Enamel and dentin formation. Cytodifferentiation has occured at the cusp tip, with the development of odontoblasts (o) and ameloblasts (a). The formation of dentin (d, dark

pink) initiates the terminal differentiation of the ameloblasts and thereby enamel matrix (e, dark purple) formation. The lighter pink material is unmineralized predentin. In this

specimen it is possible to see all of the phases of cytodifferentiation by following changes in the inner enamel epithelium and dental papilla (dp) from the cervical loop at the tips of the epithelial diaphragm (arrows) toward the cusp tip. Also note the stellate reticulum (sr)

has collapsed near the cusp tip bringing the dental follicle cells into close proximity with the ameloblasts and straatum intermedium (si).

Enamel and dentin formation are also shown in this micrograph. The dental follicle (f) is separated from the ameloblast layer (a) by outer enamel epithelium, stellate reticulum and stratum intermedium. Notice that these layers have collapsed in order to bring the capillaries located in the follicle closer to the ameloblasts. The ameloblasts are in their repolarized position next to the stratum intermedium (i). Next to the enamel layer a narrow zone of ameloblastic cytoplasm appears to be separated from the main mass of the cell by a terminal bar apparatus, which is actually tight junctions between neighboring ameloblasts. The narrow clear zone of apical cytoplasm below the terminal bar is called a Tomes' process, and the arrows demonstrate the 'pits' made by the Tomes' processes. Enamel matrix (e) has just begun to form next to the zone of mantle dentin (m), which was the very first dentin produced by the odontoblasts. Regular dentin (r, red) is visible as the mineralized zone just above the zone of predentin (p, blue). Odontoblasts are seen next to the predentin and the proximal portions of odontoblastic processes form white lines extending into the predentin. Both mantle and regular dentin are called primary dentin, to differentiate them from secondary dentin that is laid done after the root is completed.

This light micrograph shows another view the anatomical relations ships between the stellate reticulum (sr), stratum intermedium (si), ameloblasts (a), dentin (d), predentin (p) and odontoblasts. To the left, the columnar cells of the inner enamel epithelium are still

considered to be preameloblasts. They have undergone repolarization, but are not azctively secreting enamel matrix. The ameloblasts on the right are actively secreting enamel matrix (arrow), and have attained their characteristix histological appearance.

Next to the enamel layer a narrow zone of ameloblastic cytoplasm appears to be separated from the main mass of the cell by a terminal bar apparatus, which is actually

tight junctions between neighboring ameloblasts.

• The ameloblasts are actively secreting enamel matrix. Next to the enamel layer a narrow zone of ameloblastic cytoplasm appears to be separated from the main mass of the cell by a terminal bar apparatus, which is actually tight junctions between neighboring ameloblasts. The narrow clear zone of apical cytoplasm below the terminal bar is called a Tomes' process (white arrow). Within the ameloblast cell bodies you can see secretory granules (black arrow) that are released by exocytosis (merocrine secretion). The enamel matrix immediately mineralizes to about 30% mineral by weight.

• The crown of this molar has been completely formed, but maturation of the enamel matrix is incomplete. Areas that are completely mineralized (x) appear white, whereas enamel matrix that is only partially mineralized (arrow) is stained purple. Enamel maturation begins at the cusp tips, but only after the full thickness of enamel matrix has been laid down. The secretory ameloblasts then differentiate into maturation ameloblasts, and water and proteins are removed and calcium and phosphate are pumped into the matrix. As a result the matrix goes from 30% to approximately 96% mineral by weight. The remainder is water (~3%) and protein (mainly enamelins, ~1%).

• This is radiograph showing the appearance of newly forming crowns, and the radiodense material represents the maturing enamel. The clear space around the crown is the dental follicle, and the radiodense line demarcates the wall of the bony crypt.

• This is a light micrograph showing the reduced enamel epithelium which is tightly adherent to the enamel surface. The reduced enamel epithelium is made up of all four layers of the enamel organ; ameloblasts (a), and the stratum intermedium, stellate reticulum and outer enamel epithelium (asterisk). It functions to protect the enamel surface during eruption, and it forms the primary junctional epithelium as the tooth penetrates the oral epithelium and enters the oral cavity.

Enamel rods are visible in this ground section Each enamel rod consists of rod segments (white arrow) approximately 4 µm in length that represent daily incremental secretory activity by the ameloblast. The rods (direction indicated by black arrow) are demarcated from one another by interrod substance, in which the orientation of enamel crystallites is more oblique to the long axis of the rod.

Enamel rods are in this ground section are shown in cross-section, and they appear as "keyhole" or fish-scale" shaped structures. The rods are demarcated from one another by interrod substance, which is characterized by a different orientation of enamel crystallites than is found in the rod cores. The boundary between rod core and interrod substance is called the rod sheath.

• This cross-sectional light micrograph that demonstrates enamel tufts (blue arrows) and enamel lamellae. Enamel spindles are too small to be seen in this low power view. You can, however, see the scalloped nature of the DEJ. In cross-section the lines of retzius (red arrows) resemble growth rings in a tree trunk. Lines of Retzius  represent differences in ameloblastic activity that occur regularly over a period of 7 to 9 days. Changes in the shape of Tomes' processes lead to differences in the proportion of rod core to interrod substance at each line.

• This light micrograph demonstrates both Hunter-Schreger banding (h) and the lines of Retzius (white arrow). As groups of ameloblasts form enamel rods (oriented approximately perpendicular to the dentin (d) they vary in their orientation which you see as the alternating light and dark bands that run parallel to the enamel rods (black arrow). The lines of Retzius, on the other hand, run at an oblique angle to the direction of the rods. The lines of Retzius result from a periodic (7-9 days) change in ameloblast function, and thus, in enamel matrix formation.

• Hunter-Schreger bands can visualized either by reflected light, or as in this case by the use of polarizing filters. When the filter is shifted 90 degrees (pointer in the right frame compared to left) the light and dark bands also shift position demonstrating the structural nature of this phenomenon.

• Perikymata are the suface manifestations of the lines of Retzius. In this specimen you can easily discern the regularity of that changes in ameloblast function that must occur every 7-9 days during enamel formation.