WEEK THREE - ODONTOGENESIS Flashcards
list the stages of odontogenesis
- initiation 6-7wk in utero
- bud stage 8th wk
- cap stage 9-10th wk
- proliferations + beginning of histodifferentiation - bell stage 11-12th wk
- completes histodifferentiation + morphodifferentiation - crown stage
- cytodifferentiation + apposition + maturation - root formation
- eruption
briefly describe what happens during initiation
6-7th wk in utero = thickened band of epitheliam forms [primary epithelial band] from ectoderm lining
- 2 horshoe shaped bands - one for each arch
each band = 2 subdivisions
1. vestibular lamina
2. dental lamina
dental lamina
- epithelial outgrowths at site where future deciduous teeth will be [20 enlargement sites appear] –> dental placode
- mesenchymal cell accumulation around outgrowth
describe what happens during bud stage
- 8th wk in utero
- localised growth of dental lamina –> ectoderm [rounded bud shape] that penetrates ectomesenchyme below
- proliferation of ectomesenchyme cells around base of bud
describe what happens during cap stage
- 9-10th wk in utero
- recognised by beginning of cell proliferation and histogenesis [cell differentiation]
forms:
enamel organ
formation of tooth bud in cap shape w deep central depression = future enamel
dental papilla
within concavity of enamel organ = condensed mass of ectomesenchyme = future dentin and pulp
dental sac around sides of enamel organ = condensed mass of ectomesenchyme = future cemetum, PDL, alveolar bone
enamel knots determine cusp morphology
describe what happens during bell stage
-11-12 wk in utero
- regonised by
futher proliferation + histogenesis
- external enamel epithelial EEE [cuboidal cells]
- stellate reticulum [star shaped called w processes attached tgt]
- stratum intermedium [flattened cell over IEE]
- internal enamel spithelium IEE [columnar - elongates near cusp tips]
- outer + central cells of dental papilla
- morphogenesis [changes shape to give tooth appearances]
- dental lamina breakdown
stratum intermediate + IEE = tooth enamel formation
what is apposition in crown stage
enamel, dentine, cemetum = initially secreted as matrix in layers [partially calcified extracellular substance
IEE cells diffenritate –> preameloblasts –> undergo repolarisation
–> stims odontoblastic differentiation from mesenchymal cells in dental papilla –> also repolarises
- odontoblasts secrete dentine matrix [predentine] = starts dentinogensis
basement membrane between preameloblasts + odontoblasts disintegrates
- preameloblasts contact w newly formed dentine
- triggers amelogenesis
- enamel matrix secreted from Tomes Process
- mineralisation of disintegrating basement membrane = forms DEJ
cells become striated or have ruffled border [demonstrates that ameloblasts function changes from production to transportation
enamel organ loses connection w oral epithelium due to dental lamina breakdown
what is maturation in crown stage
mineralisation of matrix
describe dentinogenesis
odontoblast cells secrete predentine matrix
TWO PHASES
1. collagen matrix formed [predentine]
2. dentine formed
- deposition of calcium phosphate crystals in matrix
- crystals grow/spread/coalesce until matix fully calcified
- mineral density ^
as new predentine is deposited daily, the previous predentine calcifies –> dentine
- only the newly formed predentine along pulp border is uncalcified
describe amelogenesis
- commences at cusps as ameloblasts lay incremental deposition of enamel matrix to the surface
- small Ca2 phosphate crystals from enamel organ deposited –> matrix begins mineralisation
- initial depo of crystals = 25% of enamel mineral content and maturation = 70%
- maturation = crystals grow // after maturation = amelobalsts secrete organic cuticle on enamel surface and attaches via hemidesmosomes
- ameloblasts shorten/contract – > join EEE = reduced enamel epithelium which remains until eruption
describe root formation
- occurs AFTER crown completion and eruption commencement
- where EEE + IEE cells meet = **cervical loop **
- loop elongates away from crown to enclose more dental papilla tx –> forms Hertwig’s root sheaths [HERS] aka epithelial root sheath
- IEE cells –> odontoblasts induce dentine formation in root + determines shape
- HERs disintegrates =creating epithelial rests of malassez [ERM]
describe cementum formation
HERs disintegrates =creating epithelial rests of malassez [ERM]
- allows ectomesenchyme cells of dental follicle contact w dentin –> differentiation into cementoblasts that secrete cementoid –> calcifies into cementum
Describe the development of Multirooted Teeth
- during elongation of cervical loop –> long, tongue flaps develop dividing loop into 2/3 openings
- on pulpal floors - disintegration of HERS = induction of odontoblasts= deposition of cementoid –> calcification to cementum
Describe PDL formation
Dental follicle cells differentiate into fibroblasts → form collagen fibres
Fibers embed in cementum and other end embeds in alveolar bone
Fiber bundles increase in density as teeth erupt into functional occlusion
describe what happens during the pre-eruptive phase
- movements related to tooth eruption begin before crown formation
- incl all movements of primary/perm crowns from early initiation and finished crown completion
- phase = finished with early root formation
describe what happens during the pre-functional phase
Root formation/elongation needs SPACE → space provided by commencement of UPWARDS tooth movement [eruption]
- Cementum, dentin, root development
Movement = incisally/ occlusally through bony crypts/foramina
Eruption pathway → tissue is removed by macrophages + bone is remodelled by osteoclasts + osteoblasts
REE proliferates upwards - oral epithelium proliferates downwards = cells fuse
Crown tip penetrates fused epithelial layers - still covered by cuticle layer
- Occlusal movement until contact w opposing crown
Deciduous roots are resorped to make way for permanent eruption sequence
