Oral histology

Histology of enamel

**It s the hardest tissue in the body Epithelially derived from the ectoderm layer **It is protective layer of the tooth **Ameloblasts are the cells which are responsible for it's formation

Physical properties of E

1.E is the hardest tissue in the human body , it's also brittle & subject to fracture specially if the underlying dentin is carious, causing a weak foundation. 2. E is very hard because it is composed of 96% mineral (inorganic) in a form of hydroxyapitite crystal and 4%organic & water 3. E is white to grayish white but it's appear slightly yellow because it is translucent & the underlying yellow dentin is seen through the thinner regions 4. E is semipermeable, it can permit exchange ions & molecules 5- E has a property of acid solubility

Structure of enamel

1-The basic units of E are the rods (prisms( A-The number of E rods range from 5 million in lower lateral incisors to 12 million in upper first molar. B- Direction; Run oblique direction and wavy course C-length greater than the thickness D- Appearance; have aclear crystalline appearance

E- Rod has a key – hole shaped consists of the head and tail

2- The rods separated from each other by interrod substance

3- Rod sheath

Is a narrow space containing organic materials more than the rod it self but less calcified , act as boundary between the rod and interrod ( A thin peripheral layer) It seen darker than the rod.


4- Gnarled enamel
In the cuspal region , the enamel rods be very wavy forming intertwining bundles, this is referred to gnarled enamel.

5- Stria of Retzius

**Incremental growth lines seen in enamel as a result of enamel development **When viewed microscopically in cross-section , they appear as concentric rings **In a longitudinal section, they appear as a series of dark bands When striae of retzius appear in the surface of the enamel as waves or grooves, they called as Perikymata, which represent the external manifestation of striae of retzius Neonatal line is an enlarged stria of retzius appears due to the sudden change in the invironment and nutrition THIS LINE is darkest and thickest stria of retzius

6- Hunter Schreger bands

**These are an optical phenomena produce by changes in the direction of rods. **These bands appear as dark & light alternating zones clearly seen in longitudinal ground section ** They are found in the inner two third of the E.

7- Enamel tufts

**These are tuft like, contain organic material arise from DEJ up to (1/5-1/3 ) of E thickness. **They contain greater concentration of E protein's than the rest of E ( Hypomineralized) **They believed to occur developmentally due to changes in the direction of rods.

8- E lamellae

**Thin sheets of enamel proteins (Hypomineralized) , extends from the enamel surface towards the dentino-enamel juction (DEJ). **Represent a pathway for initiating caries in enamel.

9- Enamel spindles

**Narrow spindle shape, short, straight, thin and dark structures. **Extensions of the dentine tubules across dentino-enamel junctions into enamel **Found most commonly beneath the cusps **Help to make the edge more permeable