At 8 weeks of intrauterine life, clumps of mesenchymal cells induce the dental lamina to form swellings known as enamel organs. Each enamel organ will be responsible for the development of each tooth (Fig. 1a).
As the enamel organs grow and increase in size, the inner aspect becomes concave, resembling skull caps. By the late cap stage, at 12 weeks of intrauterine life, cells on the inner aspect of the enamel organ change from cuboidal to columnar, forming the inner enamel epithelium. The outer layer of cells remains cuboidal and is known as the outer enamel epithelium (Fig. 1b).
Beneath the inner enamel epithelium the condensation of mesenchymal cells is termed the dental papilla; this will eventually become the pulp. A fibrous capsule surrounds each enamel organ and this is termed the dental follicle; this will eventually become the periodontal ligament.
By 14 weeks of intrauterine life (Fig. 1c) the enamel organ consists of the following:
■ Inner enamel epithelium: cells lining the inner surface of the enamel organ which, at the bell stage, are columnar in shape. The inner enamel epithelium defines the shape of the crown and eventually the cells differentiate into enamel-forming cells (ameloblasts).
■ Stellate reticulum: this lies between the stratum intermedium and the outer enamel epithelium. It consists of star-shaped cells which protect the underlying dental tissues; it also maintains the shape of the tooth.
■ Outer enamel epithelium: these cells line the outer surface of the enamel organ. They are cuboidal in shape and maintain the shape of the enamel organ. The outer enamel epithelium meets with the internal enamel epithelium at the cervical loop. Eventually the inner and outer enamel epithelium grows downwards at the cervical loop forming Hertwig’s root sheath which maps out the shape of the root.