Amelogenesis and Enamel Structure PDF

# Amelogenesis and Enamel Structure ## Amelogenesis - Two processes are involved in the development of enamel: - **Enamel matrix formation**: The ameloblast begins their activity secretion when a small amount of dentin has been laid down. An island of enamel matrix is deposited along the pre-d...

# Amelogenesis and Enamel Structure ## Amelogenesis - Two processes are involved in the development of enamel: - **Enamel matrix formation**: The ameloblast begins their activity secretion when a small amount of dentin has been laid down. An island of enamel matrix is deposited along the pre-dentin. A thin continuous layer of enamel is formed along the dentin as enamel deposition proceeds. - **Mineralization and maturation of the Enamel matrix**: Mineralization of the Enamel matrix takes place in two stages: - **First stage**: Immediately partial mineralization occurs in the organic matrix segments. It is deposited in the form of crystalline apatite. - **Maturation stage**: The gradual completion of mineralization takes place. The process of maturation starts from the height of the crown and progresses cervically. It begins before the matrix has reached its full thickness. ## Lifespan of Ameloblast Cells - Depending on their functions, the lifespan of the cells of the inner enamel epithelium (ameloblast) can be divided into six stages: 1. Morphogenic 2. Organizing and differentiating 3. Formative 4. Maturative or mineralizing 5. Protective 6. Desmolytic ### Morphogenic Stage - The connective tissue of the dental papilla is separated from the inner enamel epithelium by a delicate basal lamina. - The ameloblast is short and columnar, with large oval nuclei that nearly fill the cell body. ### Organizing and Differentiating Stage - The inner epithelium interacts with the adjacent connective tissue cells of the dental papilla, which differentiate into odontoblasts. - In this stage, the cells of the inner enamel epithelium become longer with the nucleus moving to the upper part of the cell. ### Formative (Secretory) Stage - The ameloblast enters their formative stage after the first layer of dentin has been formed. - The presence of dentin seems to be necessary for the inducing ameloblast's further differentiation and the formation of Tome's processes at their base. - **Tome's processes**: secretion from the proximal part gives rise to inter-rod enamel, and secretion from its distal part gives rise to rods enamel. - At the end of this secretory stage, Tome's process will be lost, and thus the last layer of enamel will be rod-less. ### Maturation Stage - This stage has two phases: - **Transitional phase**: Ameloblasts become shorter and the overall number is reduced by programmed cell death (apoptosis). By the end of this stage, the ameloblast will be reduced to 50%. - **Maturation proper phase**: Influx of calcium and phosphate with reduced enamel proteins. Ameloblasts modulate between two phenotypes, depending on the morphology of their distal ends: - Ameloblasts with ruffled border - Ameloblasts with smooth distal borders. - This forms two morphologically different types: - Ruffled-ended ameloblasts (80% of maturation ameloblasts) - Smooth-ended ameloblasts (20%) ### Protective Stage - When the enamel has completely developed and has fully calcified, the ameloblast with cells of the stratum intermedium and outer enamel epithelium arrange themselves in a well-defined layer of cells called reduced enamel epithelium. - The function of reduced enamel epithelium is: - Protection of the mature enamel by separating it from the connective tissue until the tooth erupts. - If connective tissue contacts with the enamel, anomalies may develop, such as the enamel may be either resorbed or covered by a layer of cementum. ### Desmolytic Stage - The reduced enamel epithelium proliferation seems to destroy the connective tissue separating it from the oral epithelium. - Fusion of these two epithelia can occur. - The epithelial cells elaborate enzymes that are able to destroy connective tissue fibers by desmolysis and help in eruption. ## Enamel Structure - Enamel is the hardest calcified tissue of the body, covering the anatomic crown of the tooth. - In contrast to other calcified structures of the body, Enamel is ectodermal in origin, has 96% inorganic component, and unique organic constituents which do not contain collagen. - Structure of enamel is studied using ground sections. - Decalcified sections are not of much use because enamel is lost during decalcification due to its high mineral content. ## Enamel Rods - Enamel rods are the basic structural units of enamel. - They run from the dentinoenamel junction to the outer surface of enamel and follow somewhat tortuous course. - In a longitudinal section, enamel rods appear to be divided into segments by dark lines. - These dark lines across the enamel rods are called cross striations. - Each segment is 4 microns wide, which is the increment of enamel deposited daily. - In cross section, enamel rods may resemble a fish scale or keyhole pattern, with a head and a tail. - Head represents the rods, and tail represents the inter-rod region. - The head portion is directed towards the occlusal aspect, and tail to the cervical region of the tooth. ## Striae of Retzius - Incremental lines of enamel, representing the successive apposition of enamel. - These structural lines appear as brownish bands in ground sections. - In the region of the incisal edge and cusps, they surround the dentin while in the cervical region, they are seen as oblique lines extending from DEJ towards the outer surface deviating in an occlusal direction. - In transverse sections of teeth, incremental lines are seen as concentric rings. - These lines are hypocalcified and reflect variation in structure and mineralization. ## Neonatal Line - A prominent incremental line that separates the enamel that is formed before birth (prenatal enamel) and after birth (post natal enamel). - The incremental line becomes prominent because of an abrupt change in the environment that occurs at the time of birth. - The neonatal line is seen in all deciduous teeth and first permanent molars, where enamel is formed partly before birth and partly after. ## Enamel Lamellae - Leaf-like structures that extend from the outer surface of enamel towards the dentin. - They are hypocalcified structures and are formed in planes of tension. - There are three types: - **Type A**: Composed of poorly calcified enamel rods. This type is restricted to enamel. - **Type B**: Consists of degenerated cells and may extend into dentin. - **Type C**: Filled with organic matter derived from saliva. This type is formed after the eruption of the tooth and may be extended into the dentin. ## Enamel Tufts - Ribbon-like structures extending from the dentinoenamel junction into enamel to a distance of one-third to one-fifth of enamel thickness. - In thick sections, these ribbon-like structures arising from different planes are projected to one plane, giving the appearance of a tuft of grass. - They are hypocalcified structures containing greater concentration of organic components. - Enamel tufts are better visualized in transverse sections. ## Enamel Spindle - Odontoblastic processes crossing the dentinoenamel junction and extending to the enamel. - They are spindle-shaped structures extending from the dentinoenamel junction to enamel to a distance of approximately 10 microns. - They appear dark in a ground section under transmitted light, because the organic content of the spindle is lost and is replaced by air. ## Hunter-Schreger Band - Appear as alternate light and dark bands, seen under reflected light. - Occur due to abrupt change in direction of enamel rods. ## Gnarled Enamel - Enamel rods follow a wavy course as they extend from the dentinoenamel junction towards the outer surface. - In the region of the cusps and incisal edges, the arrangement of enamel rods is more complicated. - The enamel rods are more wavy and irregular, and intertwine with each other in this region. - This arrangement creates an optical appearance referred to as gnarled enamel. - This particular arrangement of the rods in the cuspal and incisal regions makes enamel stronger to withstand masticatory stress. # Dentin and Dentinogenesis ## Dentinogenesis - Dentin formation begins at the late bell stage of development of the tooth. - Dentinogenesis takes place in two phases: - **Collagen matrix formation**: Both the collagen and the other components of the extracellular matrix are secreted by odontoblasts. This continues until the crown is formed and the teeth erupt and move into occlusion, and completion of root development. The initial dentin formed along the cusp tips is called Korff's fibers. - **Mineralization (calcification)**: Initially mineralization starts by deposition of very fine plates of hydroxy apatite on the surface of collagen fibrils and the ground substance. ## Histological Features of Dentin - Depending on its time of development, dentin is classified as: - **Primary dentin**: The developmental dentin that is formed before root formation and during eruption. - **Secondary dentin**: Formed after root completion and complete eruption of teeth. It contains fewer dentinal tubules than the primary dentin and grows much slower than primary dentin. - **Tertiary dentin**: Produced in reaction to a stimulus, such as attrition, caries, or restorative dental procedures. It can be reactionary or reparative: - **Reactionary dentin**: Deployed by the pre-existing odontoblasts - **Reparative dentin**: Deployed by newly differentiated odontoblast ### Types of Dentin - **Predentin**: The first deposited layer of un-mineralized matrix of dentin. - **Mantle dentin**: The outer layer of dentine that mineralizes first. It lies near the DEJ. It is formed by newly differentiated odontoblasts - **Circumpulpal dentin**: The bulk of dentin underlying the mantle dentin. It is formed before the complete formation of the root. - **Peritubular dentin**: This dentin forms the walls of the dentinal tubules. - **Intertubular dentin**: It is located between the dentinal tubules and its highly mineralized (hypercalcified). - **Predentin**: Is located adjacent to the pulp tissue. It is the first formed dentin and is not mineralized. - **Interglobular dentin**: Dentine separating the mantle dentin and circumpulpal dentin is a hypo-mineralized zone. - **Tomes granular layer**: In the roots near the cemento-dentinal junction, there are hypo-mineralized areas of dentin around the dentinal tubule called the Tomes granular layer. ## Types of Dentin (cont.) - **Innervation of dentin**: Dentin is a highly sensitive tissue because the dentinal tubules contain numerous nerve endings in the predentin near the pulp in close association with the odontoblasts process within the tubules. - **Incremental lines**: The organic matrix of dentine is deposited incrementally at a daily rate of 4µm. - **Incremental lines of Von Ebner**: Appear as fine striations in dentin; they run at right angles to the dentinal tubules. These lines reflect the daily rhythmic, recurrent deposition. - **Contour lines of Owen**: Formed because of disturbances in the matrix and mineralization process. - **Neonatal line**: It is a line that separates between the prenatal and postnatal dentin. ## Secondary Curvatures - Contour lines of Owen - formed when secondary curvature of adjacent tubules coincide together. ## Age and Functional Changes - With advancing age and for functional requirements, a number of changes are seen in dentin: - **Formation of secondary dentin** - **Sclerotic dentin**: Dentin that has become translucent due to calcification of the dentinal tubules as a result of injury or normal aging. - **Formation of reparative dentin (Tertiary dentin)** - **Dead tracts**: Zone in dentin where the odontoblasts disintegrate and the tubules are empty and are filled with air, resulting in a loss of the odontoblasts processes. ## Dentin Structure - Dentin is a hard bone-like tissue that is present in the crown as well as in the root of teeth. - In the crown, dentin is covered by enamel and in the root, it is covered by cementum. - Unlike enamel, dentin is a vital tissue containing the cell processes of odontoblasts. ### General Characteristics of Dentin - It is covered by enamel on the crown and cementum on the root and rounded by the entire pulp. - Dentin is pale yellow in color, becoming darker with age. - Dentin is slightly elastic. - The dentin is permeable because it is traversed by tubules. - Dentin consists of 30% organic material (collagenous fibrils) and 70% inorganic material (hydroxyapatite), neurons. ## Structures of Dentin - **Dentinal Tubules**: Dentin is packed with dentinal tubules, which span from the outer surface of the dentin to the area near the pulp, and follow S-shaped path. These tubules contain fluid and cellular structure. - **Odontoblast**: A biological cell of neural crest origin. Dentin formation occurs by the cells known as odontoblasts. They differentiate from the ectomesenchyme cells of the dental papilla. Odontoblasts are large columnar cells. - **Odontoblast process**: Its extension of odontoblasts cells, the odontoblast process forms during dentinogenesis. Dentinal tubules contain odontoblastic process and nerve fibers. - The odontoblastic processes have numerous side branches that exist in the later branches of the dentinal tubules. ## Function of Odontoblast Processes - Cause the secretion of hydroxy apatite crystals and mineralization of the matrix secreted by the odontoblast cell. - Secretion of tubular dentin. - Maintenance of dentinal tubule fluid. - Secretion of sclerotic dentin. - Maintaining dentin sensitivity.