Enamel rod

The enamel is composed of enamel rods or prisms, rod sheaths, and in some regions a cementing interprismatic substance. The enamel prisms are cylindrical, in longitudinal section, therefore the term rods is more apt. The number of enamel rods has been estimated as ranging from 5 million in the lower lateral incisors to 12 million in the upper first molars.

From the dentinoenamel junction the rods run somewhat tortuous courses outward to the surface of the tooth. The length of most rods is greater than the thickness of the enamel because of the oblique direction and the wavy course of the rods. The rods located in the cusps, the thickest part of the enamel, are longer than those at the cervical areas of the teeth. The increase in the area of enamel at the surface compared to their area at the DE junction is not due to increase in the number of prisms but it is explained on the basis of increased prism diameter near the tooth surface and the oblique orientation of prisms towards the surface. It is stated generally that, as observed with the light microscope, the diameter of the rods averages 4 μm, but this measurement necessarily varies, since the outer surface of the enamel is greater than the dentinal surface where the rods originate. It is claimed that the diameter of the rods increases from the dentinoenamel junction toward the surface of the enamel at a ratio of about 1:2.

The enamel rods normally have a clear crystalline appearance, permitting light to pass through them. In cross-section under the light microscope they occasionally appear hexagonal. Sometimes they appear round or oval. In cross-sections of human enamel, many rods resemble fish scales. 3D images obtained from confocal laser scanning microscope shows arcade outlines were seen near DE junction and keyhole-shaped outlines were seen at the enamel surface.

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