Figure 158

Diagram showing the basic organization and structure of enamel rods. The enamel rod is a thin structure extending from the denti-noenamel junction to the surface of the enamel. Where the enamel is thickest, at the tip of the crown, the rods are longest, measuring up to 2,000 /xm. On cross section, the rods reveal a keyhole shape. The upper ballooned part of the rod, called the head, is oriented superiorly, and the lower part of the rod, called the tail, is directed interiorly. Within the head, most of the enamel crystals are oriented parallel to the long axis of each rod. Within the tail, the crystals are oriented more obliquely.

enchyme where the upper and lower jaws will develop. At the site of each future tooth, there is a further proliferation of cells that arise from the dental lamina, resulting in a rounded, cellular, bud-like outgrowth, one for each tooth, that projects into the underlying mesenchymal tissue. This outgrowth, referred to as the bud stage, represents the early enamel organ (Fig. 15.10a). Gradually, the rounded cell mass enlarges and then develops a concavity at the site opposite where it arose from the dental lamina. The enamel organ is now referred to as being in the cap stage (Fig. 15.10b). Further growth and development of the enamel organ results in the bell stage (Fig. 15.10, c and d). At this stage the enamel organ consists of four recognizable cellular components:

• Outer enamel epithelium, made up of a cell layer that forms the convex surface

• Inner enamel epithelium, made up of a cell layer that forms the concave surface

• Stratum intermedium, a cell layer that develops internal to the inner enamel epithelium

• Stellate reticulum, made up of cells that have a stellate appearance and occupy the inner portion of the enamel organ

The mesenchymal cells within the "bell" adjacent to the inner enamel epithelial cells become columnar and have an epithelial type appearance. They will become odontoblasts and form the dentin of the tooth. The inner enamel epithelial cells of the enamel organ will become ameloblasts. Along with the cells of the stratum intermedium, they will be responsible for enamel production. At the early stage, just prior to dentinogenesis and amelogenesis, the dental lamina degenerates, leaving the developing tooth pri-mordium detached from its site of origin.

Dental enamel is formed by a matrix-mediated biomin-eralization process known as amelogenesis. The major stages of amelogenesis are

• Matrix production, or secretory stage. In the formation of mineralized tissues of the tooth, dentin is produced first. Then, partially mineralized enamel matrix (Fig. 15.11) is deposited directly on the surface of the previously formed dentin. The cells producing this partially mineralized organic matrix are called secretory-stage ameloblasts. As do osteoblasts in bone, these cells produce an organic proteinaceous matrix by activity of the rough endoplasmic reticulum (rER), Golgi apparatus, and secretory granules. The secretory-stage ameloblasts continue to produce enamel matrix until the full thickness of the future enamel is achieved.

• Matrix maturation. Maturation of the partially mineralized enamel matrix involves the removal of organic material as well as continued influx of calcium and phosphate into the maturing enamel. Cells involved in this second stage of enamel formation are called maturation-stage ameloblasts. Maturation-stage ameloblasts differentiate from secretory-stage ameloblasts and function primarily as a transport epithelium, moving substances into and out of the maturing enamel. Maturation-stage ameloblasts undergo cyclical alterations in their morphology that correspond to cyclical entry of calcium into the enamel.

Secretory-stage ameloblasts are polarized columnar cells that produce enamel

The secretory-stage ameloblast lies directly adjacent to the developing enamel. At the apical pole of each ameloblast is a process, Tomes' process, which is surrounded by the developing enamel (Fig. 15.12). A cluster of mitochondria in the base of the cell accounts for the

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