Development Of Teeth (Odontogenesis) Lec 2 PDF

DEVELOPMENT OF TEETH (ODONTOGENESIS) lec 2 by shadan muhammad  CAP STAGE (PROLIFERATION) : The epithelial bud continues to proliferate into the ectomesenchyme. Immediately adjacent to the epithelial ingrowth, the cellular density inc...

DEVELOPMENT OF TEETH (ODONTOGENESIS) lec 2 by shadan muhammad  CAP STAGE (PROLIFERATION) : The epithelial bud continues to proliferate into the ectomesenchyme. Immediately adjacent to the epithelial ingrowth, the cellular density increases. This process is known as the condensation of the ectomesenchyme. When the embryo is 20 weeks old ,deciduous dentition is at various stages of development. After continuous division and differentiation, the size and shape of the enamel organ changes from knob-like to cap like. EARLY CAP STAGE OF TOOTH DEVELOPMENT  CONT. This developmental stage is called as the cap stage. Invagination on the inner surface of tooth bud results from unequal division leading to cap stage. Cap stage is characterized by: - The outer and inner enamel epithelium - The stellate reticulum.  OUTER AND INNER ENAMEL EPITHELIUM The cells of the outer enamel epithelium are cuboidal and cover the convexity of the cap whereas the cells of the inner enamel epithelium are tall, columnar and cover the concavity of the cap. Basement membrane separates the inner enamel epithelium from the dental papilla and outer enamel epithelium from the dental sac or dental follicle.  STELLATE RETICULUM (ENAMEL PULP) Polygonal cells present in the center of the enamel organ between the inner and the outer enamel epithelium. The proteinaceous fluid- containing albumin gives a cushion-like consistency to the stellate reticulum that supports and protects the delicate enamel- forming cells. Although it is not fully developed until the late bell stage. THE CENTER OF E.ORGAN SHOWING STELLATE RETICULUM. 1-PERIPHERAL ENAMEL EPITHELIUM 2- ECTOMESENCHYME CONDENSED AROUND E.ORGAN CAP STAGE OF TOOTH DEVELOPMENT  Dental Papilla The proliferating epithelium of the enamel organ exerts influence on the ectomesenchyme (neural crest cells) to proliferate. The ectomesenchymal cells are partly covered by the invaginated portion of the inner enamel epithelium. On condensation, it is called as the dental papilla. It is the primordium of the pulp and is responsible for the formation of dentin and pulp. Cont. In dental papilla, mitotic cell division takes place along with proliferation of new blood capillaries. The peripheral cells near the inner enamel epithelium increase in size and differentiate to form the Odontoblasts. Dental Sac (Dental Follicle) Along with the development of enamel organ and dental papilla, their surrounding areas at the margins, showed cell division resulting into condensation and fibrous development in this zone. This results in the formation of the dental sac. The cells of dental sac are responsible for the formation of the cementum and the periodontal ligament. The entire tooth and its supporting structures are formed by epithelial enamel organ, the dental papilla and the dental sac. Cont. Dental sac is the capsular structure consisting of circular arrangement of fibers. These fibers with root development are differentiated into various types of periodontal fibers, which on one end are embedded in the alveolar bone and the other end in the developing cementum. Transitory structures : During the early stages of tooth development ,three transitory structures may be seen , they are: 1- Enamel knot : It’s a localized mass of cells in the center of inner E. epith., E. Knot thought to play a role in the formation of crown pattern by outlying the enamel fissure. E.Knot soon disappears and seems to contribute cells to the E.cord , although it’s transitory, recent studies of Enamel knot  Cont. it may represent an important signaling center during tooth development. unlike adjacent cells, those within E.Knot are non proliferative and produce molecules associated with signaling in other sites. 2- Enamel cord : it’s a strand of early bell stage of development. it arises in the increasingly high enamel organ as a vertical extension of the E.Knot ,it’s termed E. septum when E. Cord extend from E. knot to outer E. epith. 3- enamel niche :(corner.slit) It’s an apparent structure in the histologic section, created because the dental lamina is a sheet rather than a single strand and often contains a concavity filled with C.T., a section through this arrangement creates the impression that tooth germ has a double attachment to the oral epithelium by two separate strands. Enamel niche early bell stage of tooth development  Bell Stage Is known for Histodifferentiation and morphodifferentiation that takes place. During this stage , tooth crown assumes it’s final shape (morphodifferentiation ),and the cells that will be making the hard tissues of crown (Ameloblasts and odontoblasts ) acquire their distinctive phenotype (Histodifferentiation ). The dental organ is bell- shaped during this stage, majority of it’s cells are called stellate reticulum because they have star-shaped appearance. Cont. Bell stage is divided into: 1- Early bell stage 2- Late bell stage 1- Early Bell stage Cells on the periphery of enamel organ separate into four important layers : a. Inner enamel epithelium b. Stratum intermedium c. Stellate reticulum d. Outer enamel epithelium Bell stage 1- early bell stage : A- Inner Enamel Epithelium : Consists of a single layer of tall columnar cells, which differentiate into specialized cells called ameloblasts, before amelogenesis. These are characterized by high glycogen content. Ameloblasts are attached by junctional complexes laterally and to stratum intermedium by desmosomes. Ameloblasts exert an organizing influence on the underlying ectomesenchymal cells , which differentiate into odontoblasts. B- Stratum Intermedium : a layer of squamous cells present in between the inner enamel epithelium and stellate reticulum. These cells are intimately attached by desmosomes and gap junctions. They have a high degree of metabolic activity due to developed cytoplasmic organelles, acid mucopolysaccharides, glycogen deposits and an enzyme, alkaline phosphatase ,which is important in the mineralization of the enamel. Cont. The cells of this layer concern in the transport of materials to and from E. forming cells in inner E. epith. (ameloblasts ). it’ is essential for the formation and calcification of enamel. It is absent in the root part of the tooth. Inner enamel epithelium and stratum intermedium are considered as a single functional unit. Bell stage: deciduous mandibular first molar tooth germ /(l.s.) c- Stellate Reticulum : The star-shaped cells of the stellate reticulum have long processes, which anastomose with the processes of adjacent cells. There is continuous expansion in the size of the stellate reticulum because of increased amount of intercellular fluid. Just prior to the beginning of enamel formation, at the height of the cusp or incisal edge, the stellate reticulum collapses and gets mixed up with the cells of the stratum intermedium. Thank you