Crown stage of tooth development

The enamel organ consists the inner and the outer enamel epithelia and the stellate reticulum. The enamel organ encloses the the dental pulp.

The outer enamel epithelium forms the outer surface of the enamel organ and consists of cuboidal ectodermal cells.

The inner enamel epithelium consists of columnar ectodermal cells that form the inner surface of the enamel organ in close proximity to the ectomesenchymal cells of the papilla. The cells differentiate into ameloblasts that secrete enamel.

The location where the inner enamel epithelium/pre-ameloblasts and the outer enamel epithelium meet is called the cervical loop. This structure will elongate during the root phase of tooth development to form the Hertwig’s epithelium root sheath.

The ectodermal cells within the center of the enamel organ secrete glycosaminoglycans that draw in water, causing the cells to appear star-shaped. The large stellate reticulum allows space for the morphodifferentiation of the crown of the tooth. At the cusp to this tooth the stellate reticulum is condensed with the other layers of the enamel organ to form the reduced enamel epithelium.

The dental follicle is formed by ectomesenchymal stem cells that surround the outer most aspect of the apical part of the tooth germ. These cells differentiate into cementoblasts that deposit cementum, fibroblasts that deposit collagen fibers as part of the periodontal ligament, and osteoblasts that deposit the alveolar bone.

During the crown stage of tooth development the ectomesenchymal cells of the dental papillae differentiate into the dental pulp organ that will form the dental pulp

The cusp is the pointed or rounded projection on the occlusal or incisal surface of the tooth that first emerges through the gingiva.

During the crown stage, ectomesenchymal pre-odontoblasts fully differentiate into odontoblasts that deposit pre-dentin, the organic matrix of dentine into the basement membrane that separates the ameloblasts (ectodermal cells) from these ectomesenchymal cells. Processes extend from the odontoblasts through dentinal tubules toward the enamel or cementum. Odontoblasts are retained in the dental pulp throughout life.

Dentin is deposited similarly to bone: the organic matrix is deposited first and provides a framework on which hydroxyapatite crystals are laid down to form dentin. The organic matrix that is deposited by the odontoblasts that reside in the tooth pulp is called pre-dentin. There is always a layer of organic pre-dentin between the mineralized dentin and the odontoblast cell bodies throughout the life of the tooth.

Dentin is deposited similarly to bone: the organic matrix is deposited first and provides a framework on which hydroxyapatite crystals are laid down to form dentin. Dentin makes up the bulk of the hard tissue of the tooth and is 70% mineralized. Dentin is the first hard tissue deposited during tooth development.

Enamel is the most mineralized tissue in the human body, being 96% mineralized with hydroxyapatite calcium phosphate crystals. Enamel is mineralized quickly after the organic matrix is deposited by ameloblasts, so there is no apparent organic matrix layer, as there is in dentin.

The columnar ectodermal cells on the inner aspect of the enamel organ that deposit the organic matrix of enamel are fully differentiated ameloblasts. During the crown stage of tooth development the pre-ameloblasts fully differentiate ameloblasts that deposit enamel starting at the cusp of the tooth and moving apically.

Starting at the cusp of the tooth, the cell layers of the enamel organ will condense together forming a reduced enamel epithelium. The reduced enamel epithelium contains the inner enamel epithelium/pre-ameloblasts/ameloblasts, the stratum intermedium, the stellate reticulum, and the outer enamel epithelium. The reduced enamel epithelium will allow nutrients from blood vessels on the outside of the outer enamel epithelium (red arrows) to be supplied to the pre-ameloblasts/ameloblasts once hard tissue is deposited.