Teeth Development and root formation

Dr asmaa s

Dentition
Primary dentition – develops during prenatal period20 teethPermanent dentition – develops as the jaw grows and matures32 teethperiod in between – during the preteen yearsmixed dentition period

THERE ARE MULTIPLE STAGES IN TOOTH DEVELOPMENT – but three major ones actually initiation stage – 6th to 7th weekbud stage – 8th week cap stage – 9th to 10th weeksbell stage – 11th to 12th weeksapposition stage – vaires per toothmaturation stage – varies per tooth

Tooth formation: Initial stages

The initiation of tooth formation starts around the 37th day of gestation.
Primary epithelial bands: Horseshoe-shaped bands that appear approximately around the 37th day of development, one for each jaw. -there are two subdivisions: vestibular lamina and dental lamina -the dental lamina develops a series of epithelial outgrowths - grow deep into the mesenchyme-develops in the future spot for the dental arches-will form the midline for these arches-arches then form posteriorly from this point-the ingrowths represent the future sites for each deciduous tooth-the vestibular lamina cells rapidly enlarge and then degenerate – forms a cleft that becomes the vestibule of the oral cavity -involves the physiologic process of induction-induction of ectodermal tissues by the developing mesenchyme-mechanisms remain unknown-at the 6th week the stomatodeum is lined with ectoderm – outer portion is theoral epithelium-this gives rise to the primary epithelial bands-also is a developing mesenchyme which contains neural crest cells that have migratedto the area-a basement membrane separates the developing oral epithelium and mesenchyme

Bud Stage

marked by the incursion of epithelium into the mesenchyme period of extensive proliferation and growth of the dental lamina forms into buds or oral masses that penetrate into the mesenchyme each tooth bud is surrounded by the mesenchyme buds + mesenchyme develop into the tooth germ and the associated tissues of the tooth this developing tooth forms from both the ectoderm and mesenchyme and from neural crest cells that have migrated into the mesenchyme
Tooth bud Oral epithelium Mesenchyme

Cap Stage

characterized by continuation of the ingrowth of the oral epithelium into the mesenchyme. tooth bud of the dental lamina proliferates unequally in different parts of the budforms a cap shaped tissue attached to the remaining dental laminalooks like a cap sitting on a ball of condensing mesenchymeoccurs for the primary dentition (during the fetal period)this stage marks the beginning of histodifferentiation (differentiation of tissues)the tooth germ also begins to take on form – start of morphodifferentiationa depression forms in the deepest part of each tooth bud and forms the cap or enamel organ (or dental organ) – produces the future enamel (ectodermal origin)below this cap is a condensing mass of mesenchyme – dental papilla – produces the future dentin and pulp tissue (mesenchymal origin)the basement membrane separating the dental organ and the dental papilla becomes the future site for the dentinoenamel junction (DEJ)remaining mesenchyme surrounds the dental/enamel organ and condenses to form the dental sac or the dental follicle


Cap stage
together the enamel organ + dental papilla + dental follicle is considered the developing tooth germ or tooth primordium these primordium will be housed in the developing dental arches and will develop into the primary dentition

Bell Stage

Continuation of histodifferentiation and morphodifferentiation cap shape then assumes a more bell-like shapedifferentiation produces four types of cells within the enamel/dental organ 1. inner enamel epithelium2. outer enamel epithelium3. stellate reticulum4. stratum intermediumduring the bell stage the dental lamina is separated from the dental organthe dental papilla undergoes differentiation and produces two types of cells1. outer cells of the DP – forms the dentin-secreting cells (odontoblasts)2. central cells of the DP – forms the primordium of the pulpdental sac increases its collagen content and differentiates at a later stage than the EO and DP

Differentiation of the Enamel/Dental organ

outer enamel epithelium (OEE) – cuboidal shapeprotective barrier during enamel productionmay also be called the outer dental epitheliumvery little cytoplasmcells are separated from the dental follicle by a basement membraneinner enamel epithelium (IEE) short, columnar cellsdifferentiates into the enamel secreting cells = ameloblastsseparated from the dental papilla below it by a basement membrane alsocells accumulate large amounts of glycogenmay also be called the inner dental epithelium the IEE and OEE are continuousregion where they connect – curved rim of the EO = cervical loopstellate reticulumstar-shaped cells in many layerscenter of the enamel organforms a network = reticulumsupports production of enamelstratum intermediuminner layer of compressed flat to cuboidal cellsvery high levels of the enzyme alkaline phosphatasesupports production of enamel cervical loop
IEE
OEE

Bell Stage

the cells in the center of the enamel organ begin to synthesize and secrete GAGs this pulls water into the EO increasing amount of fluid in the EO forces the central cells apart however, they remain connected via cellular processes which makes them star shaped = stellate ret.
B = inner dental epithelium


Bell stage – early crown formation the dental papilla is separated from the enamel organ by a basement membraneimmediately below this BM is a region called the acellular zonethis is where the first enamel proteins will be laid downthe dental lamina begins to break up into discrete islands of epithelial cells (epithelial pearls)– separates the oral epithelium from the developing tooththese pearls may form cysts and delay eruption or they may develop into supernumerary teeththe IEE completes its folding and you can begin to identify the shape of the future crown pattern

Tooth development so far

Cap and Bell stages & Permanent teeth

during the cap stage the development of the permanent dentition begins – anterior teeththe primordia for these teeth appears as an extension off the developing dental lamina penetrates into the mesenchyme lingual to the primary primordiumits site of origin is called the succesional dental laminathese permanent teeth are called succedaneous teeth (anterior teeth and the premolars)teeth that form with the primary tooth buds (primary predecessors)permanent molars are non-succedaneous - they are formed by posterior proliferation of the dental lamina.

Appositional stage

secretion of enamel, dentin and cementumthese tissues are initially secreted as a matrix that is partially calcified – serves as a framework for later calcificationtime period variesmultiple inductions occur between the ectodermal tissues of the enamel organ and the mesenchymal tissues of the dental papillae and dental sacthese inductions are crucial for the production of enamel, dentin and cementumthese interactions are mediated by the basement membrane found in between these ectodermal and mesenchymal tissues

Maturation stage

characterized by the completion of calcification

Ameloblasts and Odontoblasts

ameloblaststhe cells of the IEE assume a more columnar shape or they elongatedifferentiate into pre-ameloblaststhis differentiation is characterized by the repolarization of these PAs – movement of the nucleus away from the basement membranethis repolarization is critical to the differentiation of the PAscontinued differentiation and maturation results in the formation of ameloblaststhe pre-ABs induce the cells of the dental papilla to differentiate alsoodontoblastsdifferentiation by the mesenchyme of the dental papillaoccurs after differentiation of pre-ABs beginsresults because the pre-ABs induce differentiation of the mesenchymal cells alsoalso undergo repolarization – mirror image of the pre-ABs (see Figure 6-12 and 6-13)after differentiation – the ODs then start dentinogenesisbegin to deposit predentin on the side of their basement membrane – forms a layer immediately below the BM and above the cells (figure 6-13)therefore dentin formation begins before enamel synthesisexplains why dentin is thicker than enamel

At 1 the epithelium is separated from the dental papilla by an acellular zone. At 2 the cells of the inner dental epithelium have elongated, and the acellular zone begins to be eliminated as odontoblasts differentiate from ectomesenchymal cells in the tooth pulp. At 3 the odontoblasts retreat toward the center of the pulp, leaving behind formed dentin. At 4 the cells of the inner dental epithelium, now ameloblasts, begin to migrate outward and leave behind formed enamel. before dentin forms – cells of the EO receive blood supply from vessels of the dental laminaas dentin forms, it cuts of this papillary source of blood/nutrientsthis causes a drastic reduction in the amount of nutrients that reach the EObut the ABs require extensive nutrients to form enamel – stellate reticulum collapses and invagination of the OEE – this brings in blood supply from peripheral vessels found outside the tooth

Dentinoenamel junction

after OD differentiation and the initiation of dentinogenesis – the BM between the pre-ABs and ODs disintegratesthis allows direct contact between the pre-ABs and ODs – results in the completion of pre-AB differentiation to mature ABsABs then begin amelogenesis – apposition of enamel matrixreplaces the disintegrating BMeach ameloblast forms a tapered portion that faces the disintegrating BM - called a tome or Tome’s processupon contact of the enamel matrix and dentin – the disintegrating BM begins to mineralize – forms the dentinoenamel junction or DEJthe ODs form cellular process as they retreat toward the dental papilla (figure 6-14) that penetrate the forming predentin = dentinal tubulesmineralization of the developing dentin and enamel is distinct for each type of tissuethe cell bodies of the ODs remain in the pulp tissuethe cell bodies of the ABs participate in tooth eruption and will disappear shortly after

Timetable for tooth development

Entire primary dentition initiated between 6 and 8 weeks of embryonic development. Successional permanent teeth initiated between 20th week in utero and 10th month after birth permanent molars between 20th week in utero (first molar) and 5th year of life (third molar)


ROOT FORMATION
takes place as the crown in completely shaped and the tooth begins to erupttherefore the tooth forms from the “top down” – i.e. crown to rootroot formation is through the formation of a cervical loopthe CL is the most cervical portion of the enamel/dental organ – two layers consisting of IEE and OEEthe CL begins to grow down into the dental sac forms a Hertwig's root sheathrim of the sheath = epithelial diaphragm – encloses the developing primary alveolar foramenalso grows down to encompass all but the basal portion of the pulpthis sheath shapes the root and induces dentin formation in the root area by the ODs of the dental papillathis sheath lacks the stellate reticulum and stratum intermediumis capable of differentiating into ODs BUT NOT ABs A, The root is beginning to form as an extension of the inner and outer dental epithelia in the cervical loop region (circles) which form a bilayered structure called Hertwig’s epithelial root sheath. B. formation of dentin by odontoblasts above the root Sheath

Root Dentin

The root of the tooth is composed by dentin and cementumdentin forms when the outer cells of the dental papilla are induced to differentiation into ODssimilar to what occurs at the crown areainfluenced by the IEE of Hertwig’s root sheaththe ODs then undergo dentinogenesis and secrete predentinafter dentin formation – the BM disintegrates along with the Hertwig’s sheath

Cementum and Pulp formation

cementogenesis in the root area also occurs upon degradation of the H. root sheaththe degradation allows contact of the dental sac cells with the dentin surface – induces the formation of cementoblast cellsthe CBs cover the root dentin and undergo cementogenesis – laying down cementoidthe CBs do NOT leave cellular processes within the cementum but many CBs become entrapped in the forming cementumthese entrapped CBs are called cementocytesonly upon mineralization of the cementoid can it be called cementumthe region of contact between cementum and root dentin = dentinocemental junction or DCJwhile the cementum is forming - the central cells of the dental papilla form the pulp (Chapter 13)

Periodontal ligament

the surrounding tissues of the tooth also develop as the crown and root form the mesenchyme of the dental sac condenses to form the periodontal ligament forms adjacent to the new cementum involves synthesis of collagen and bundling into fibers ends of these fibers insert into the outer layer of cementum and surrounding alveolar bone the cells of the disintegrating H. root sheath develop into discrete islands of epithelial cells become epithelial rests of Malassez (figure 6-22) cells become located in the mature periodontal ligament no known function they can be identified in the periodontal ligament and are responsible for the development of radicular cysts.

Multirooted teeth

anterior teeth, premolars and molars all begin as a single root – root trunkroot of the posterior teeth divides from the trunk into the correct number of root branchesdifferential growth of the H. root sheath results in the division of the root trunk into two or three roots

Primary tooth eruption

the epithelium remains attached to the tooth at the cervical portion i.e. as the tooth pierces the oral epithelium, the cells of the reduced dental epithelium and the oral epithelium form the initial junctional epithelium (thin dotted line) this will eventually form the dentogingival junction

References

Ten Cate's Oral Histology Human Oral Embryology and Histology. Reviewed by Mark W. J. Ferguson Permar's Oral Embryology and Microscopic Anatomy by Rudy C Melfi Articles from internet