ENAMEL - Al-Mustansiriya University

ENAMEL - Al-Mustansiriya University

ENAMEL PHYSICAL CHARACTERSTICS On the cusps of human molars and premolars the enamel attains a maximum thickness of about 2 to 2.5mm, thinning down to almost a knife edge at the neck of the tooth.

It is the hardest calcified tissue in the human body. It is brittle in nature. The specific gravity of enamel is 2.8.

The color of the enamel covered crown ranges from 2 yellowish white to grayish white. Incisal areas may have a bluish tinge where the thin edge consists only of a double layer of enamel.

Acts like a semipermeable membrane, permitting complete or partial passage of certain molecules: C14- labeled urea, I, etc. The function of enamel is to form a resistant covering of the teeth, rendering 3 4

CHEMICAL PROPERTIES Consists mainly of inorganic material (96%) and only a small amount of organic substance and water.

The relative space occupied by the4% organic material and water and the entire enamel is almost equal. 5 STRUCTURE The enamel is composed of enamel rods or prisms,

rod sheaths, and in some regions a cementing interprismatic substance. The number of enamel rods ranges from 5 million in the lower lateral incisors to 12 million in the upper first molars. From the dentinoenamel junction the rods run somewhat

tortuous courses outward to the surface of the tooth. The length of most rods is greater than the thickness of enamel because of the oblique direction & the wavy course of the rods. 6 7 SUBMICROSCOPIC STRUCTURE

The most common pattern is a key-hole or paddleshaped prism in human enamel. The rods measure about 5 um in breadth & 9 um in length. The bodies of the rods are nearer occlusal & incisal surfaces, whereas the tails point cervically.

The apatite crystals are oriented parallel to the long axes of the rods in their bodies or heads & deviate about 65 degrees from their axes as they fan out into the tails of the prisms. The rods are segmented because the enamel matrix is formed in a rhythmic manner.

8 KEY HOLE SHAPED ENAMEL ROD 9 HUNTER- SCHREGER BANDS The more or less regular change in the direction of rods may be regarded as a functional adaptation, minimizing the risk of cleavage in the axial

direction under the influence of occlusal masticatory forces. The change in the direction of rods is responsible for the appearance of the HunterSchreger bands.

These are alternating dark and light strips of varying widths seen in longitudinal ground 10 11 INCREMENTAL LINES OF RETZIUS

Appear as brownish bands in ground sections of the enamel. Illustrate the successive apposition of layers of

enamel during the formation of crown. In longitudinal sections they surround the tip of the dentin. In cervical parts of the crown they run obliquely. From the dentinoenamel junction to the surface they deviate occlusally. In transverse sections of a tooth, they appear as concentric circles. 12 13

14 15 16 The enamel of the deciduous tooth develops partly before & partly after birth.

The boundary between the two portions of enamel in the deciduous teeth is marked by an accentuated incremental line of retzius, the neonatal line or neonatal ring. 17 HYPOCALCIFIED STRUCTURES IN E.

1. ENAMEL LAMELLAE Thin leaf like structures that extend from the enamel surface toward the dentinoenamel junction. May sometimes extend to dentin.

Consist of organic material, with but little mineral content. Develop in planes of tension. 18 Three types of lamella

May be a site of weakness in a tooth & may form a road of entry for bacteria that initiate caries. 19 20 2.ENAMEL TUFTS

Arise at the dentinoenamel junction & reach into the enamel. Resemble tufts of grass when viewed in ground sections.

Is a narrow, ribbon like structure. Consists of hypocalcified interprismatic substance. Act to prevent enamel fractures, stress-shielding to 21 increase compliance with dentin.

enamel rods & 22 3. ENAMEL SPINDLE

Represent extension of odontoblast process through enamel before its fully mature 24 DENTINOENAMEL JUNCTION The surface of the dentin at the dentinoenamel junction is pitted.

Into the shallow depressions of the dentin fit rounded projections of the enamel. This relation assures the firm hold of enamel cap on dentin.

Is scalloped. The convexities of the scallops are directed toward the dentin. 25 26 CLINICAL CONSIDERATIONS

Ameloblasts are lost after the enamel has been laid down ,therefore enamel could not be repaired by itself ,it only repaired by filling material. 27

AMELOGENESIS Begins first at cusp tips Then sweeps down crown slopes Stops at CEJ AMELOGENESIS - LIFE CYCLE OF AMELOBLASTS Functional stages in life cycle of ameloblasts: 1. Morphodifferentiation 2. Histodifferentiation

3. Secretory (initial) 4. Secretory (Tomes process) 5. Maturation (ruffle-ended) 6. Maturation (smooth-ended) 7. Protective AMELOBLAST Ameloblasts are derived from oral epithelium tissue of ectodermal origin. Their differentiation from preameloblasts (whose origin is from inner enamel epithelium) is a result of signaling from the ectomesenchymal cells of the dental papilla.

The ameloblasts will only become fully functional after the first layer of dentin (predentin) has been formed by odontoblasts. Then the cells be part of the reduced enamel epithelium after enamel maturation and then are subsequently lost during tooth eruption AMELOBLAST IN FORMATIVE STAGE Each ameloblast is a columnar cell approximately 4 micrometers in diameter, 40 micrometers in length and is hexagonal in cross section.

The secretory end of the ameloblast ends in a six-sided pyramid-like projection known as the Tomes' process. The angulation of the Tomes' process is significant in the orientation of enamel rods, the basic unit of tooth enamel. AMELOGENESIS - MATURATION STAGE Maturation Process: 1. Removal of water & organic material

2. Introduction of additional inorganic material Process = Modulation cyclic creation, loss, and recreation of highly invaginated ruffle-ended apical surface on ameloblasts AMELOGENESIS - MATURATION STAGE Ameloblasts incorporation of inorganic material

Ruffle-ended Ameloblasts exit of protein fragments & water Smooth-ended AMELOGENESIS - MATURATION STAGE

AMELOGENESIS - MATURATION STAGE Enamel hardens before tooth erupts Results from growth in width, thickness of crystals Amelogenesis slow process almost mature enamel, most mineral removed

AMELOGENESIS - ENAMEL PROTEINS Enamel Proteins: 1. Contributing to appositional growth, thickness enamel * Amelogenin (main protein in forming enamel) * Ameloblastin * Enamelin Amelogenesis - Enamel Proteins Amelogenin

vs Ameloblastin PROTECTIVE STAGE Full thickness of enamel complete, enamel mature Ameloblast layer & papillary layer form reduced enamel epithelium

What is the enamel space? ENAMEL - STRUCTURE Crystals hexagonal contour to older mature crystals recently

formed thin crystals ENAMEL - STRUCTURE Aapd.org/publications/peddent/ Note: rod, interrod crystals same, but divergent orientation ENAMEL - STRUCTURE Crystals

hexagonal contour to older mature crystals recently formed thin crystals ENAMEL - STRUCTURE

Crystal Profiles hexagons with unequal-sided peaks (un) & equal-sided peaks (eq) x 300,000 (rat incisor) THANK YOU

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