the ectomesenchyme. The cells of the epithelium show minimal changes and the ectomesenchymal cells surround the epithelial bud. Due to the ectomesenchymal condensation (a process in which the epithelial bud propagates into the ectomesenchyme), the density of the cells increases near the epithelial outgrowth. This condensation is owed to the increased mitotic activity carried in the cells of the tooth bud and mesenchymal cells surrounding it. These buds develop at the distal side of the dental lamina and each bud represents a group of cells at dental lamina's end. The epithelial part is separated from the mesenchyme by a basement membrane. The ectomesenchyme surrounding the tooth bud is called the dental follicle or sac whereas the area directly subjacent to the condensation is called the dental papilla. The dental follicle is ultimately responsible for the formation of cementum, periodontal ligament (PDL), and alveolar bone. The dental papilla is responsible for the formation of dental pulp and dentin.
1.1.2 Key Identifying Features
The enamel organ at this stage appears roughly ovoid to spherical with poor histodifferentiation and morphodifferentiation. A typical tooth bud consists of centrally located polygonal (multiple‐shaped) cells and peripherally arranged columnar cells.
1.1.3 Clinical Significance
The successful development of the tooth depends on the interaction of epithelial and mesenchymal components. If these parts grow individually, neither will differentiate further [1]. This epithelial–mesenchymal interaction starts in the bud stage; therefore, any problem affecting the bud stage could seriously affect the development of teeth.
1.2 Cap Stage
Figure 1.4 H and E stained section showing the cap stage of tooth development.
Figure 1.5 H and E stained section showing the cap stage of tooth development.
1.2.1 Description
The cap stage is the second stage of tooth development. As the tooth bud matures, it takes part of dental lamina along with it, which is called the lateral lamina. The tooth bud grows non‐uniformly, and the growth is more in certain areas and less in others. This stage is called the cap stage as the epithelial outgrowth looks like a cap which is sitting on top of the condensed ectomesenchyme (dental papilla). During this stage, greater differentiation is seen in the central and peripheral cells. The central polygonal cells change into the stellate reticulum cells which have a somewhat star‐shaped appearance due to greater intake of water, pushing the cells apart but retaining their desmosomal attachments. The peripheral cells change into external and inner enamel epithelium. The outer enamel epithelium cells are cuboidal whereas the inner epithelial cells are tall and columnar. These layers of epithelial cells are separated from the dental follicle and dental papilla by a basement membrane. Another structure called the enamel knot is formed during this stage which represents a collection of cells in the center of the inner enamel epithelium. It is a transitory structure which is believed to contribute cells to the enamel cord (strand of cells).
1.2.2 Key Identifying Features
The enamel organ resembles a cap present on top of the dental papilla. The dental follicle and dental papilla become more recognizable during this stage compared to the bud stage.
1.2.3 Clinical Significance
It is believed that the blood supply of the tooth is established during the cap stage. The blood vessels first enter through the dental follicle, then move into the dental papilla [2]. Any disruptions during this stage could severely affect the vascular supply of the tooth and in turn, its maturation, vitality, and eruption.
1.3 Early Bell Stage
Figure 1.6 H and E stained section showing the early bell stage of tooth development (green circle, cervical loop; arrows, dental papilla).
Figure 1.7 H and E stained section showing the early bell stage of tooth development (arrow, dental follicle).
1.3.1 Description
During this stage, the enamel organ resembles a bell. It is during this stage that the tooth crown will undertake its final shape (morphodifferentiation) and ameloblasts along with odontoblasts are histodifferentiated. The region where the outer and inner enamel epithelial cells meet at the border of enamel organ is called the cervical loop. In between the stellate reticulum and the inner enamel epithelial cells, some of the cell population differentiates and forms a new layer of cells called stratum intermedium. The cells of the inner enamel epithelium and stratum intermedium work collaboratively to form the enamel tissue. The enamel organ during the early bell stage clearly shows its four diverse layers: outer enamel epithelium, inner enamel epithelium, stellate reticulum, and stratum intermedium. During this stage, the enamel organ loses its contact with the oral epithelium as the dental lamina is broken down. This connection is restored during the process of tooth eruption. The remnants of dental lamina are called epithelial rest of Serres. In the early bell stage, the enamel knot disappears and the enamel cord appears between the stratum intermedium and stellate reticulum.
1.3.2 Key Identifying Features
On histological sections, bell‐shaped enamel organ dissociated from the oral epithelium can be seen clearly. The tooth germ during this stage is enclosed by dental follicle. The cervical loop is also very prominent and easily recognizable.
1.3.3 Clinical Significance
Many important structures appear during this stage. It is believed that enamel cord facilitates the change from the cap to bell stage [1]. The cervical loop is responsible for the formation of Hertwig's epithelial root sheath (HERS) [3].
1.4 Late Bell Stage
Figure 1.8 H and E stained decalcified section showing the late bell stage of tooth development.
Figure 1.9 H and E stained decalcified section showing the late bell stage of tooth development (white arrow, enamel; black arrow, dentin).