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 ameloblasts
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 dentin • 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 tooth: 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 ODONTOBLAST 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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