Amelogenesis 2024 PDF

INTRODUCTION Tooth develops from a genetically instigated, localised and interaction between an area of oral ectoderm and underlying mesenchymal cells which have been derived from the neural crest cells. Stages are defined by the morphology of the epithelial component of the developing...

INTRODUCTION Tooth develops from a genetically instigated, localised and interaction between an area of oral ectoderm and underlying mesenchymal cells which have been derived from the neural crest cells. Stages are defined by the morphology of the epithelial component of the developing tooth that becomes the enamel organ. Origin and formation of enamel is amelogenesis Innermost layer of the enamel organ, the internal dental epithelium plays a vital role in amelogenesis whereby it deposits and later modifies the enamel.  INTRODUCTION Enamel matrix formation begins soon after the beginning of dentin formation and closely follows the progress of dentin formation (reciprocal induction) Begins at the early crown stage of tooth development. Involves the differentiation of the cells of internal dental epithelium at the tips of the cusp outlines and then sweeping down the slope until all cells of the epithelium have differentiated into enamel forming cells or ameloblasts.  DEVELOPMENT OF AMELOBLAST Described in five main stages: Presecretory Secretory Transition Maturation Post-Maturation  PRESECRETORY STAGE Involves activities of the ameloblast before the secretory activity. During bell stage, cells of IDE, cease to divide and differentiate into ameloblast. Cells are cuboidal or low columnar with centrally located nuclei. Golgi elements are in the proximal portion of the cells. Mitochondria and other cytoplasmic components are scattered throughout the cells  Changes are also occurring in the outer mesenchymal cells of the dental papilla which are differentiating into odontoblasts. The internal dental epithelium with epithelial layers forms a basal lamina separating it from underlying mesenchyme. This lamina marks the position of the future enamel-dentine junction. The basal lamina supporting the ameloblasts disintegrates after the deposition of a layer of predentin and thus the future odontoblast and ameloblast are in contact to allow inductive signalling Cells of the internal dental epithelium differentiate into ameloblasts. Pre – ameloblast synthesise and secrete small amounts of proteins similar to those that will form the enamel matrix This brings about changes to the cell such as: they elongate and their nuclei shift proximally toward the stratum intermedium. Golgi complex increases in volume and migrates from its proximal position to occupy a major portion of the central core of the cell. Amount of rough endoplasmic reticulum increases significantly Most of the mitochondria of the cells cluster in the proximal region. Thus the ameloblast becomes a highly polarized cell  SECRETORY STAGE Aka formative stage Enamel proteins are synthesised in the rough endoplasmic reticulum. It is then taken to the golgi complex where it is condensed and packaged into membrane bound secretory granules. Granules then migrate to the distal extremity of the cell, and the contents, which is the enamel matrix (partially mineralised) are released against the newly formed mantle dentine. Hydroxyapatite crystals are randomly packed in the first formed enamel which interdigitate with the crystals of the dentine  As the first structureless enamel is formed ameloblast migrate away from the dentin surface and each ameloblast develops short conical projections known as Tome’s process. Tome’s processes jut into the newly forming enamel, giving the junction between the enamel and ameloblasts a “picket-fence” or saw toothed appearance Although the cytoplasm of the ameloblast continues into the Tome’s process, the distinction between the process and cell body is clearly marked by the distal terminal web junctional complex. Process contains the secretory granules whereas the cell body cytoplasm contains abundant synthetic organelles. Enamel matrix formation  Secretion of enamel protein by secretory granules occurs through narrow channels that are located mainly at two regions within the Tome’s process, namely at its distal end and at the proximal part of its lateral surface, near the junction of the adjacent processes.  Proximal secretory sites completely encircle the Tome’s process Since the timing of proximal and distal sites are not simultaneous, walled pits are formed occupied by the distal end of the processes. This mechanism produces rod like structures of the enamel Hence highly coordinated activity of the Tome’s process creates structure of the enamel. Tome’s process persists until the formation of the final few increments of enamel.  TRANSITION STAGE Initial deposition of enamel is high in water and protein and low in mineral. Period that ameloblasts change from a secretory to matured form is the transition stage. Enamel secretion stops and most of the amelogenin is removed. Height of the ameloblast is reduced.  Through apoptosis ameloblast number is reduced to about 50% With the remaining ameloblast, cell organelles associated with protein synthesis are reduced Enamel organ becomes invaginated by blood vessels  MATURATION At this stage the entire thickness of enamel has formed but it is still immature and partially mineralized. Process by which enamel changes to its final form, is termed maturation which involves: Removal of water Removal of amelogenin-type enamel proteins Removal of organic matrix, reducing protein content to 1% Ameloblast change whereby the tome’s process is lost and organelle content reduced.  Mineral is added which squeezes the protein and water out. This is where there is rapid influx of calcium and phosphate ions. Rapid crystal growth occurs occupying the space that was taken up by water and organic matrix.. Growth of crystals continues to a point of compaction so that there is very little space between them.  POST – MATURATION STAGE Ameloblast become flattened. A amorphous layer of protein, primary enamel cuticle , separates the cells from the enamel. This maybe the material extruded from the enamel during maturation or may be the last product of the fading ameloblast. Cells contain hemidesmosomes and a basal lamina reappears (important in the formation of dentogingival junction) Other layers (steallate reticulum, stratum intermedium and external enamel epithelium) of the enamel organ merge with the dental follicle.  During eruption, this reduced enamel epithelium protects the enamel surface from the follicular connective tissue. At this stage, cells are able to modify enamel composition. If fluoride is available, it is incorporated.  ABNORMALITIES Adontia - total lack of tooth development. Amelogenesis imperfecta - abnormal tooth enamel formation (AMELX, ENAM, KLK4, MMP20). Dentinogenesis imperfecta - discoloured teeth with an opalescent sheen, dentin does not support enamel (dentin sialophosphoprotein mutation) Dens evaginatus - dental anomaly mainly affecting premolars in people of Mongolian origin. Hypodontia - lack of development of one or more teeth. Hypohidrotic ectodermal dysplasia - maldevelopment of one or more ectodermal-derived tissues. Microdontia - small teeth.  REFERENCE Nanci. A (2008) Ten Cate‘s Oral Histology – Development, Structure and Function,