Odontogenesis Lecture Notes PDF

ORAL HISTOLOGY PETRA UNIVERISTY Development of teeth (Odontogenesis) The primitive oral cavity or stomatodeum is lined by strat...

ORAL HISTOLOGY PETRA UNIVERISTY Development of teeth (Odontogenesis) The primitive oral cavity or stomatodeum is lined by stratified squamous epithelium which is two or three cell layers in the thickness and is called oral ectoderm. Underling the oral ectoderm is an embryonic connective tissue originates from the neural crest and is called ectomesenchyme. Odontogenesis is initiated by factors in the epithelium of the 1stbranchial arch, but with time this potential assumed by ectomesenchyme. Ping-Pong interactions were required between epithelium and connective tissue.  Dental lamina Around 6thw.i.u life a continuous band of the proliferated epithelium forms at the site of future maxillary and mandibular dental arches. These bands are horse shoe shaped and are called primary epithelial bands. The primary epithelial band gets divided into two parts: a- The vestibular lamina (lip furrow band): The vestibule arises from the proliferation of the vestibular lamina. The cells inside the lamina divide enlarge and rapidly degenerate to form the vestibule. b- Dental lamina: The ectoderm at certain areas of the dental lamina proliferates and forms knob like structures that grows into the underlying ectomesenchyme. Each of these knobs represents one of the future deciduous teeth and is called enamel or dental organ.  Morphological stages and functional activities of tooth development: 1- Bud stage (Initiation and proliferation) 2- Cap stage (Proliferation and Histo-differentiation) 3- Bell stage: (Histo-differentiation and Morpho-differentiation). 4- Early crown (advanced or late bell stage): (Apposition) 5- Late crown: (Successive apposition) 6-Root formation.  Bud stage This is the initial stage of tooth formation where the enamel organ resembles a small bud. During the bud stage, the enamel organ consists of peripherally located low columnar cells and centrally located polygonal cells. The surrounding ectomesenchymal cells proliferate resulting in their condensation in two areas: DR. ATEF ISMAIL AHMED 1 ORAL HISTOLOGY PETRA UNIVERISTY a- The area of condensation immediately below the enamel organ is the dental papilla. Condensation of cells of D. papilla due to local grouping of cells that fails to produce extracellular substance and not separated b- The condensation area that surrounds tooth bud and dental papilla is the dental sac (dental follicle).The dental papilla and dental sac are not well defined during this stage. The enamel organ along with the dental papilla and the dental sac is collectively called tooth germ. The enamel organ eventually helps in the enamel formation. The dental papilla forms the dentine and pulp while the dental sac forms cementum, periodontal ligament and alveolar bone (supporting structures).  Cap stage During this stage the tooth bud continued to proliferate resulting in a cap shaped enamel organ. This is characterized by a shallow invagination on the undersurface of the bud. The ectomesenchymal condensation (dental papilla and dental sac), are well defined during this stage. The dental papilla is separated from the inner enamel epithelium by a thin basement membrane. A similar basement membrane separates between the outer enamel epithelium and the dental sac. - The outer cells of the cap covering the convexity are cuboidal and are called the outer enamel (dental) epithelium. The cells lining the concavity of the cap are tall columnar and are called the inner enamel (dental) epithelium. The central area of the enamel organ between the outer and inner enamel epithelium which consisted of polygonal cells, acquire more intercellular fluid and forms a cellular network called the stellate reticulum or enamel pulp. - Stellate reticulum is a group of star-shaped cells that form a network inside the dental organ. Contrary to other epithelial structures, the stellate reticulum cells are separated by large amounts of extracellular products (e.g. glycosaminoglycans). They expand further due to continued accumulation of fluid. The increase in extracellular fluid forces the cells apart. The cells of this area are having large processes that anastomose with those of the adjacent cells.  Enamel knot, enamel cord and enamel niche: The enamel knot is a localized condensation of non-dividing epithelial cells in the inner enamel epithelium of the tooth. Each tooth has single (primary) knot except molars have two (secondary) knots. It is often continuous as a pillar of cells towards the outer enamel epithelium. This column of cells that divides the enamel organ into two is called the enamel cord or septum. The enamel knot and cord are believed to be involved in determining the initial position of the first cusp of the tooth DR. ATEF ISMAIL AHMED 2 ORAL HISTOLOGY PETRA UNIVERISTY during crown formation. Enamel cord may act as a mechanical tie. Enamel knot may act as signaling center, producing a unique set of signaling molecules (FGF) for tooth development. Enamel cord and knot are transient structures undergoes programmed cellular death (apoptosis) at the onset of the early bell stage. The enamel niche is an apparent structure created by the plane of section during histologic sectioning. It is seen as mesenchymal structures surrounded by the dental epithelium at the site of attachment of enamel organ to the dental lamina.  Early Bell stage Two important events occur during this stage. First, the dental lamina fragments. Second, the inner enamel epithelium completes its folding. Due to continuous uneven growth of the enamel organ it acquires a bell shape by deepening of the undersurface of the enamel organ. The cells of the inner enamel epithelium differentiate into tall columnar cells called preameloblasts. Preameloblasts secrete enzymes that degrade the basement membrane. The cells of the inner enamel epithelium exert an inductive influence on the underlying mesenchymal cells of the dental papilla to differentiate into odontoblasts. They are cuboidal cells that later assume a columnar shape and produce dentin. The junction between the inner enamel epithelium and odontoblasts outlines the future dentino-enamel junction.  Late bell stage (Early crown): During the late bell stage the two major hard tissues of the tooth are formed namely the enamel and dentin. At the site of the future cusp tips the short columnar cells of the inner enamel epithelium elongate and became tall columnar, with the nuclei placed close to the stratum intermedium. The undifferentiated ectomesenchymal cells increase rapidly in size and differentiate into odontoblasts. The differentiation of odontoblasts is initiated by the organizing from the cells of the inner dental epithelium thus in the absence of the inner enamel epithelium dentin will not form. - The odontoblasts begin to elaborate the organic matrix that ultimately mineralizes to form dentin. As the secretion of organic matrix occurs the odontoblasts start moving towards the center of the dental papilla leaving behind a cytoplasmic extension. Once the initial dentin has formed, the cells of the inner enamel epithelium differentiate to form ameloblasts and start secreting the organic matrix of enamel against the newly formed dentin. This organic matrix gets mineralized to form the enamel. The ameloblasts move away from the dentin as the enamel is gradually secreted. DR. ATEF ISMAIL AHMED 3 ORAL HISTOLOGY PETRA UNIVERISTY - A mutual interaction and reciprocal induction is exhibited between the various cells of the dental germ. The odontoblasts differentiate under the influence of the inner enamel epithelium. Similarly enamel formation starts under the influence of dentin. - As the enamel formation starts, the stellate reticulum collapses thereby reducing the distance between the outer and inner enamel epithelium. The cells of the outer enamel epithelium flatten to form low cuboidal cells, and thrown into folds which are rich in capillary network. This provides a source of nutrition for the enamel organ (Reverse of nutritional source). - The dental sac exhibits a circular arrangement of its fibers and resembles a capsule around the enamel organ. The fibers of the dental sac form the periodontal ligament fibers that span between the root and bone. The inner end of the outer dental epithelium meets at the rim of the dental organ. This zone is called cervical loop or area of reflection. - In summary, this stage is characterized by the beginning of apposition of the mineralized tissues of the crown, dentin forms first followed by enamel. It should be emphasized that while apposition starts at the cusp tips or growth centers, a gradient of differentiation exists in a cervical direction along the sides of the developing crown.  Late crown stage This stage is characterized by more incremental apposition (meaning the mineralized tissue is laid down in successive layers or increments) of dentin and enamel.  Root formation After the crown of the tooth is completely formed, a structure called the epithelial root sheath of Hertwig is derived from both the outer and inner dental epithelia at the site where these layers are continuous with each other at the cervical loop region. The root sheath prior to elongation in an apical direction forms an epithelial diaphragm which is a horizontal extension of the epithelial cells. The inner enamel epithelium of the root sheath induces differentiation of adjacent dental papilla cells into odontobalsts which form the root dentin. The sheath also determines the shape and number of the roots. Once dentin is laid down, the sheath disintegrates at that site allowing dental sac cells to come in contact with root dentin. These cells differentiate into cementoblasts which lay down cementum, fibroblasts which lay down periodontal ligament fibers and osteoblasts which lay down alveolar bone. DR. ATEF ISMAIL AHMED 4 ORAL HISTOLOGY PETRA UNIVERISTY - Remnants of the epithelial root sheath persist in the periodontal ligament throughout life. These rests are called the epithelial cell rests of Malassez. Some of these rests differentiate into cementoblasts. The wide open apical foramen is gradually reduced by apposition of dentine and cementum at the root apex. - In case of multirooted teeth the epithelial diaphragm develops tongue like extensions which grow inwards and divides the root trunk into two or three hollow tubes which form as many roots. Root formation in permanent teeth is usually completed approximately 2 -3 years after the tooth erupts. Apposition of root dentin and cementum continues after eruption of the teeth.  Function of the dental lamina: Dental lamina plays an important role in the development of dentition. 1- The deciduous teeth are formed by direct proliferation of the dental lamina. 2- The permanent molars develop as a result of its distal proliferation. 3- The permanent teeth that replace deciduous teeth develop from lingual extension of the dental lamina namely successional laminae.  Fate of the dental lamina During the bell stage the dental lamina breaks up by mesenchymal invasion which first penetrates its central portion and divides it into lateral lamina and the dental lamina proper. The dental lamina proper (successional lamina) proliferates only at its deeper margin, which becomes free and situated lingually to the enamel organ and forms the primordium of the permanent tooth. Fragmentation of lateral dental lamina into discrete clusters of epithelial cells and is called epithelial rests of Serre's (epithelial pearls) that normally degenerate and resorbed. If any persist, they may form small cysts (eruption cysts) over the developing tooth and delay eruption. These pearls may be the key to tooth regeneration?  Life span of the dental lamina: The dental lamina is active from the 6th week in utero to 5 years of age (when the 3rd permanent molar is initiated). The entire dental lamina is not active for five years; only portions of the dental lamina are active at certain times when they give origin to dental organs. DR. ATEF ISMAIL AHMED 5