Late bell stage of tooth development

The outer enamel epithelium consists of cuboidal ectodermal cells that form the outer surface of the enamel organ.

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. During the late bell stage, these cells first differentiate into pre-ameloblasts and then into fully differentiated ameloblasts, capable of depositing enamel. At this stage, pre-ameloblasts are indistinguishable from the cells of the inner enamel epithelium, and they have not yet differentiated into ameloblasts.

The stratum intermedium consists of a transient population of 2 to 3 layers of cells located between the inner enamel epithelium and the stellate reticulum. These cells are derived from the inner enamel epithelium and play a critical role in the differentiation of ameloblasts.

Starting at the cusp of the tooth, the cell layers of the enamel organ condense into the reduced enamel epithelium. It consists of the inner enamel epithelium (pre-ameloblasts, ameloblasts), the stratum intermedium, cellular remnants of the stellate reticulum, and the outer enamel epithelium. The reduced enamel epithelium allows nutrient transfer to the pre-ameloblasts and ameloblasts once enamel formation begins and also aids in tooth eruption.

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

During the bell stage, the ectomesenchymal stem cells adjacent to the inner enamel epithelium differentiate into columnar-shaped pre-odontoblasts and eventually odontoblasts. During this process, the basement membrane of the pre-ameloblasts is degraded allowing for the reciprocal signaling necessary for the development of odontoblasts and ameloblasts. Odontoblasts will secrete pre-dentin, the organic matrix of dentin that will eventually become mineralized. The remainder of the dental papilla forms the dental pulp.

Basement membrane of the inner enamel epithelium.

During the bell stage, the ectomesenchymal stem cells adjacent to the inner enamel epithelium differentiate into columnar-shaped pre-odontoblasts and eventually odontoblasts. During this process, the basement membrane of the pre-ameloblasts is degraded allowing for the reciprocal signaling necessary for the development of odontoblasts and ameloblasts. Odontoblasts secrete pre-dentin, the organic matrix of dentin that will eventually become mineralized. The remainder of the dental papilla forms the dental pulp.