4 development.pdf

development of teeth Dr Sandeep Gupta Oral Pathologist Learning objectives At the end of the lecture student should be able to inform about Role of dental lamina in tooth formation Various stages of tooth development Various changes in the process of calcification and...

development of teeth Dr Sandeep Gupta Oral Pathologist Learning objectives At the end of the lecture student should be able to inform about Role of dental lamina in tooth formation Various stages of tooth development Various changes in the process of calcification and tooth form Overview The tooth is formed from the ectoderm and ectomesenchyme. At 37 days of development, a continuous band of thickened epithelium forms around the mouth in the presumptive upper and lower jaws. are roughly horseshoe-shaped Correspond in position to the future dental arches of the upper and lower jaws  Each band of epithelium, called the primary epithelial band, quickly gives rise to two subdivisions They in-grow into the underlying mesenchyme colonized by neural crest cells. These are the dental lamina The second subdivision forms the vestibular lamina Initiation of tooth development starts with formation of localized thickenings or placodes within the primary epithelial bands  Teeth are formed in relation to the alveolar process. Ten Epithelial thickenings in Dental lamina Enamel organs: Series of 10 local thickenings on dental lamina in each alveolar process. Each placode forms one deciduous tooth. Ectomesenchymal cells accumulate around these outgrowths.  From this point, tooth development proceeds in three stages: the bud, cap, and bell. These terms are descriptive of the morphology of the developing tooth. Stages in formation of tooth Bud stage : Characterized by formation of a tooth bud. The epithelial cells begin to proliferate into the ectomesenchyme of the jaw.  The bud stage is represented by the first epithelial incursion into the ectomesenchyme of the jaw The supporting ectomesenchymal cells are packed closely beneath and around the epithelial bud The epithelial bud continues to proliferate into the ectomesenchyme This increases cellular density increases immediately adjacent to the epithelial outgrowth This process is classically referred to as a condensation of the ectomesenchyme. Bud stage (Initiation) CONDENSATION Bud stage Cap stage : Formation of dental papilla. The enamel organ & dental papilla forms the tooth germ. Formation of ameloblasts. Formation of odontoblasts.  Enamel organ in cap stage shows an unequal rate of proliferation in different parts instead of uniform expansion. This leads to a stage where the enamel organ looks like a cap. The cells of the enamel organ in the convex portion of the cap are cuboidal in shape and form the outer enamel epithelium The cells in the concavity of the cap are columnar in shape and form the inner enamel epithelium.  The cells in the center of the enamel organ (between the outer and the inner enamel epithelium) synthesize glycosaminoglycans That absorb water as it is hydrophilic causing stretching of intercellular bridges This gives them appearance like stars So it is termed as stellate reticulum (star-shaped branch like network) The fluid in stellate reticulum acts as shock absorber and protects enamel forming cells  The cells in the centre of the concavity of the ‘cap’ form a knob-like enlargement projecting towards the underlying dental papilla. This structure is called the primary enamel knot. Enamel knot extends vertically running across the center of the enamel organ - called the enamel cord  Dental Papilla in cap stage Cells of the dental papilla appear more crowded in this stage. The dental papilla also shows signs of becoming more vascular. Dental Sac in cap stage The dental sac appears more condensed and fibrous.  Enamel Organ in bell stage As the enamel organ further invaginate with growth in the margins, it takes the shape of a bell. In this stage, the crown of the tooth gets its final shape (morpho-differentiation) Cells that form the hard tissues of the crown (the ameloblasts that form the enamel and the odontoblasts that form dentin) acquire histo- differentiation. Bell stage:  enamel organ can be identified in this stage as four layers: Inner enamel epithelium Stratum intermedium Stellate reticulum Outer enamel epithelium Some layers of squamous cells seen between the inner enamel epithelium and stellate reticulum are called stratum intermedium.  Stratum intermedium exhibit a high enzyme alkaline phosphatase activity. This layer plays an important role in regulating the formation of enamel. Inner enamel epithelium: A single layer of columnar cells differentiates into tall columnar cells called ameloblasts before the formation of enamel (amelogenesis).  Stellate reticulum- star-shaped They serve to protect the underlying inner enamel epithelial cells. After a layer of dentin is formed, the inner enamel epithelium does not get their nutritional from the dental papilla. The stellate reticulum collapses before the formation of enamel to provide nutrition from the dental follicle  Outer enamel epithelium: The cuboidal cells are connected to the adjacent cells by junctional complexes. It is smooth in the initial stages. As the stellate reticulum collapses before enamel formation, it gets into folds This brings the capillaries present in the dental follicle closer to the just formed ameloblasts.  Dental papilla in bell stage There is formation of acellular zone between enamel organ and dental papilla The peripherally placed undifferentiated ectomesenchymal cells of the dental papilla increase in size before the enamel formation begins. They are initially cuboidal and later become columnar, occupy the acellular zone and differentiate into odontoblasts.  The basement membrane which separates the enamel organ and dental papilla just before dentin formation is called membrana preformativa.  Dental Follicle in bell stage Collagen fibrils occupy the extracellular spaces between the fibroblasts of the dental follicle These collagen fibrils orient themselves circularly around the enamel organ and the dental papillae. These fibers of the dental follicle that later differentiate into periodontal fibers Crown Pattern Determination The other events that occur during the bell stage are: The breaking up of dental lamina Determination of the crown pattern of the tooth (morphodifferentiation) In bell stage, the dental lamina (which joins the enamel organ to the oral ectoderm) is invaded by the surrounding cells of the dental follicle.  Causes separation of the enamel organ from the oral ectoderm. Then clusters of epithelial cells usually degenerate, If the cells persist in this region form the epithelial islands (cell rests of Serres). Sometimes, cysts are formed over the developing tooth by these cell rests, delaying the eruption of the tooth.  Advanced Bell Stage Advanced bell stage is characterized by the beginning of mineralization and root formation.  Mineralization The peripheral ectomesenchymal cells of the dental papilla are differentiated into odontoblasts This occurs under the influence of the inner enamel epithelial cells. The line separating the newly differentiated odontoblasts and the inner enamel epithelium outlines the dentinoenamel junction. (membrana preformativa)  Odontoblasts begin to differentiate and form organic matrix of dentin along the dentinoenamel junction It generally begins in the region of future cusp. This matrix proceeds in pulpal and apical direction and mineralizes later. After the first layer of dentin is formed, the inner enamel epithelial cells elongate and exhibit the reversal of polarity  Turn into functional ameloblasts. These ameloblasts produce an organic matrix of enamel against the newly formed dentin. This organic matrix mineralizes to become the initial layer of enamel. As the enamel formation proceeds from the dentinoenamel junction towards the outer surface, the ameloblasts move away from dentin in coronal and cervical region. Reciprocal Induction An important step in the development of tooth is the terminal differentiation of ameloblasts and odontoblasts They form two principal hard tissues of the tooth: enamel and dentin.(histodifferentiation) Differentiation of odontoblasts - from the ectomesenchymal cells of the dental papilla This is influenced by the cell of inner enamel epithelium  Once the odontoblasts are differentiated - a single layer of dentin matrix is laid down This process stimulate inner enamel epithelial cells to differentiate into functional ameloblasts The differentiation of ameloblasts and odontoblasts is interdependent (reciprocal induction) ROOT FORMATION When dentinoenamel line reaches the future cementoenamel junction, root development begins. From the cervical portion of the enamel organ Hertwig’s epithelial root sheath formation occurs plays an important role in the determination of the shape, length, size and number of roots Also, it initiates radicular dentin formation.  Sheath is a double layer of cells composed only of the inner and outer enamel epithelia. inner enamel epithelial layer of the sheath influence the formation of a layer of odontoblasts from the outermost portion of radicular dental papilla. This layer of odontoblasts lays down the first layer of radicular dentin.  After the first layer of radicular dentin is formed, the cells of the dental follicle/sac proliferate They invade the double layer of sheath, dividing it into epithelial strands. Connective tissue cells of the dental follicle meet the surface of the newly formed radicular dentin. They differentiate into cementoblasts from the cells of the dental follicle, To deposit cementum on the surface of the radicular dentin.  Along with, Cells of the Dental follicle like Fibroblasts differentiate to form the periodontal ligament Osteoblasts differentiate to form the alveolar bone Most of the cells of sheath are removed by invasion by the cells of the dental follicle, some remnants are left behind. These remnants are called cell rests of Malassez They may be found as strands of epithelial cells in the periodontal ligament of erupted teeth May contribute to odontogenic tumors an cysts  Formation of a Single-Rooted Tooth During root development, the plane of the epithelial diaphragm remains fixed, and the lengthening of the root is accompanied by the movement of the crown in an axial direction. With the increase in the length of the root sheath, more root is formed with the formation of the cementum, periodontal ligament and alveolar bone  Formation of a Multi-Rooted Tooth In case of multirooted teeth, tongue-like extensions develop on the horizontal diaphragm due to differential growth of the diaphragm. lower molars show two such extensions The upper molars show three such extensions, The free ends of which grow towards each other and fuse. Thank You