Enamel

Histology

Physical Characteristics of Enamel

Protection

Enamel is the outer layer covering the crown of the tooth. It’s thickest on the cusps of molars and premolars (about 2-2.5 mm) and thins out at the neck of the tooth. The enamel is thicker on the lingual surfaces of upper molars and the buccal surfaces of lower molars to withstand the pressure during chewing.

Hardness

Enamel is the hardest tissue in the human body due to its high mineral content. This hardness makes it ideal for chewing but also makes it brittle, especially when it’s not supported by healthy dentin.

Variations in Strength

The enamel near the surface of the tooth is harder and has a higher elasticity compared to the enamel closer to the dentin-enamel (DE) junction.

Physical Properties

Density

Enamel's density decreases from the surface to deeper layers.

Electrical Resistance

Enamel is an insulator and does not conduct electricity at room temperature.

Permeability

Enamel can act like a semipermeable membrane, allowing certain molecules to pass through.

Color

The color of enamel can range from yellowish-white to grayish-white, influenced by its thickness and translucency.

Chemical Properties of Enamel

Composition

Enamel is 96% inorganic (mostly hydroxyapatite crystals) and 4% organic material and water.

Proteins

The organic material includes proteins like amelogenins (which make up 90% of enamel proteins) and nonamelogenins (about 10%). These proteins help in the formation of enamel.

Inorganic Material

The main component is hydroxyapatite, a mineral with a chemical formula of Ca₁₀(PO₄)₆(OH)₂. The crystals are arranged in a way that forms enamel rods or prisms.

Mineral Content

The enamel has various minerals, with oxygen, calcium, and phosphorus being the most common. The concentration of these minerals changes as you move from the surface of the enamel towards the dentin.

Water and Pores

Water is found within the hydroxyapatite crystals, between the crystals, and surrounding the rods. Pores in the enamel, especially at the rod boundaries, are also filled with water.

Mineralization

Enamel mineralizes right after it's secreted, with the mineral content increasing smoothly over time. This process is faster in enamel compared to dentin or bone. 

Enamel Structure

1. Enamel Rods: Enamel is made up of tiny structures called enamel rods (or prisms). These rods are like small cylindrical tubes, making the term "rods" more appropriate. The number of rods varies in different teeth, ranging from about 5 million in lower lateral incisors to 12 million in upper first molars. The rods start from the dentinoenamel junction (DEJ) and run outward to the tooth's surface in a slightly wavy pattern. Rods in the cusps (the highest points of the teeth) are longer than those near the tooth's neck (cervical area) due to the thicker enamel there. The rods become wider as they approach the tooth surface, not because there are more of them, but because their diameter increases. The average rod diameter is about 4 µm, but this can vary depending on where you measure. Near the surface, the rods can have a hexagonal shape but might also appear round or oval. Recent advanced imaging techniques have shown that the rods change shape from the DEJ to the surface, starting with an arcade shape near the DEJ and becoming keyhole-shaped at the surface.

Ultrastructure: Some aspects of enamel rods are too small to be seen with a light microscope and require an electron microscope. Human enamel often shows rods surrounded by a sheath (rod sheath) and separated by a substance between rods (interrod substance). A common pattern in human enamel is a keyhole or paddle-shaped prism. When viewed in a cross-section, rods can resemble fish scales. The "bodies" or "heads" of the rods align with the long axis of the tooth, while the "tails" point toward the tooth's neck. Apatite crystals, which make up the rods, generally align with the rod's long axis but can deviate by up to 40 degrees. These crystals vary in length but are thought to range between 0.05 and 1 µm. The crystals are shaped like pyramids, with their base toward the DEJ. In cross-section, the crystals are irregular in shape, about 30 nm thick, and 90 nm wide. Earlier studies found a network of fine organic fibers throughout the rods, but recent research suggests that the organic matrix forms an envelope around each apatite crystal.

Striations: Enamel rods appear to have a striped or striated look due to segments separated by dark lines, called cross-striations. These are more visible when treated with mild acids and are more pronounced in less mineralized enamel. The striations represent a rhythmic pattern of enamel formation, with each segment being about 4 µm long. This rhythm might relate to a daily cycle in enamel formation, leading to varicosities (swelling) and changes in composition.

Direction of Rods: Enamel rods typically run at right angles to the DEJ. In the central and cervical areas of the crown, they are horizontal. Near the incisal edge (cutting edge) or cusp tips, they become more vertical. The rods are rarely straight; they follow a wavy course. In the middle of the crown, rods bend in opposite directions in adjacent thin layers (disks) of enamel, creating a pattern called "gnarled enamel." This wavy course helps the enamel resist breaking under chewing forces.

2. Hunter-Schreger Bands: These bands are light and dark stripes visible under certain lighting conditions. They are caused by changes in the direction of the enamel rods and help the enamel resist fractures. The dark bands (diazones) are rods cut lengthwise, while the light bands (parazones) are rods cut across. The angle between these zones is about 40 degrees. Some studies suggest that these bands also reflect differences in enamel calcification and organic content, though the widely accepted view is that they result from changes in rod direction.

3. Incremental Lines of Retzius: These lines are visible as brownish bands in enamel sections and represent the layers of enamel deposited over time during tooth development. In longitudinal sections, they curve around the tip of the dentin. In transverse sections, they appear as concentric rings, similar to tree growth rings. These lines reflect variations in enamel structure and mineralization that occur during its formation. Incremental lines are spaced about 25-30 micrometers apart, representing about a week's worth of enamel formation.

4. Neonatal Line: A special type of incremental line called the neonatal line marks the enamel formed before and after birth. It reflects the change in environment and nutrition when a baby is born. The prenatal enamel (formed before birth) is usually better developed than the postnatal enamel. This line is found in all primary teeth and in the first molars, as their enamel formation begins before birth.

5. Surface Structures: The outer surface of the enamel often has a structureless layer called prismless enamel, especially near the cervical area of the tooth. This layer is about 30 micrometers thick and is more heavily mineralized than the underlying enamel. The enamel surface also has microscopic features like perikymata (grooves), rod ends, and cracks (lamellae). Perikymata are wave-like grooves that reflect the underlying striae of Retzius. Rod ends are shallow concave structures, and lamellae are crack-like features that can extend into the dentin.

6. Enamel Cuticle: A delicate membrane called Nasmyth's membrane or the primary enamel cuticle covers the crown of newly erupted teeth. This membrane is thought to protect the enamel from resorption before tooth eruption. After eruption, the enamel is usually covered by a pellicle, a thin layer of salivary proteins that forms within hours of cleaning.

7. Enamel Lamellae: Enamel lamellae are thin, leaf-like structures that extend from the enamel surface toward the DEJ. They can penetrate into the dentin and are composed of organic material with little mineral content. Lamellae can develop in areas of tension where rods do not fully calcify, and they may serve as pathways for bacteria, contributing to caries formation.

8. Enamel Tufts: Enamel tufts are ribbon-like structures that extend from the DEJ into the enamel, representing areas of hypocalcified rods and interprismatic substance. They are seen more often in horizontal sections and are thought to result from spatial conditions during enamel formation. Despite their name, they do not resemble grass tufts but are instead narrow, tubular structures.

9. Dentinoenamel Junction (DEJ)

Structure and Significance:

• The surface of the dentin at the Dentinoenamel Junction (DEJ) is not smooth but pitted. These shallow pits in the dentin fit into rounded projections of the enamel, creating a firm bond between the enamel and dentin.
• When viewed in section, the DEJ appears as a scalloped line rather than a straight one. The scallops' convexities point towards the dentin, reinforcing the connection between enamel and dentin.
• The pitted structure of the DEJ is preformed before the hard tissues of the tooth develop and is visible in the arrangement of ameloblasts and the basement membrane of the dental papilla.
• At the DEJ, the crystals of dentin and enamel intermix, contributing to the strong bond between these two layers.
• The DEJ is particularly pronounced in the occlusal (chewing) areas of the teeth, where the stresses of mastication (chewing) are greatest.

Hypermineralized Zone:

• A hypermineralized zone, approximately 30 micrometers thick, can sometimes be observed at the DEJ in microradiographs of ground tooth sections. This zone is most prominent before the mineralization of the enamel is complete.

Odontoblast Processes and Enamel Spindles

Odontoblast Processes:

• Occasionally, odontoblast processes (extensions of the cells that form dentin) extend across the DEJ into the enamel.
• These processes can thicken at their ends, forming structures known as enamel spindles.

Enamel Spindles:

• Enamel spindles originate from odontoblast processes that extended into the enamel epithelium before the formation of hard tissues.
• The direction of enamel spindles follows the original direction of the ameloblasts (the cells that form enamel), which is at right angles to the dentin surface.
• However, since enamel rods (the structural units of enamel) are formed at an angle to the ameloblasts' axis, the directions of spindles and rods differ.
• In dried tooth sections, the organic content of the spindles often disintegrates, leaving spaces that appear dark under transmitted light.
• Transmission Electron Microscopy (TEM) studies have shown that enamel spindles are channels about 2 micrometers in diameter. These channels contain small needle-like crystals or granular and amorphous materials.
• Enamel spindles are similar in structure to enamel tufts (another feature of the enamel), and both are hypomineralized or partially mineralized.
• Enamel spindles are most commonly found in the cusps of teeth (the pointed parts on the chewing surface).
• Studies using Energy Dispersive X-ray Microscopy have shown that enamel tufts, lamellae, and spindles have lower calcium and phosphorus content compared to the enamel prisms (the basic units of enamel).
• Neither Scanning Electron Microscopy (SEM) nor TEM studies have found peritubular dentin, membranous structures, or lamina limitans (a boundary layer) surrounding enamel spindles.

Age-Related Changes in Enamel

• Attrition: Enamel undergoes wear due to mastication, a process known as attrition.
• Perikymata: After a tooth erupts, the fine ridges on its surface, known as perikymata, are lost over time.
• Fluoride Uptake: The outer layers of enamel can absorb fluoride, which strengthens the enamel.
• Decreased Permeability: With age, the enamel becomes less permeable, reducing its ability to absorb substances from the external environment.