Teeth have a dual origin: ◦ Ectodermal; ◦ Mesodermal: ◦ Ectomesenchymal. Combine tissue type (neural crest cells + mesenchyme) is called ectomesenchyme. In tooth development, an interaction occurs between oral ectoderm and ectomesenchyme to initiate the process. Cellular interactions, between epithelium+ectomesenchyme: Formation of dentine, enamel and cementum as well as crown; Morphogenesis is stimulated by molecular signals that control: ◦ Cell growth; ◦ Migration; ◦ Cell fate and differentiation. For every developmental event a complex and intricate cascade of gene expression take place to direct the cells to the right place and onto the proper differentiation pathway; Many of these pathways all result from epithelialmesenchymal interactions in which essentially the same molecular mediators are implicated. First process to occur during embryogenesis; Interaction between developing embryonic cells. May also be called morphodifferentiation; Development of form and specific tissues; Results from migration of embryonic cells and inductive interactions between these cells. Patterning Specification of the embryo through segmentation; Patterning is both a spatial and temporal event as exemplified by regional development of incisors, canines, premolars, and molars, which occurs at different times and involves the classical processes of induction, competence, and differentiation. Differentiation Process of specialization of embryonic cells. Growth deep within a tissue of organ; As opposed to appositional growth – growth at the periphery through the addition of additional cell layers; Initiation; Proliferation; Histodifferentiation; Morphodifferentiation; Apposition; Maturation; Eruption. An intriguing question is how dental development is initiated? The newly formed stomadeum is lined by a primitive two- or three-cell-thick layered epithelium covering an embryonic connective tissue that, because neural crest cells have migrated in it, is termed ectomesenchyme; The ectomesenchyme consists of a few spindle-shaped cells separated by a gelatinous ground substance. Formation of primary epithelial thickening, dental lamina, tooth bud & the cap, bell stage cannot take place in absence of dentally active ectomesenchyme which becomes concentrated in regions of presumptive tooth development; Inner epithelium of enamel organ induces ectomesenchyme of dental papilla to become orientated towards the intervening basal lamina and differentiate into odontoblasts. Thickening of the epithelium. Primary epithelial band; Vestibular lamina; Dental lamina. A continuous band of thickened epithelium forms around the mouth in the presumptive upper and lower jaws; This bands are roughly horseshoe shaped and correspond in position of the future dental arches of the jaws; Each band of epithelium quickly gives rise to two subdivisions: ◦ The vestibular lamina and ◦ The dental lamina just behind it. Upper epithelial band tongue lower epithelial band Upper and lower epithelial bands The formation of these thickened epithelial bands is the result not so much of increased proliferative activity within the epeithelium as of a change in orientation of the mitotic spindle and clevage plane of dividing cells. Sagittal section through the head of an embryo. A. Primary epithelial bands; B. The same structure at higher magnification; C The change in plane of clevage. Upper epithelial band Lower Epithelial band The first subdivision of the primary epithelial band is vestibular lamina; The vestibule forms as a result of the proliferation of the vestibular lamina into ectomesenchyme; Its cells rapidly enlarge and then degenerate to form a cleft that becomes the vestibule between the cheek and tooth-bearing area. The other subdivision of the primary epithelial band is dental lamina; Dental lamina is located orally to the vestibular lamina; Within the dental lamina, continued and localized proliferative activity leads to the formation of a series of epithelial outgrowths into the ectomesenchyme at sites corresponding to the positions of the future deciduous teeth. Dental lamina Vestibular lamina Vestibular proliferation leads to the formation of the vestibular lamina; Oral proliferation leads to the formation of the dental lamina. tongue Vestibular lamina Dental lamina Begin localized proliferations; ◦ Early in the medial part - incisors; ◦ Then in the lateral part - canines and molars; In the upper and lower dental lamina are formed 10 proliferations; The process is called “the formation of tooth bud” This is the beginning of the tooth development; Teeth develop from two typs of cells: ◦ Oral epithelial cells; ◦ Mesenchymal cells from the dental papilla; ◦ Neural crest cells – ectomesenchymal cells; Epithelial cells are flat on the surface of the epithelium and cylindrical immediately above to the basal lamina; The underlying ectomesenchymal cells accumulate around the epithelial autgrowths. Flat cells Cylindrical cells The controlling signals (BMP-7) in place of the thickened epithelium The proliferation is: ◦ Multiplication of cells; ◦ Increases the amount of tooth bud; The proliferation is observed: ◦ In the epithelium; ◦ In the ectomesenchym. Certain cells of the basal layer begin to proliferate at a more rapid rate then do the adjacent cells. These proliferating cells contain the entire growth potential of the teeth . Dental bud; Dental cap; Dental bell. These terms are discriptive of the morphology but we have to describe the significant fuctional changes that occur during development, such as morphogenesis and histodifferentiation; Because development is a continuous process, clear distinction brtween the transition stages is not possible. 1. Dental bud; 2. Oral epithelium; 3. Ectomesenchyme. Histological appearance of different cells with: ◦ Different form; ◦ Different size; ◦ Different function; Than are formed different layers of cells : ◦ In the epithelium; ◦ In the ectomesenchym. The differentiated cells shall be arranged under different layers; Each layer acquires a separate function; Each layer takes part in creating the structures of the tooth, in determining the shape and size of the tooth. Enamel organ - epithelial; Dental papilla: ◦ Мesenchymal; ◦ Ectomesenchymal; Dental follicle: ◦ Мesenchymal; ◦ Ectomesenchymal. Proliferation of epithelial cells; Mitotic cell division; Increasing the cells number; Increasing the size of the cells; Still no cells differentiation. The bud stage is represented by the first epithelial incursion into the ectomesenchyme of the jaw; The epithelial cells show little if any change in shape or function; The supporting ectomesenchymal cells are packed closely and around the epithelial bud. Stem cells involved in the formation of the tooth bud; This is pluripotent cell capable to become virtually any cell. At this stage there is no cell differentiation; There is only cell division and cell mitotic activity; The cells are identical in shape and size. The tooth bud now is a ball of cells; The tooth bud grows rapidly in volume. The transition from bud to cap marks the onset of morphological differences between tooth germs that give rise to different types of teeth; As the epithelial bud continues to proliferate into the ectomesenchyme, cellular density increases immediately adjacent to the epithelial outgrowth; This process referred to as a condensation of the ectomesenchyme, results from a local grouping of cells that have failed to produce extracellular substance and have thus not separated from each other. As the tooth bud grows larger, it drags along with it part of the dental lamina; So from that point on the developing tooth is tethered to the dental lamina by an extension called gubernaculum dentis or lateral lamina. Separated from ectomesenchyme factors (red) stimulated epithelial cells (blue), which in turn cause the release of others in the ectomesenchyme (pink); Immediately the bone morphogenetic protein (green) inhibits the further release of epithelial factors (blue); This is the starting signal for the cells differentiation. It is the beginning of cellular differentiation; Differentiated cells are arranged in layers; The formative elements of the tooth are already visible: ◦ The epithelial outgrowth, which resemble a cap sitting on a ball of condensed ectomesenchyme, is referred as the enamel organ (it will form the enamel of the tooth); ◦ The ball of condensed ectomesenchymal cells, called dental papilla, forms the dentin and pulp; ◦ The condensed ectomesenchyme limiting the dental papilla and encapsulating the enamel organ – the dental follicle or sac – give rise to the supporting tissues of the tooth. Because the enamel organ sits over the dental papilla like a cap, this stage of the tooth development is known as the cap stage. The cells that covering the enamel organ are outer dental (or enamel) epithelium; The cells, which are covering the recess of the enamel organ are inner dental epithelium; The center of the enamel organ is termed the stella reticulum: ◦ The cells in the center of the enamel organ synthesize and secrete glycosaminoglycans into the extracellular compartment between the epithelial cells; ◦ Glycosaminoglycans are hydrophilic and so pull water into the enamel organ; ◦ The increasing amount of fluid increases the volume of the extracellular compartement of the enamel organ, and the central cells are forced apart; ◦ Because they retain connections with each other through their desmosomal contact, they become star shaped. You can see three separate cell layers : ◦ Outer epithelium – a single row of cuboidal cells; ◦ Inner epithelium - cuboid cells adjacent to dental papilla; ◦ Stellate reticulum -star shaped cells.They are the largest volume with large intercellular spaces. Enamel knots – are cluster of nondividing epithelial cells; Enamel cord – enamel knot is extends between the inner and outer epithelia as the enamel cord; Enamel niche - spaces between the epithelium of the dental lamina and outer enamel epithelium. 1. Dental lamina 2. Dental follicle 3. Outer enamel epithelium; 4. Stellate reticulum; 5. Inner enamel epithelium; 6. Dental papilla; 7. Enamel cord - epithelial proliferation, which seemed to divide enamel organ into two parts; 8. Enamel 9. Blood vessels; 10. Enamel niche; 11. Permanent tooth bud Enamel knots are clusters of nondividing epithelial cells; The current view is that the enamel knot represents an organizational center, which orchestrates morphogenesis. Blue dots are the boundary between epithelial and ectomesenchyme cells; Red circles and arrows indicate enamel knot the command center. The primary enamel knot is under the influence of betacatenin; The progression of a bud in the cap is done under mesenchymal incentives that activate cells from the tip of the tooth bud to form enamel knot. This is the epithelial signaling center. The enamel knot is an important regulator of tooth shape; Induction of the enamel knot is required for the transition from bud to cap; Enamel knot is cluster of nondividing epithelial cells. If the enamel knot be removed, the tooth is not developing. Enamel knot - the expression of growth factors Each tooth germ has a single primary enamel knot and enamel cord at the cap stage, and as these disappear, secondary enamel knots appear at the tip of the future cusps in molars. Localized group of cells in the enamel organ; Enamel cord be located under the outer enamel epithelium; It directs and binds to enamel knot; The function of the enamel cord is obviously associated with the enamel knot. Enamel cord Enamel knot This structure is created by the plane of section cutting through a curved lateral lamina so that mesenchyme appears to be surrounded by dental epithelium. Continued growth of the tooth germ leads to the next stage – bell stage, so called because the enamel organ comes to resemble a bell as the undersurface of the epithelial cap deepens; During this stage, the tooth crown assumes its final shape (morphodifferentiation); The cells that will be making the hard tissues of the crown (ameloblasts and odontoblasts) acquire their distinctive phenotype (histodifferentiation). At the periphery of the enamel organ the cells assume a low cuboidal shape and form the outer dental epithelium; The cells bordering on the dental papilla assume a short columnar shape – inner dental epithelium; The star-shaped cells forms stellate reticulum; In the bell stage, some epithelial cells between the inner dental epithelium and the stellate reticulum differentiate into a layer called the stratum intermedium. Represents enlargement of the overall size of the tooth germ and deepening of its under surface; Cells at the center secrete an acid mucopolysaccharide into the extracellular space between the epithelial cells covering the germ, (drawing in of the water cause enlargement of germ); A zone of stretched but interconnected cells (stellate reticulum) produced at center of the germ; Epithelial cells: ◦ Next to the papilla develop into an enamel-producing layer of cells (inner dental epithelium); ◦ Along leading edge of germ form the outer dental epithelium (dental cuticle). ◦ The transition zone between outer and inner dental epithelia forms the cervical loop. This layer cover enamel organ; The cells are cuboidal shape; They are connected to dental lamina; The function of this cell layer is to transport products from dental follicle to enamel organ and to limit the growth impulses of enamel organ. In the early bell stage they have short columnar shape; The cells become tall and columnar; The cells are located directly above the basement membrane, which separates them from the cells of the dental papilla; They have highly developed intracellular organelles; At this stage they receive nutrients from the dental papilla; Cell nucleus is large and positioned above the basement membrane. These cells are called "pre ameloblasts” The functions are: ◦ They are differentiated for protein synthesis; ◦ They are preparing for secretory function; ◦ They stimulate the underlying ectomesenchyme of the dental papilla; ◦ They produce and secrete the enamel; ◦ They determine the shape of the dental crown. The star-shaped cells are conected to each other, to the cells of the outer dental epithelium, and to the stratum intermedium by attachment plaques known as desmosomes; Their cytoplasm contains all of the usual organelles, but these are distributed sparsely; This layer provides a space for the development of enamel; It transports nutrients and incentives for the ameloblasts; In the wide intercellular space they stored the nutrients and water; There takes place transformation of the different substances. The cells of stratum intermedium are connected to each other and to the cells of the stellate reticulum and inner epithelium also by desmosomes; There are openings between the cells; These cells can to be further differentiate into the cells of the neighboring layers; This is the reason this layer is called the germinative layer. First, the dental lamina (and lateral lamina) joining the tooth germ to the oral epithelium breaks up into discret islands of epithelial cells, thus separating the developing tooth from the oral epithelium; Second, the inner dental epithelium completes its folding, making it possible to recognize the shape of the future crown pattern of the tooth. Fragmentation of the dental lamina results in the formation of discrete clusters of epithelial cells that normally degenerate, but some may persist and are given the name of epithelial pearls; These may form small cysts (eruption cysts) and delayed eruption; May give rise to odontoma; Or may be activated to form supernumerary teeth. The region where the inner and outer dental epithelium meet at the rim of the enamel organ is known as the zone of reflexion or “cervical loop”; This point is where the cells continue to divide until the tooth crown attains its full size and which, after crown formation, gives rise to epithelial component of root formation; Later epithelial cells of the cervical loop proliferate to form a double layer of cells known as Hertwig`s epithelial root sheath. Cervical loops Cervical loop Hertwig's epithelial root sheath (frequently abbreviated as "HERS") is a proliferation of epithelial cells located at the cervical loop of the enamel organ in a developing tooth. Hertwig's epithelial root sheath initiates the formation of dentin in the root of a tooth by causing the differentiation of odontoblasts from the dental papilla. The root sheath will further dictate whether the tooth will have single or multiple roots. It stimulates the ectomesenchyme of the dental papilla to differentiation; It simulates the ectomesenchyme of the dental follicle to differentiation; It contains information on the number, size and shape of the dental root. So far, only the initial development of the decidious (or primary) dentition has been described; The permanent dentition also arise from the dental lamina; The tooth germs that gives rise to the permanent incisors, canines, and premolars form as a result of further proliferative activity within the dental lamina at its deepest extremity; This increased activity leads to the formation of another tooth bud on the lingual aspect of the deciduous tooth germ, which remains dormant for some time; They have not deciduous predecessors, so their tooth germs do not originate in the same way; The dental lamina burrows posteriorly beneath the lining epithelium of the oral mucosa into the ectomesenchyme; This backward extention successively form the tooth germs of the first, second , and third molars. The cessation of mitotic division within cells of the inner dental epithelium determinise the shape of a tooth; When the tooth germ is growing rapidly during the cap to bell stage, cell division occurs throughout the inner dental epithelium; Division ceases at a particular point because the cells are beginning to differentiate and assume their eventual function of producing enamel; The point at which inner epithelial cell differentiation first occurs represents the site of future cusp development, or the growth center. The zone of cell devision is indicated by the brekened area in the inner dental epithelium, and the zone where cells have differentiated by the white area. The determination of specific tooth types at their correct positions in the jaws is referred to as patterning of the dentition; Two hypothetical models have been proposed to explain how these different shapes are determined, and evidence exist tu support both: ◦ The first is the field model; ◦ The second is the clone model. The field model proposes that the factors responsible for tooth shape reside within the ectomesenchyme in distinct but graded field for each tooth family; There is a "incisal field" responsible for dental shape of the incisors; It is concentrated in the central incisal area; The stimulation strongly reduced at the end of the field. Presumptive incisor mesenchyme Incisal field Molar field The clone model proposes that each tooth class is derived from a clone of ectomesenchymal cells programmed by epithelium to produce teeth of a given pattern; Clone theory A. The molar clon has induced the dental lamina to begin tooth development. At its posterior border the clone and dental lamina grow posteriorly by means of progress zona. B. B. When a clone reaches the critical size, a tooth bud is initiated at its center. A zone of inhibition surrounds the tooth bud. C. The next tooth bud is not initiated until the progress zone of the clone has escaped its influence. The dental papilla is separated from the enamel organ by a basement membrane; The cells of the dental papilla appear as undifferentiated ectomesenchymal cells, having an uncomplicated structures with all the usual organelles; Blood vessels appear early in the dental papilla, initially in the central region along with nerve fibers associated with these vessels; The vessels bring nutrition to the rapidly growing organ; As the papilla grows, smaller vessels are also seen in the periphery of the area, bringing nutrition to the elongating odontoblasts and ameloblasts. Dental papille The ectomesenchyme of the dental papilla forms: ◦ Preodontoblasts in the early bell stage; ◦ Odontoblasts in the late bell stage; ◦ The papilla cells are believed to be significant in furthering enamel organ bud formation into the cap and bell stage; ◦ This cell density is maintained as the enamel organ grows. Dental papilla gives rise to the dentin and dental pulp; -The dental follicle is distinguished clearly from the dental papilla in that many more collagen fibrils occupy the extracellular spaces between the follicular fibroblasts; -These generally are oriented circulary around the dental organ and dental papilla; -Clusters of blood vessels are found ramifying around the tooth germ in the dental follicle adjasent to the outer dental epithelium Dental follicle is dental sac with its enclosed developing tooth. It gives rise to three important entities: cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum; Osteoblasts form alveolar bone; Fibroblasts form periodontal ligaments which connect teeth to the alveolar bone; The dental follicle is formed partly from cells at the base of the dental papilla that flatten and migrate to enclose the whole of tooth germ. Dental follicle Dental follicle brings nutrition to the enamel organ and dental papilla; It limits their growth impulses; After stimulation of the Herwig’s epithelial root sheath dental follicle forms cement, periodontium and alveolar bone. Enamel organ: ◦ Forms enamel; ◦ Furthered dental papilla and dental follicle; Dental papilla: ◦ Forms dentin and dental pulp; Dental follicle: ◦ Forms cement, periodontium and alveolar bone.