LECTURE V

1 Learning Objective

• At the end of this lecture, the students will be able to know about the;

 Ultrastructure of Enamel Rods

 Enamel Cuticle

 Enamel Lamellae

 Enamel Tufts

 Dentinoenamel Junction (DEJ)

 Enamel Spindles

2 ENAMEL CUTICLE

• Primary enamel cuticle or the Nasmyth’s membrane • A typical basal lamina under electron microscope • Probably visible with the light microscope - wavy course • Apparently secreted by the

3 Enamel Cuticle Cont’d…

• Function of the enamel cuticle • Afibrillar cementum • Pellicle

4 ENAMEL LAMELLAE

• Thin, leaf-like structures • May extend to, and sometimes penetrate into, the • Consist of organic material with little mineral content • May be confused with cracks

5 Enamel Lamellae Cont’d…

• Types of Lamellae  Type A: composed of poorly calcified rod segments  Type B: consisting of degenerated cells  Type C: filled with organic matter, presumably originating from saliva • Lamellae can be observed better in horizontal sections

6 Enamel Lamellae Cont’d… • Lamellae may develop in planes of tension  Where rods cross such a plane, a short segment of the rod may not fully calcify • If the disturbance is more severe, a crack may develop  Unerupted : filled by surrounding cells  After eruption: filled by organic substances from the oral cavity

7 ENAMEL TUFTS • Arise at the dentinoenamel junction and reach into the enamel to about one fifth to one third of its thickness • Resemble tufts of grass - erroneous • Does not spring from a single small area but is a narrow, ribbon-like structure • Consist of hypocalcified enamel rods and interprismatic substance

8 Enamel Tufts Cont’d…

• SEM studies  Revealed enamel tufts to be tubular structure with cross striations • TEM studies  Showed plate-like structures in the center of the tufts • In another study  Needle-like crystals and granular or amorphous material was found partially occluding the channels or the voids  These studies concluded that the tufts were hypomineralized structures

9 DENTINO ENAMEL JUNCTION (DEJ)

• The surface of the dentin at the DEJ is pitted • Into the shallow depressions of the dentin fit rounded projections of the enamel • appears not as a straight but as a scalloped line • convexities of the scallops are directed toward the dentin

10 DEJ Cont’d… • The pitted DEJ is preformed even before the development of hard tissues • In the DEJ, the crystals of dentin and enamel mix with each other • Is more pronounced in the occlusal area • A hypermineralized zone about 30 μm thick

11 PROCESSES AND ENAMEL SPINDLES

• Occasionally odontoblast processes pass across the DEJ into the enamel • Many are thickened at their end termed enamel spindles • Direction corresponds to the original direction of the ameloblasts—at right angles to the surface of the dentin

12 Enamel Spindles Cont’d… • In ground sections of dried teeth  The organic content of the spindles disintegrates and is replaced by air  The spaces appear dark in transmitted light • TEM studies  Showed spindles to be channels containing small needle-like crystals or granular and/ or amorphous material  They are found mainly in the cusp tip regions

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