Hard tissues, including enamel and dentin, develop during the next stage of tooth development. This stage is called the crown, or maturation stage, by some researchers. Important cellular changes occur at this time. In prior stages, all of the IEE cells were dividing to increase the overall size of the tooth bud, but rapid dividing, called mitosis, stops during the crown stage at the location where the cusps of the teeth form. The first mineralized hard tissues form at this location. At the same time, the IEE cells change in shape from cuboidal to columnar and become preameloblasts. The nuclei of these cells move closer to the stratum intermedium and away from the dental papilla as they become polarized.
The adjacent layer of cells in the dental papilla suddenly increases in size and differentiates into odontoblasts, which are the cells that form dentin. Researchers believe that the odontoblasts would not form if it were not for the changes occurring in the IEE. As the changes to the IEE and the formation of odontoblasts continue from the tips of the cusps, the odontoblasts secrete a substance, an organic matrix, into their immediate surrounding. The organic matrix contains the material needed for dentin formation. As odontoblasts deposit organic matrix termed predentin, they migrate toward the center of the dental papilla. Thus, unlike enamel, dentin starts forming in the surface closest to the outside of the tooth and proceeds inward. Cytoplasmic extensions are left behind as the odontoblasts move inward. The unique, tubular microscopic appearance of dentin is a result of the formation of dentin around these extensions.
After dentin formation begins, the cells of the IEE secrete an organic matrix against the dentin. This matrix immediately mineralizes and becomes the initial layer of the tooth's enamel. Outside the dentin are the newly formed ameloblasts in response to the formation of dentin, which are cells that continue the process of enamel formation; therefore, enamel formation moves outwards, adding new material to the outer surface of the developing tooth.