- Enamel is composed of enamel rods (prisms), rod sheaths, and interrod regions. Enamel rods are bundles of hydroxyapatite crystals that run obliquely from the dentin-enamel junction to the enamel surface. They have a hexagonal cross-section and keyhole shape longitudinally. - Rod sheaths surround the organic matrix around 2/3 of enamel rods. Interrod regions are the spaces between rods. Enamel tufts and lamellae contain organic materials and arise from the dentin-enamel junction and enamel surface, respectively. - With age, enamel wears down from mastication, decreases in permeability, and darkens in color.

10. Microscopical appearance of enamel:
Under light microscope the basic structural
unite of enamel appear composed of:
• rods or prism.
• Rod sheath.
• Inter rod region.

11. Enamel Rode (E.R.):-
• Consist of a highly packed hydroxy apatite crystals and highly organized in pattern.
• Each E.R. run from dentino-enamel junction to enamel
surface in oblique direction and a wavy course due to the
movement of ameloblast cells during amelogenesis.
• Diameter of E.R. is largest at the surface of enamel and
smaller at the dentino-enamel junction in a ratio of
about (2:1).
• Shape of E.R. in cross section under light microscope, appear
hexagonal, sometimes it appears round or oval in shape.
Cross section of human enamel has a fish scale like appearance.
• Longitudinal section of E.R. appear as a keyhole shape,
each keyhole consisted of head and tail, the head represent
rod and tail represent inter rod region. Head is at occlussal or
incisal edge and the tail is located cervically.
The direction of E.R. at the cusp and incisal edge has a
vertical direction while cervically each E.R. has an inclination
about 2º tilt it becomes horizontal cervically.

12. Rod sheath (R.S.):-
It represent the organic matrix of enamel and it form an envelop
surrounds 2/3 of head of E.R.. R.S. develops due to irregular
arrangement of the crystals and it is firstly attack by dental caries.
Inter-rod substance
A region between 2 rods called cementing substance or tail
region.

16.  Enamel tufts, they are grass like
structures arise at the dentino-
enamel junction and reach into the
enamel to about one fifth consist
of a hypoclcified enamel rods
and interpresmatic substance.
 Enamel lamellae: are thin leaf like structures arise from
enamel surface toward dentino-enamel junction. They
are filled with organic material and water. They result
from failure of maturation process.

23. Age changes in enamel
• With age, enamel become worn out
because of masticatory attrition.
• Age also causes a decrease in the
permeability of enamel.
• Discoloration: enamel become more
darker with the age.