DEVELOPMENT AND GROWTH OF TEETH – PART I: INITIATION, BUD STAGE & CAP STAGE
Author: Sanketh DS, MDS
The development and growth of teeth is a complex process of interactions between the primitive oral epithelium and the underlying ectomesenchymal cells. The epithelium is derived from the ectoderm of the first pharyngeal arch while the cells of the ectomesenchyme are neural crest in origin. Since the neural crest cells are primarily derived from the ectoderm and eventually give rise to mesenchymal tissues, they are called ectomesenchymal cells. Tooth development, though a continuous process, can be divided into three stages called the bud, cap and bell stages. These stages are so named because of the shape the enamel organ assumes in each stage.
Early events (Initiation)
After about six weeks of development of the embryo, a proliferation of cells in the form of a thickened band appears in the presumptive or developing upper and lower jaws. They are horse-shoe shaped and correspond to the position of the future upper and lower jaws. This thick band of proliferation is called the primary epithelial band. This epithelial band bifurcates into a lingual process called the dental lamina and a buccal or facial growth called vestibular lamina. It is on the dental lamina that the teeth will form and the vestibular lamina would eventually give rise to the oral vestibule between the teeth and cheeks.
Now, certain points in the dental lamina would develop an additional growth of cells called placodes that protrude into the underlying mesenchyme. These points, 20 in number (both upper and lower) correspond to the sites of the future deciduous teeth. The permanent tooth germs develop from a lingual extension of the dental lamina and are called successional lamina. It has to however be noted that permanent molars do not develop from a successional lamina and arise from the dental lamina itself, posterior to the primary molars.
The placode proliferates and takes the shape of a bud, giving rise to the name “Bud Stage” which is the first stage of tooth development. During the process of tooth bud formation, there is also a dense condensation of cells surrounding the tooth bud. These cells are called ectomesenchymal cells and are derived from the neural crest cells.
The tooth bud is referred to as the enamel organ due to the fact that it would eventually give rise to the enamel of the tooth.
The enamel organ in the bud stage comprises of low columnar cells peripherally and polygonal cells centrally.
Rather than all cells dividing uniformly in the tooth bud, there is a differential proliferation of cells or an unequal rate in the proliferation of cells. This leads to a concavity or an infolding of the epithelium resulting in the bud shaped enamel organ becoming cap shaped! In the cap stage, the enamel organ comprises of three different types of cells called the inner & outer enamel epithelium and the stellate reticulum.
1. Inner enamel epithelial cells are low columnar and line the invagination of the “cap” on the inside while the periphery of the cap is lined by cuboidal outer enamel epithelium.
2. In between these cells, filling the enamel organ are cells called stellate reticulum. These cells were the polygonal shaped cells of the enamel organ during the bud stage. These polygonal cells synthesize and secrete glycosaminoglycans into the extracellular compartment between the cells. Glycosaminoglycans are hydrophilic and hence attract water from the adjacent ectomesenchyme into the enamel organ. When water starts filling the extracellular compartment of the enamel organ, the central polygonal cells start to separate and are pushed apart. However, these cells maintain contact with each other at desmosomal junctions and this gives them a star shaped appearance. They are thus called stellate reticulum.
3. The ectomesenchyme cells condensed just below and within the concavity of the cap are called dental papilla and the cells surrounding the enamel organ and the dental papilla constitute the dental sac or follicle. The dental papilla would eventually form the dentin and pulp and the dental follicle would form the cementum, periodontal ligament and the alveolar bone.
Together the enamel organ, dental papilla and the dental follicle constitute the tooth germ or the dental organ.
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Nanci A. Tencate’s Oral Histology. Development, Structure and Function. 8th ed. Elsevier;2013.
Kumar GS. Orban’s Oral Histology and Embryology.13th ed. Elsevier;2011.
Histologic slide of tooth in cap stage:
Histology of developing tooth. Tooth is in Cap Stage/Dozenist/CC-BY-SA 3.0
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