Enamel : Appearance, Composition, Structure and Organization
Appearance - Mostly gray and semitranslucent, but very opaque white when dry. Covers the crown of the tooth.
Highly mineralized crystalline structure 95-98% inorganic matter by weight; hydroxyapatite (HA) is largest mineral constituent, present 90-92% by volume .
1-2% by weight organic content .
4% by weight water .
Structure and Organization-
Ameloblasts are the cells that are involved in the formation of enamel but are not present after tooth erupts and throughout life- none vital tissue .
Enamel is made up of rods or prisms, and rod sheaths and a cementing inter-rod substance in some areas.
Rods run from DEJ (dentinoenamel junction – interface of dentin and enamel) to the external surface of the tooth.
Rods are intertwined, densely packed and run a wavy course approximately perpendicular to the DEJ.
Rods: 4-8μm diameter.
In transverse section, rods have a rounded head or body and a tail (look like goldfish), forming a repetitive series of interlocking prisms; rounded head of each prism (rod) lies between the narrow tail portions of 2 adjacent prisms; usually the rounded head is oriented incisally or occlusally, and the tail cervically .
Each Rod (Prism) is made up of millions of crystallites .
Each Rod is surrounded by and organic sheath which contains millions of crystallites .
Each crystallite is surrounded by an organic matrix also .
Crystal orientation: tightly packed, distinct pattern which gives strength to enamel prisms .
Crystals are long hexagonal needle-like in shape, with average 1600Å length by 200-400 Å width .
Head region: crystals are straight on .
Tail and periphery regions: tilted on the side up to 65° .
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