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Tooth formation occurs in the 6th week of intrauterinelife with the formation of primary epithelial band. Atabout 7th week the primary epithelial band divides intoa lingual process called dental lamina & a buccal processcalled vestibular lamina. All deciduous teeth arises fromdental lamina, later the permanent successors arisefrom its lingual extension & permanent molars from itsdistal extension
• The primitive oral cavity, or stomodeum, is lined by stratified squamousepithelium called the oral ectoderm• The oral ectoderm contacts the endoderm of the foregut to form thebuccopharyngeal membrane
• Membrane ruptures at about 27th day of gestation and the primitive oralcavity establishes a connection with the foregut• Most of the connective tissue cells underlying the oral ectoderm are ofneural crest or ectomesenchyme in origin 3= Connective tissue cells• These cells instruct the overlying ectoderm to start the tooth development,which begins in the anterior portion of the future maxilla & mandible andproceeds posteriorly
• 2- 3 weeks after the rupture of buccopharyngeal membrane, certain areas ofbasal cells of oral ectoderm proliferate rapidly, leading to the formation ofprimary epithelial band• The band invades the underlying ectomesenchyme along each of the horse-shoe shaped future dental arches.
• At about 7th week the primary epithelial band divides into an inner (lingual)process called Dental Lamina & an outer ( buccal) process called VestibularLamina • The dental lamina serves as the primordium for the ectodermal portion of the deciduous teeth • Later during the development of jaws, permanent molars arise directly from the distal extension of the dental lamina
• The successors of the deciduous teeth develop from a lingual extension of thefree end of the dental lamina opposite to the enamel organ of each deciduousteeth.
The lingual extension of the dental lamina is named the successional lamina &develops from the 5th month in utero ( permanent central incisor) to the 10thmonth of age (second premolar)
FATE OF DENTAL LAMINA• It is evident that total activity of dental lamina exceeds over a period of atleast 5 yrs• As the teeth continue to develop, they loose their connection with the dental lamina•They later break up by mesenchymal invasion, which is at first incomplete and does notperforate the total thickness of the lamina
• Fragmentation of the dental lamina progresses toward the developing enamelorgan• Any particular portion of the dental lamina functions for a much briefer periodsince only a relatively short time elapses after initiation of tooth developmentbefore the dental lamina begins to degenerate• However the dental lamina may still be active in the third molar region after ithas disappeared elsewhere, except for occasional epithelial remnants
ANODONTIA• Anodontia, also called anodontia vera, isa rare genetic disorder characterized bythe congenital absence ofall primary or permanent teeth• It is of following types1. Complete anodontia/ total anodontia2. Partial anodontia/ sub-Total anodontia COMPLETE• Forms-1. True anodontia2. Psuedo anodontia3. False anodontia PARTIAL
SUPERNUMERARY TEETHHyperdontia is the condition ofhaving supernumerary teeth, or teeth whichappear in addition to the regular number ofteethSupernumerary teeth can be classified byshape and by position. The shapes include:• Supplemental(where the tooth has anormal shape for the teeth in that series);• Tuberculate (also called "barrel shaped");• Conical (also called "peg shaped");• Compound odontome (multiple smalltooth-like forms);• Complex odontome (a disorganized mass ofdental tissue)When classified by position, a supernumerary toothmay be referred to as a mesiodens, a paramolar, or adistomolar.
VESTIBULAR LAMINA• Labial and buccal to the dental lamina in each dental arch, another epithelialthickening develops independently• It is Vestibular Lamina also termed as lip furrow band• Subsequently hollows and form the oral vestibule between the alveolar portion ofthe jaws and the lips and cheeks.
• At certain points along the dental lamina each representing the location of one of the10 mandibular & 10 maxillary teeth, ectodermal cells multiply rapidly & little knobsthat grow into the underlying mesenchyme• Each of these little down growths from the dental lamina represents the beginning ofthe enamel organ of the tooth bud of a deciduous tooth• First to appear are those of anterior mandibular region• As the cell proliferation occurs each enamel organ takes a shape that resembles a cap
DENTAL PAPILLAOn the inside of the cap, the ectomesenchymal cells increase in number. Thetissue appears more dense than the surrounding mesenchyme and representsthe beginning of the dental papilla B = Dental Papilla
DENTAL SAC/ DENTAL FOLLICLESurrounding the combined enamel organ or dental papilla, the third part of the toothbud forms. It is known as dental sac/follicle and it consists of ectomesenchymal cells andfibres that surrounds the dental papilla and the enamel organ. C= Dental sac
• Thus the tooth germ consists of ectodermalcomponent- the enamel organ, theectomesenchymal components- the dental papilla& the dental follicle• The enamel is formed from the enamel organ,the dentin and the pulp from the dental papillaand the supporting tissues namely the cementum,periodontal ligament & the alveolar bone from thedental follicle• During & after these developments the shape ofthe enamel organ continues to change• The depression occupied by the dental papilladeepens until the enamel organ assumes a shaperesembling a bell• The dental lamina becomes longer, thinner &finally loses its connection with the epithelium ofthe primitive oral cavity
MORPHOLOGICAL PHYSIOLOGICAL1. Dental lamina Initiation2. Bud stage3. Cap stage Proliferation4. Early bell stage Histodifferentiation5. Advanced bell stage Morphodifferentiation6. Formation of enamel and dentin matrix Apposition
BUD STAGE / PROLIFERATION• This is the initial stage of tooth formationwhere enamel organ resembles a small bud• During the bud stage, the enamel organconsists of peripherally located low columnarcells & centrally located polygonal cells• The surrounding mesenchymal cellsproliferate, which results in theircondensation in two areas• The area of condensation immediatelybelow the enamel organ is the dental papilla• The ectomesenchymal condensation thatsurrounds the tooth bud & the dental papillais the tooth sac
• The dental papilla as well as the dental sac are not well defined during the bud stage,they become more defined during the subsequent cap & bell stages• The cells of the dental papilla form the dentin and pulp while the dental sac formscementum & periodontal ligament
CAP STAGE / PROLIFERATION• As the tooth bud continues to proliferate, it does not expand uniformly into alarge sphere• Instead unequal growth in different parts of the tooth bud leads to the capstage which is characterized by a shallow invagination on the deep surface of thebud
OUTER & INNER ENAMEL EPITHELIUM• The peripheral cells of the cap stage arecuboidal , cover the convexity of the cap &are called the outer enamel epithelium• The cells in the concavity of the capbecome tall columnar cells & represent theinner enamel epithelium
•The outer enamel epithelium is separated from thedental sac, & the inner enamel epithelium from thedental papilla, by a delicate basement membrane
STELLATE RETICULUM• Polygonal cells located between the outer and the inner enamel epithelium, begin toseparate due to water being drawn into the enamel organ from the surrounding dentalpapilla• As a result the polygonal cells become star shaped but maintain contact with eachother by their cytoplasmic process• As the star shaped cells form a cellular network, they are called the stellate reticulum
• The cells in the center of the enamel organ are densely packed and form theenamel knot• This knot projects toward the underlying dental papilla
• At the same time a vertical extension of the enamel knot,called the enamel cord occurs
• The function of enamel knot & cordmay act as a reservoir of the dividingcells for the growing enamel organ• The enamel knot act as a signalingcenters as many important growthfactors are expressed by the cells ofthe enamel knot & thus play animportant role in determining theshape of the tooth• The ectomesenchymalcondensation i.e the dental papilla &the dental sac are pronouncedduring this stage of dentaldevelopment
BELL STAGE / HISTODIFFERENTIATION• Due to continued uneven growth of theenamel organ it acquires a bell shape• In bell stage crown shape is determined• It was thought that the shape of the crownis due to pressure exerted by the growingdental papilla cells on the inner enamelepithelium• This pressure however was shown to beopposed equally by the pressure exerted byfluid present in the stellate reticulum• The folding of enamel organ to causedifferent crown shapes is shown to be due todifferent rates of mitosis & difference in celldifferentiation time
INNER ENAMEL EPITHELIUM• The inner enamel epithelium consists of a single layer of cells that differentiate prior toamelogenesis into tall columnar cells called ameloblasts• These elongated cells are attached to one another by junctional complexes laterally &to cells in the stratum intermedium by desmosomes• The cells of the inner enamel epithelium exert a strong influence on the underlyingmesenchymal cells of the dental papilla, which later differentiate into odontoblasts
STRATUM INTERMEDIUM• A few layers of squamous cells form the stratum intermedium , between the innerenamel epithelium & the stellate reticulum• These cells are closely attached by desmosomes & gap junctions• This layer seems to be essential to enamel formation
STELLATE RETICULUM• The stellate reticulum expands further due to continued accumulation of intra-cellularfluid• These star shaped cells, having a large processes anastomose with those of adjacentcells• As the enamel formation starts., the Stellate reticulum collapses to a narrow zonethereby reducing the distance between the outer & inner enamel epithelium
OUTER ENAMEL EPITHELIUM• The cells of the outer enamel epithelium flatten to form low cuboidal cells• The outer enamel epithelium is thrown into folds which are rich in capillary network,this provides a source of nutrition for the enamel organ• Before the inner enamel epithelium begins to produce enamel. Peripheral cells ofthe dental papilla differentiate into odontoblasts• These cuboidal cells later assumes a columnar form & produce dentin
DENTAL LAMINA• Dental lamina is seem to extend lingually and is termed successional dental laminaas it gives rise to enamel organs of permanent successors of deciduous teeth• The enamel organs of deciduous teeth in the bell stage show successional lamina &their permanent successor teeth in the bud stage
DENTAL SAC• The dental sac exhibits a circulararrangement of fibres & resembles acapsule around the enamel organ• The fibres of the dental sac formthe periodontal ligament fibres thatspan between the root & the bone• The junction between the innerenamel epithelium & odontoblastsoutlines the future dentino-enameljunction
DENTINOGENESIS IMPERFECTA• Dentinogenesis imperfecta (hereditaryOpalescent Dentin) is a geneticdisorder of tooth development.• This condition causes teeth to bediscolored (most often a blue-gray oryellow-brown color) and translucent. Teethare also weaker than normal, making themprone to rapid wear, breakage, and loss.• These problems can affect both primary(baby) teeth and permanent teeth.• This condition is inherited inan autosomal dominant pattern, whichmeans one copy of the altered gene ineach cell is sufficient to cause the disorder.
ADVANCED BELL STAGE / MORPHODIFFERENTIATION Characterized by the commencementof mineralization & root formation The boundary between the innerenamel epithelium & odontoblastsoutline the future dentinoenameljunction Formation of dentin occurs first as alayer along the future dentinoenameljunction in the region of future cusps &proceeds pulpally & apically After the first layer of dentin is formed,the ameloblasts lay down enamel overthe dentin in the future incisal & cuspalareas
The enamel formation then proceedscoronally & cervically in all the regionsfrom the dentinoenamel junction towardthe surface The cervical portion of enamel organgives rise to Hertwig Epithelial RootSheath (HERS) This HERS outlines the future root &thus responsible for the size, shape ,length & number of roots
FUSION• The phenomenon of toothfusion arises through union of twonormally separated tooth germs, anddepending upon the stage ofdevelopment of the teeth at the time ofunion, it may be either complete orincomplete.• However, fusion can also be the unionof a normal tooth bud to asupernumerary tooth germ. In thesecases, the number of teeth is fewer ifthe anomalous tooth is counted as onetooth.
GEMINATIONGemination arises when twoteeth develop from one toothbud and, as a result, thepatient has an extra tooth
FORMATION OF ENAMEL & DENTIN MATIX ( APPOSITION)• Apposition is the deposition of the matrix of the hard enamelstructures• Appositional growth of the enamel & dentin is a layer likedeposition of an extracellular matrix. This type of growth istherefore additive• Appositional growth is characterised by regular & rhythmicdeposition of the extracellular matrix, which is of itself incapable offurther growth
ENAMEL HYPOPLASIAEnamel hypoplasia is thedefect of the teeth in whichthe tooth enamel is hard butthin and deficient in amountThis is caused by defectiveenamel matrix formationwith a deficiency in thecementing substance
AMELOGENESIS IMPERFECTA• Amelogenesis imperfecta presents withabnormal formation of the enamel orexternal layer of teeth. Enamel is composedmostly of mineral, that is formed andregulated by the proteins in it. Amelogenesisimperfecta is due to the malfunction of theproteins in the enamel:ameloblastin, enamelin, tuftelin, amelogenin• People afflicted with amelogenesisimperfecta have teeth with abnormal color:yellow, brown or grey. The teeth have ahigher risk for dental cavities and arehypersensitive to temperature changes. Thisdisorder can afflict many number of teeth.
Dens- In- Dente ( DENS INVAGINATUS)• Represents a defect of tooth in which afocal area on the tooth surface is folded orinvaginated pulpally to a variable extent• Defect in generally localized to a singletooth & interestingly maxillary lateralincisors are more commonly affected• Bilateral involvement is often seen &sometimes defect can involve multiple teethinvolving the supernumeraries• In case of pulp involvement with orwithout apical pathology, endodontictreatment should be attempted. However inmore severe form extraction should be done
DENS EVAGINATUS• Dens evaginatus is a condition foundin teeth where the outer surface appearsto form an extra bump or cusp.• Premolars are more likely to beaffected than any other tooth. This maybe seen more frequently in Asians• The pulp of the tooth may extend intothe dens evaginatus.• There is a risk of the dens evaginatuschipping off in normal function• Hence this condition requiresmonitoring as the tooth can lose its bloodand nerve supply as a result and mayneed root canal treatment.
TALON CUSP• A talon cusp, also known as an"eagles talon", is an extra cusp on ananterior tooth.• Of all cases, 55% occur on thepermanent maxillary lateral incisor,and 33% occur on thepermanent maxillary central incisor.They are found rarely in primary teeth• Whenever the lingual pits arepresent restorative treatments shouldbe done to prevent caries• When talon cusp interferes withnormal occlusion preventive careshould be taken by performingendodontic treatment
• The development of roots begin afterenamel & dentin formation has reachedthe future cementoenamel junction• The enamel organ plays an important rolein root development by forming HERS,which models the shape of the root• HERS consists of outer & inner enamelepithelium only• As the first layer of the dentin has beenlaid down, the epithelial root sheath losesits structural continuity and is closerelation to the surface of the root
•Its remnants persists as anepithelial network of strands orclumps near the external surface ofthe root• These epithelial remnants arefound in the periodontal ligamentof erupted teeth and are called asrests of mallasez
• Prior to the beginning of root formation,the root sheath forms the epithelialdiaphragm• The outer & the inner enamel epitheliumbend at the future cementoenamel junctioninto a horizontal plane, narrowing the widecervical opening• The proliferation of the cells of theepithelial diaphragm is accompanied by theproliferation of the cells of the connectivetissues of the pulp, adjacent to thediaphragm• The free end of diaphragm does not growinto the connective tissue but theepithelium proliferates coronal to theepithelial diaphragm
• Connective tissue of the dental sacsurrounding the root sheath proliferates& invades the continuous doubleepithelial layer dividing it into network ofepithelial strands• The rapid sequence of proliferation &destruction of Hertwig’s root sheathexplains the fact that it cannot be seen asa continuous layer on the surface ofdeveloping root• In the last stages of the rootdevelopment, the proliferation of theepithelium in the diaphragm lags behindthat of the pulpal connective tissue• The wide apical foramen is reduced firstto the width of the diaphragmaticopening itself & later is further narrowedby opposition of dentin & cementum tothe apex of the root
• Differential growth of the epithelialdiaphragm in the multirooted teeth causesthe division of root trunk into 2 or 3 roots• During the general growth of enamelorgan, expansion of its cervical openingoccurs in such a way that long tongue likeextensions of the horizontal diaphragmdevelop• Before division of the root trunk occurs,free ends of the horizontal epithelial flapsgrow towards each other & fuse• The single cervical opening is divided into2 or 3 openings
• On the pulpal surface ofthe dividing epithelialbridges, dentin formationstarts • On the periphery of each opening, root development follows in the same way as described for single rooted teeth
DILACERATION• Dilaceration refers to an angulation or asharp bend or curve anywhere along theroot portion of a tooth• Condition probably occurs subsequent totrauma or any other defect of developmentwhich alters the angulation of the toothgerm during root formation• Can easily be detected by radiographs• Care should be taken during extractionsince these teeth are more prone to fracture
CONCRESCENCEConcrescence is a conditionof teeth wherethe cementum overlying the rootsof at least two teeth join together.The cause can sometimes beattributed to trauma or crowding ofteeth.Radiographic diagnosis ismandatory before attempting toothextraction
CONCLUSIONSince development of tooth forms the base ofdentistry, a thorough understanding and a soundknowledge is required by a dentist regarding thedevelopment stages of tooth & the anomalies relatedto it, so as to identify & treat them in a proper fashion.
REFERENCES• Orban’s, Textbook of oral histology & embryology• Ten cates, Textbook of oral histology• Shaefer’s. Textbook of oral pathology• Avery JK: Embryology of the tooth. J Dent Res 30:490,1951• Avery JK: Primary induction of tooth formation. JDent Res33:702,1954