Enamel PDF - Oral Histology

ORAL HISTOLOGY PETRA UNIVERISTY Enamel Enamel is a hard mineralized tissue that covers the anatomic crown of a tooth. It is an ectodermal derivative and is th...

ORAL HISTOLOGY PETRA UNIVERISTY Enamel Enamel is a hard mineralized tissue that covers the anatomic crown of a tooth. It is an ectodermal derivative and is the hardest tissue in the body. Mature enamel is the only tissue that is totally acellular. After enamel is completely formed, no more enamel can be deposited.  Physical properties  The high mineral content makes the enamel the hardest tissue in the body. This hardness enables it to withstand the heavy forces applied during mastication. It is greater at the surface and decrease towards the DEJ, and at the cusps and incisal ridges and decreases cervically.  The nature of its structure and the hardness makes enamel very brittle having a low tensile strength. Thus the underlying layer of resilient dentin helps in maintaining the integrity of the enamel.  Enamel has a certain degree of permeability demonstrated by dyes and radio-active isotopes. It acts as semi-permeable membrane for certain ions and dyestuff of small molecular size through pores between the crystals. Permeability is mainly from saliva and less from the pulp across the dentin.  Enamel is translucent and its color varies from light yellow to grayish white. The color is influenced by the thickness of enamel. The thinner areas appear more yellowish as the underlying dentine is seen through the enamel. Enamel of deciduous teeth is whiter because it is less mineralized so less translucent  The thickness of enamel varies from 2.5 mm at the cusp tips to less than 100 µm at the cervical part of the tooth and along the pits and fissures.  The density or the specific gravity of enamel is 2.8- 3.0 gram\ml and has a knoop hardness number of 343.  Chemical properties - Enamel is composed of both organic and inorganic substances. By weight mature human enamel is made of 96% inorganic and 4% organic material and water. By volume, the organic matter and water are nearly equal to the inorganic content. - The inorganic content is basically hydroxyapatite crystals (92-98%) and various ions such as strontium, magnesium, lead and fluoride. DR. ATEF ISMAIL AHMED 1 ORAL HISTOLOGY PETRA UNIVERISTY - The organic material is mostly tyrosine rich amelogenin protein and non amelogenin proteins (enamelins and ameloblastins). It forms a fine network between the crystals or compressed forming an envelope surrounding each crystal.  The histologic structure of the enamel The basic unit structure is the enamel rod (prism). The enamel rod is a very long, thin structure extending from the dentino-enamel junction to the surface of enamel. The enamel rods follow a tortuous course thus the length of an enamel rod may be greater than the thickness of enamel. Enamel rods appear to be encased in a rod sheath and the sheathed rods are cemented together by an inter-rod substance. Of these three structures, the enamel rod is the most highly mineralized and the rod sheath the least mineralized. Diameter: The enamel rods have an average diameter of 4-5 µm. The diameter of the rod increases from the dentino-enamel junction towards the surface (8μ). Number: The number of enamel rods varies from 5-7.5 million in the mandibular incisors to 12.5 million in the upper molars. Length: The average length of the rod in the mid crown region is 0.2 mm. the rods are longer at the region where the enamel is thick i.e. at the cusp tips and shorter at the cervical region. Shape: In cross section the enamel rods by E.M. may be round, oval or hexagonal in shape. But most often they have key-hole or paddle shaped configuration. The tails resides between two heads of the adjacent rods. The bodies of the rods are closer to the occlusal or incisal surface while the tail points in a cervical direction. By L.M rods appear clear and structureless. Arrangement: The enamel rods are arranged perpendicular to the dentino-enamel junction, except in the cervical region where they are inclined towards the gingiva. In the deciduous teeth, the rods in the cervical and middle parts are more horizontally arranged. The rods at the cusps or incisal edges are twisted (gnarled enamel), but at pits and fissures they are converge in their outward course. Regular change in rods direction is due to functional adaptation. Crystals: The enamel rod is made of up of numerous crystals. The crystals in the central region of the rod are arranged with their long axis parallel to the longitudinal axis of the rod. Crystals that are peripherally placed flare laterally (65) °. Striations: the enamel rods consist of segments because the enamel matrix is formed in a rhythmic pattern. These segments are separated by dark lines representing the daily rate secretary activity of ameloblasts. It gives the enamel the striated appearance (4µ) in length. DR. ATEF ISMAIL AHMED 2 ORAL HISTOLOGY PETRA UNIVERISTY  Structural features of enamel The enamel exhibits a number of features that characterize it as a more complex tissue. The following are some of the features of the enamel.  Incremental lines of Retzius: The lines of Retzius represent the incremental nature of enamel deposition. The lines of Retzius are seen in ground sections are comparable to the concentric growth rings seen in the cross-section of tree trunks. In longitudinal sections of the teeth, these lines are seen as brownish bands that surround the tip of dentin. The striae of retzius are more frequently seen in the permanent teeth and less in deciduous and prenatal enamel. The occurrence of a few striae is considered normal, but when they are in great numbers or as a broad band it indicates periods of metabolic disturbance or disturbance in amelogenesis. These lines represent a hypomineralized or weekly rhythmic formation of the enamel.  Neonatal line: This line is accentuated stria, which represents the disturbances at birth. Neonatal lines can be seen in primary dentition and the first molars of the permanent dentition. This line demarcates the prenatal and the postnatal enamel. The prenatal enamel is more regular and lightly calcified due to undisturbed and even formation. While the postnatal enamel is irregular and less calcified.  Hunter Schreger bands: The Hunter Schreger bands are alternating dark (diazones) and light zones (parazones) seen in longitudinal ground sections when viewed under reflected light. Their appearance gets reversed when viewed under transmitted light. These bands originate at the dentinoenamel junction and pass outward traversing more than half of the enamel ( 4 ). They are not seen towards the enamel surface. These bands are an optical 5 phenomenon.  Gnarled enamel: The enamel rods below the cuspal and incisal region appear irregular, twisted and intertwisted; this type of enamel is called gnarled enamel. Unlike the hunter schreger bands, gnarled enamel extends throughout the thickness of the enamel at the cusp tips and incisal edges. This arrangement of enamel rods is believed to aid in resisting the high masticatory loads that the cusps have to bear.  Enamel lamella: Enamel lamellae are thin, leaf like structures extend from the enamel surface towards the DEJ. The enamel lamellae are primarily organic and represent DR. ATEF ISMAIL AHMED 3 ORAL HISTOLOGY PETRA UNIVERISTY improperly mineralized enamel, which is a failure of the process of removal of the organic matrix and water during mineralization. They can extend in a longitudinal or in a radial direction and can be seen running vertically on the crown from the incisal or cuspal area to the cervical region. Enamel lamellae are seen in areas of tension where a short segment of the rod is not fully calcified. These areas may develop into a crack occurred prior to eruption. Cracks that occur after eruption are filled with organic material from the oral cavity. The enamel lamellae are more frequently seen in cervical areas. They may be a site of weakness, which may form the pathway for bacterial invasion. The enamel lamellae are classified into three types: - Type A: they are lamellae of un-erupted teeth composed of poorly calcified rod segments. They are less common and are restricted to enamel. - Type B: they are lamellae of un-erupted teeth consisting of degenerated cells. Type B is less common and may reach the dentin. - Type C: they are cracks seen in erupted teeth that are filled with organic matter or debris from saliva and is usually not cellular. Type C is more common and may reach the dentin.  Enamel tufts: Enamel tufts are organic structures that originate at the DEJ and extend into for about ⅓ of its thickness. The enamel tufts resemble tufts of grass and are areas where developing enamel proteins are not completely removed during maturation. Thus in these areas there is slightly more matrix at the prism borders and possibly more matrix between the crystals of the enamel rods. Developmentally, they are formed due to abrupt changes in the rod direction which lead to different ratio of interred and rod enamel, creating less mineralized and weakened planes.  Enamel spindles: Enamel spindles are odontoblastic processes that went astray crossed the DEJ and got entrapped in the enamel matrix. They are developmentally derived from dentin and have an organic content higher than surrounding enamel. The spindles unlike tufts or lamellae do not line up as rows, but are randomly distributed along the DEJ. They appear club shaped in ground sections and dark under transmitted light. N.B. Tufts and spindles may play role in cementing enamel to dentin.  Dentino-enamel junction: The dentinoenamel junction is a scalloped interface between the enamel and dentin. Small curved projections of enamel fit into small concavities of the dentin. The DEJ in a fully developed tooth separates the mineralized layers of enamel and DR. ATEF ISMAIL AHMED 4 ORAL HISTOLOGY PETRA UNIVERISTY dentin. During tooth development, a basal lamina separates the secretory fronts of the odontoblasts and ameloblasts. This layer is lost as the tissues undergo mineralization.  Surface structures of enamel  Perikymata: Perikymata are transverse wave like grooves. They are external manifestation of the striae of retzius where they end in shallow furrows.  Enamel cuticles: The enamel cuticle or Nasmyth’s membrane is a structureless membrane seen on the crown of tooth and adhering firmly to its surface. It is mostly seen in newly erupted teeth and is lost due to mastication. It is about 0.5 to 1.5 µm thick. The enamel cuticles are structurally described as basal lamina similar in appearance to the basal lamina found at the junctions of the epithelium and connective tissue. It is actually made of two cuticles, primary enamel cuticle and secondary enamel cuticle. 1- Primary enamel cuticle is the last product of the ameloblasts and it becomes mineralized. 2- The secondary enamel cuticle covers the primary cuticle and is a product of the reduced enamel epithelium and is not mineralized.  Rod ends: Enamel rod ends are concave depressions and vary in shape and depth.  A prismatic enamel: The outermost and the innermost enamel is rodless as it is produced without tom’s processes. At the surface it is 30 µm thick in 70% of permanent teeth and in all deciduous teeth. The crystals are parallel to each other and oriented at right angles to the striae of Retzius. This kind of enamel is more mineralized, found commonly at cervical areas.  Cracks: They are narrow, fissure like structures. They are the outer manifestations of enamel lamellae.  Life history of ameloblasts: Ameloblasts show different morphological features and organelle content during different stages of enamel formation. Accordingly, ameloblasts are assigned to stages according to functions that start before enamel apposition and continue through enamel formation as well as after enamel is completely formed. These stages collectively are referred to as the life cycle or history of ameloblasts. 1- Morphogenetic (morphogenic) stage 2- Differentiating or organizing stage DR. ATEF ISMAIL AHMED 5 ORAL HISTOLOGY PETRA UNIVERISTY 3- Secretory stage 4- Transitional stage 5- Maturative and modulation stage 6- Protective stage: 7- Desmolytic stage  Amelogenesis 1- Enamel matrix secretion (Early stage of enamel formation): 2- Enamel maturation (Late Stage of enamel formation): DR. ATEF ISMAIL AHMED 6 ORAL HISTOLOGY PETRA UNIVERISTY Enamel rods A, Stria of Retzius; B, DEJ A, Stria of Retzius; B, Enamel tuft; C, Enamel lamella; D, DEJ A, Gnarled enamel; B, Neonatal line; Perikymata C, Dentin; D, Dentino-enamel junction DR. ATEF ISMAIL AHMED 7 ORAL HISTOLOGY PETRA UNIVERISTY A, Enamel spindle; B, Enamel tuft Hunter-Shreger bands A B A-Rod sheath B- Rod core Enamel spindles DR. ATEF ISMAIL AHMED 8 ORAL HISTOLOGY PETRA UNIVERISTY A- Gnarled enamel Primary enamel cuticle B- Enamel spindles Reduced dental epithelium Enamel formation Tom’s process DR. ATEF ISMAIL AHMED 9