Teeth Development PDF

The primitive oral cavity or stomedeum is lined by stratified squamous epithelium termed ectoderm. Most of the connective tissue underlying oral ectoderm are neural crest cells or ectomesenchyme. These cells are thought to instruct the overlying oral ectoderm after epithelial signal to start tooth...

The primitive oral cavity or stomedeum is lined by stratified squamous epithelium termed ectoderm. Most of the connective tissue underlying oral ectoderm are neural crest cells or ectomesenchyme. These cells are thought to instruct the overlying oral ectoderm after epithelial signal to start tooth development. It begins in the anterior portion of both maxilla and mandible and proceeds posteriorly. *Proliferation of epithelium and high mitotic index in the ectomesenchyme* ***[DENTAL LAMINA]***: at certain areas of basal cells of oral ectoderm proliferates more rapidly than do adjacent cell. This was preceded by proliferation of ectomesenchymal cells in the horse shoe shaped dental arches. This leads to formation of dental lamina or band of epithelium along each of the horseshoe future dental arch. **[FUNCTION :]** 1. The dental lamina serves as primordial for ectodermal portion of the deciduous teeth later in the development of the jaws. 2. The permanent molars arises directly from distal extension of dental lamina 3-The successors of the deciduous teeth develop from a lingual extension of the free end of dental lamina opposite to the enamel organ of each deciduous tooth. \`Figure\_017 ***[FATE OF DENTAL LAMINA] :*** As the teeth continue to develop they lose their connection to the dental lamina.They latter break up by mesenchymal invasion which is at first incomplete & does not perforate the total thickness of the dental lamina. Remnants of the dental lamina persists as epithelial pearls or islands within the jaw as well as in the gingival \[epithelial pearls of serres \]. [T00TH DEVELOPMENT]: At certain points along the dental lamina each representing the location of one of 10 mandibular & maxillary deciduous teeth. The ectodermal cells of the dental lamina take instruction again from neural crest cells to multiply more rapidly and form little knob **[(bud)]** that grow into the underlying mesenchyme, The first to appears those of the anterior mandibular region. As all proliferation continues each enamel organ increases in size and change in shape. As it develops, it take one shape resemble **[a cap]** with the outside of the cap toward the oral surface. As in the depression inside the cap, ectomesenchymal cells that proliferates & condensed than adjacent mesenchyme and represents the beginning of the dental papilla. Surrounding the combined enamel organ and dental papilla , the third part of the tooth which is the dental sac \[ it cosists of ectomesenchymal cells and fibers\]. During and after these developments the shape of the enamel organ continued to change. The depression occupied by the dental papilla deepens until enamel organ assumes a shape resembling **[a bell]**. As this development takes place, the dental lamina has thus far connected the enamel organ to oral ectoderm. It breaks up and the tooth germ loses connection with the epithelium of the primitive oral cavity. ***[DEVELOPMENTAL STAGES :]*** They are named after the shape of epithelial part of the enamel organ and are called bud, cap ,bell and advanced bell stages. **[BUD STAGE :]** The epithelium of the dental lamina is separated from underlying ectomesenchyme by [basement membrane]. At specific location ectomesenchymal proliferation instruct the basal portion of dental lamina to proliferate to form round or ovoid swelling called **[bud]**. This swelling corresponding to future position of deciduous teeth. In bud stage the [enamel organ consists of short columnar cells peripherally located and polygonalcells centrally located].  **[CAP STAGE :]** The ectomesenchymal cells continued to proliferate forming ball and are called dental papilla. Also, the epithelium proliferate in an attempt to surround this growing ball. [The ectomesenchymal cells still exerts pressure in the center of the enamel organ leading to unequal proliferation and iscalled cap stage]. The ectomesenchymal cells migrate to encapsulate the E.O. &D.P.and are called **[dental follicle]**. The cells in the center of enamel organ begin to secrete acidic MPS into the intercellular spaces separating them. This substance is intensely hydrophilic. So pulled water into this intercellular spaces and become star shaped and are called **[stellate reticulum]**. **[The Histologic features of the cap stage :]** **[Outer enamel epithelium]** **[Enamel] organ [ stellate reticulum]** **[Inner enamel epithelium]** **[Dental papilla]** **[Dentall follicle]** Capture4.JPG **[BELL STAGE]**: The epithelium is essential for the initiation of tooth development, regulating the key of epithelial-mesenchyme interactions that control morphogenesis and histodifferentiation of enamel organ. [The epithelial signal stop at the end of cap stage. At the bell stage the ectomesencyme determines the shape of the crowen pattern.] -The bell stage constitutes of two phases: 1-[EARLY BELL STAGE] \[before mineralization\] [2-LATE BELL STAGE]: \[after mineralization \] Four different types of epithelial cells can be distinguished. Also, the role of ectomesenchyme indetermination of tooth shape, size appear in bell stage. **The four layers are :** inner and outer enamel epithelium as well as stellate reticulum and stratum intermedium. During this stage the dental organ show more histodifferentiation & morphodifferentiation than the cap stage. A new layer of cells is differentiated between the inner enamel epithelium and stellate reticulum which is the stratum intermedium. It consists of 2:3 layers of flattened cells rich in alkaline phosphatase enzyme.. This layer seems to be essential forenamel formation. The inner enamel epithelium consist of a single layer of short columnar cells. The cells elongates on toward the dental papilla resulting in disappearance of the cell free zone. As the IEE. Come in contact with D.P. cells, they exert an organizing influence on the undifferentiated ectomesenchymal cells to differentiate to odontoblasts \[ the process is known as induction\]. The smooth convex surface of the O.E.E. is laid in folds. Between the folds , the adjacent mesenchymal of dental sac forms papillae that contain capillary loops and thus provide a rich nutrional supply for the intense metabolic activity for the a vascular enamel organ. ***[THE HISTOLOGICAL PATTERN OF ADVANCED BELL STAGE :]*** The advanced bell stage is marked by the beginning of dentin mineralization. During this stage ,the boundary between the I.E.E. &odontoblasts outline the future dentino-enamel junction. In addition the cervical loope give rise to epithelial root sheath of hertwig,s which mould the root & important in root formation. ***Enamel organ :*** After mineralization of dentin , the main source of nutritional supply of enamel organ from dental papilla is cut off and the dental organ react to compensate the needs from nutritive materials from the C.T. of dental sac surrounding it. **[( reverse of nutritional source)]** **[outer enamel epithelium]** **Inner enamel epithelium** **Stellate reticulum** **Stratum** **intermedium** **Dental papilla : [it is designated as dental pulp by beginning of dentinformation.]** **Dental follicle** : **[The mesenchyme showed increased formation ,condensation and become more cellular.]** **[Dental lamina:]** The dental lamina joining the tooth germ breaks up by invasion of the mesenchymal tissue of the dental sac.So , the developing tooth germ lose connection with the oral epithelium. [Remenants of dental lamina may persist in the gingival and jaw and called epithelial rests of serre,s \[serres pearls\]]. These epithelial rests normally degenerates or may form small cysts over the developing tooth and delay eruption. Capture7.JPG ***[HISTOPHYSIOLOGIC STAGES]***: 1. INITIATION 2. PROLIFERATION 3. -HISTODIFFERENTIATION 4. MORPHODIFFERENTIATION 5. APPOSITION : Differential growth of the epithelial diaphragm in multirooted teeth causes the division of the root trunk into two or three roots. That long tongue like extensions of the horizontal diaphragm develop. Two such extensions are found in the germs of lower molars and three in the germs of upper molars.. If the continuity of Hertwig,s root sheath is broken or is not established prior to dentin formation, a defect in the dentinal wall of the pulp ensures.This accounts for the development of accessory root canals opening on the periodontal surface of the root  Scan23  11