MNU Histology first term(1).PDF

1  Enamel Enamel is the ectodermal non-collagenous tissue covering the anatomical crown of human teeth. It is the most highly mineralized tissue in the body; it forms a protective covering of the teeth to resist the powerful masticatory forces. Enamel is an acell...

1  Enamel Enamel is the ectodermal non-collagenous tissue covering the anatomical crown of human teeth. It is the most highly mineralized tissue in the body; it forms a protective covering of the teeth to resist the powerful masticatory forces. Enamel is an acellular, inert, non-vital and insensitive tissue, when destroyed by any means usually wear or caries it cannot be replaced or regenerated. Chemical characteristics: Enamel is consisting of: 96% inorganic content: a crystalline calcium phosphate hydroxyapatite and various ions, strontium, magnesium, lead and fluoride. 4% organic material: (non-collagenous proteins) and water. Physical characteristics: Extremely hard tissue. Brittle therefore an underlying layer of resilient dentin is necessary to maintain its integrity. Translucent. Varies in color from light to yellow to grayish white (the underlying yellow dentin is seen through the thinner regions). Varies in thickness from 2.5 mm over the working surfaces to a feather edge at the cervical line. Enamel act as a semipermeable membrane. 2 Histological structure of enamel: Because of its high crystalline nature, the structure of enamel is extremely difficult to study in decalcified sections, since only an empty space can be seen in areas formerly occupied by enamel, because the minerals have been dissolved and the organic material washed away. 1- Enamel prisms (rods): Number: It reaches about five million in the lower central incisor and up to twelve millions in the upper first molar. Direction: The direction of the rods in general is perpendicular to the dentin surface. In deciduous teeth the enamel rods are directed roughly vertical in the cusp tip or incisal ridge regions, then they become oblique (towards the occlusal surface) in the middle third of the crown and horizontal in the cervical region, where the enamel ends abruptly as a thick layer. In permanent teeth, the direction of the enamel rods is similar to the deciduous teeth in the occlusal and middle thirds, while in the cervical region the rods deviated from horizontal to be directed obliquely rootwise. This is because the enamel of permanent teeth ends as a knife edge at the cervix. 3 Course: The course of individual enamel rods is wavy from the amelodentinal junction outwards, but just before they reach the outer surface of the enamel they become straight. Under the cusp tips and incisal ridges, the course of the enamel rods is more complicated where they become twisted and braided together to give maximum strength to the areas which are more subjected to the masticatory forces. The enamel in these areas is called gnarled enamel. Diameter: The diameter of the enamel rods differs from the amelodentinal junction to the outer surface of enamel. The ratio between the diameter of the rods at the inner and outer enamel surfaces is 1:2 where the diameter at the ADJ is 3-4 microns, it may reach up to 8microns at the outer surface. This is because of the marked difference in the surface area between the inner and outer enamel surfaces. Enamel structure by the light microscope: The enamel rods whether cut longitudinally or transversely appear clear and structureless, because they are formed of tightly packed mass of hydroxyapatite crystals that allow light to pass through. The peripheral part of the enamel rod is called prism sheath and forms an incomplete envelope around the prism. It is less calcified than the rod itself i.e. it has a higher organic content. Enamel rods are separated from each other by the interprismatic substance, which is as highly calcified as the enamel rods. When a longitudinally ground section of enamel is treated with mild etching solution, the cylindrical clear crystalline enamel prism appears to be divided horizontally into equal segments by less calcified dark lines, 4 giving the enamel rods a striated appearance. These lines are called cross striation or short increments. 5 Hunter Schreger bands: The bands of Hunter and Schreger are an optical phenomenon produced solely by changes in rod direction. They are seen most clearly in longitudinal ground sections viewed by reflected light and are found in the inner two thirds of the enamel. These bands appear as dark (diazones) and light (parazones) alternating zones that can be reversed by altering the direction of incidence illumination. Scanning electron microscopy clearly reveals the difference in orientation of groups of rods within these zones. It may be due to: 1. Change in the direction of enamel rods. 2. Variation in calcification of the enamel. 3. Alternate zones having different permeability and organic material. 4. Optical phenomenon. 6 Incremental lines of enamel: Cross striation: Each enamel rod demonstrates closely positioned striations along its length known as cross-striations or short increments. These are thought to be formed by the daily rhythm of the ameloblast laying down more and less mineralized enamel. The striations are approximately 3-4 µm apart. This distance represents one day of enamel deposition. Scanning electron microscopy reveals alternating constrictions and expansions. Incremental lines of Retzius: During development of enamel, variations in the metabolism cause variations in the amount of organic material deposited in the enamel. This causes changes in the coloration of the enamel that is laid down at that time so that alternating dark (higher organic material) and light (less organic material) banding occurs. These bands are called Stria of Retzius. In Longitudinal Section: Seen as a series of dark bands reflecting successive enamel-forming fronts. At the middle and cervical parts: they run obliquely and deviate occlusally reach the enamel surface and become represented as a series of transverse depressions (perikymata). At cusp tips and incisal ridges: these bands form semicircle as they do not reach the surface (surround the tip of dentin). In Cross Section: Seen as concentric rings. Neonatal line: The neonatal line is a dark stria of Retzius that occurs at the time of birth. It is due to the stress of birth. The neonatal line is usually the darkest and thickest stria of Retzius. 7  Present only in deciduous teeth and first permanent molars (enamel develops partly before and partly after birth). Separate enamel formed before birth from enamel formed after birth. The quality of prenatal enamel is better than the postnatal enamel (more protected condition and constant nutrition of the fetus). 8 Hypocalcified structures of enamel: Amelodentinal junction: The junction between enamel and dentin is seen as a scalloped profile in cross section with the convexities of the scallops directed towards the dentin, these scallops provide undercuts that was thought at one time to be the cause of the firm attachment between enamel and dentin. However, the elevations of the scallops occasionally seem to be absent and in this case the ADJ appears smooth and still enamel and dentin are firmly attached. It seems that the cause of the firm attachment between enamel and dentin is due to the interdigitation at the ADJ between the fibrils of the first formed layer of dentin and the fibrils of the organic matrix of the first formed layer of enamel, and consequently the interdigitation between the hydroxyapatite crystals that are deposited during the mineralization of both structures. Enamel tufts: Enamel tufts are less mineralized areas of enamel in the inner third of enamel adjacent to the ADJ. They resemble tufts of grass. Occur developmentally because of abrupt changes in the direction of groups of rods that arise from different regions of the scalloped ADJ. Project from ADJ for a short distance into 1/5-1/3 enamel thickness, they appear to be branched and contain greater concentration of enamel proteins than the rest of enamel. Best seen in transverse thick sections of enamel. 9 Enamel spindles: The enamel spindles appear as short, straight, thin, dark structures which extend for only short distances into enamel. They are residual tubules formed when odontoblastic processes extend across the ADJ during odontogenesis before enamel forms. In ground sections, the odontoblastic processes disintegrate and are replaced by air which appears dark in transmitted light. Enamel lamellae: Enamel lamellae are fissure-like defects and extend for varying depth from the surface of enamel and consist of linear longitudinal oriented defects. True lamellae may contain enamel proteins or organic debris from oral cavity. Types: Developmental (true Lamellae): Type A Non Developmental (False Lamellae) Or Cracks: Type B Type C Accidental crack 10 Surface structures in Enamel: Outer Structureless enamel: ( prismless enamel ) In the surface layer of enamel no prism are visible and the apatite crystals are arranged parallel to one another and perpendicular to the incremental lines of retzius. The high degree of mineralization of the outer 15 µm of the enamel probably reflects events occurring during the late maturation stage as well as post-eruptive mineral accumulation from saliva. Commonly occur at the cervical area more than the cusp tip or incisal edge. Found in all deciduous teeth and 70% of permanent teeth. Perikymata: It is the external manifestation of the incremental lines of retzius. Represented as shallow furrows where the striae of retzius end in the outer surface of enamel. Continuous around the tooth and parallel to the CEJ. There are 30 perikymata/mm in cervical area and decrease toward the occlusal surface. 11 Rod end: They are concave depression vary in depth and shape. They are shallow at the cervical region and deepen near the incisal or occlusal surface Cracks: They are narrow fissure like structure found on almost all enamel surfaces and extend for varying distance. They are actually the outer edge of enamel lamellae. Completely disappear by careful decalcification. Enamel perikymata 12