FIGURE 16.10 • Diagrams and photomicrographs of a developing tooth. a. In this bud stage, the oral epithelium invaginates into the underlying mesenchyme, giving origin to the enamel organ (primordium of enamel). Mesenchymal cells adjacent to the tooth bud begin to differentiate, forming the dental papilla that protrudes into the tooth bud. b. Tooth bud in cap stage. In this stage, cells located in the concavity of the cap differentiate into tall, columnar cells (ameloblasts) forming the inner enamel epithelium. The condensed mesenchyme invaginates into the inner enamel epithelium, forming the dental papilla, which gives rise to the dentin and the pulp. c. In this bell stage, the connection with the oral epithelium is almost cut off. The enamel organ consists of a narrow line of outer enamel epithelium, an inner enamel epithelium formed by ameloblasts, several condensed layers of cells that form the stratum intermedium, and the widely spaced stellate reticulum. The dental papilla is deeply invaginated against the enamel organ. d. In this appositional dentin and enamel stage, the tooth bud is completely differentiated and independent from the oral epithelium. The relationship of the two mineralized tissues of the dental crown, enamel and dentin, is clearly visible. The surrounding mesenchyme has developed into bony tissue. e. In this stage of tooth eruption, the apex of the tooth emerges through the surface of the oral epithelium. The odontoblastic layer lines the pulp cavity. Note the developed periodontal ligaments that fasten the root of the tooth to the surrounding bone. The apex of the root is still open, but after eruption occurs, it becomes narrower. f. Functional tooth stage. Note the distribution of enamel and dentin. The tooth is embedded in surrounding bone and gingiva. g. This photomicrograph of the developing tooth in the cap stage (comparable to b) shows its connection with the oral epithelium. The enamel organ consists of a single layer of cuboidal cells forming the outer enamel epithelium, the inner enamel epithelium has differentiated into columnar ameloblasts, and the layer of cells adjacent to the inner enamel epithelium has formed the stratum intermedium. The remainder of the structure is occupied by the stellate reticulum. The mesenchyme of the dental papilla has proliferated and pushed into the enamel organ. At this stage, the forming tooth is surrounded by condensed mesenchyme, called the dental sac, which gives rise to periodontal structures. 300. h. This photomicrograph shows the developing crown of an incisor, which is surrounded by the outer enamel epithelium and remnants of the stellate reticulum. It is comparable to d. The underlying lighter-stained layer of dentin is a product of the odontoblasts. These tall columnar odontoblasts have differentiated from cells of the dental papilla. The pulp cavity is filled with dental pulp, and blood vessels permeate the pulp tissue. 40.