Structure and Features of enamel
Structure of enamel
Enamel is made up of millions of enamel prisms or rods, which run from the amelo-dentinal junction to the enamel surface. Each prism is made up of a large number of enamel crystallites. When viewed under a light microscope each prism resembles the rounded ‘head’ portion of a keyhole (Fig. ). The enamel crystallites run parallel to the long axis of the prism and in the ‘tail’ portion the enamel crystallites are inclined away from the long axis of the enamel prism.Enamel is laid down in layers which produce incremental growth lines. After each successive layer the ameloblasts retreat so as not to be trapped within their matrix. Some growth lines mark daily deposits which are about 4 ìm thick; these are called cross striations.
Features of enamel
The following features of enamel are significant:
■ Brown striae of Retzius: these are brown lines indicating variations in weekly deposits that run obliquely from the amelo-dentinal junction towards the enamel surface. When the striae emerge on to the enamel surface a series of grooves may be seen; these are termed perikymata grooves.
■ Hunter-Schreger bands: when viewed under a light microscope, broad dark and light bands can be seen running obliquely from the amelo-dentinal junction to two thirds of the thickness of the enamel. They are curved with the convexity of the curve always facing rootwards.
■ Neonatal line: since this line marks the disruption in amelogenesis at birth, it can only be seen in primary teeth and first permanent molars. It can provide an important forensic landmark.
■ Lamellae: these are sheet-like faults that run vertically through the entire thickness of the enamel.