Early tooth development:-
Teeth are formed from:-
Oral epithelium (dental lamina) which forms the enamel.
Ectomesenchyme (neural crest) which form the dentin, pulp, cementum and periodontal ligament.
At 6 weeks, a condensation of connective tissues occurs to form the primary epithelial band (horse shoe shape) that form upper and lower jaws.
At 35 days of development, epithelial tissues form ectomesenchymal tissues.
Note:-
Ectoderm is epithelial tissues.
Mesoderm is connective tissues.
Ectomesenchymal cell is ectoderm in origin and mesoderm in its behavior.
The role of neural crest:-
The neural tube formed by invagination of the overlying cells.
The neural crest cells migrate from the dorsal midline region of the neural tube.
In exiting from neural crest cells invade the developing branchial arches.
In a series of reciprocal inductive interactions with early epithelium, form tooth promodia (undeveloped tooth).
Dental ectomesenchyme formed from neural crest:-
From: the posterior midbrain.
To: lesser extent from the anterior hindbrain.
Failure of neural crest ectomesenchymal cells to migrate normally to their sites during craniofacial development leads to serious developmental defects, including the absence of teeth (anodontia) and underdeveloped jaw bones (micrognathia).
Subsets of cranial neural cells give rise to:-
Chndrocytes which give condyles.
Esteoblasts which give bones.
Periodontal ligament fibroblasts which give periodontal ligaments.
Cementoblasts which give cementum.
Odontoblasts which give dentin.
Final phenotype differentiation regulated by interaction of the ectomesenchymal cells with extrinsic factors (growth factors and substrate adhesion molecules in the oral microenvironment.
The molecular code for each tooth type appears to reside in specific sets of homeobox genes.
Ectomesenchyme may be forth germ layer, merging of ectoderm and mesoderm cells.
Primary epithelial band:-
At 6 week intrauterine (i.u), embryo stomodeum lined by primitive two or three cells thick layers of epithelium covering embryonic connective tissue.
It has been suggested that the zone of epithelial thickening contains the genetic determinants for the initiating signals that regulates the number and position of the future tooth bud.
Primary epithelial band proliferates to:-
Vestibular lamina.
Dental lamina.
Vestibular lamina:-
It is a wedge of epithelial cells facial to dental lamina.
It proliferates into ectomesenchyme. Then degenerates at the center to form cleft which becomes the oral vestibules.
Dental lamina:-
Formation:-
After later stage of development and in the region of future alveolar process of each jaw, the epithelium of the primary epithelial band invaginated into the underlying mesenchyme to form dental lamina.
It is also horse shoe shaped band that transverse the circumference of the lower and upper jaws.
This process begins in the distal (molar) region and lateral in the midline.
Then the dental lamina shows signs of additional differential growth reflecting the determination of incisor, canine and molar domains.
Deep notches in the dental lamina are present between the incisor and canine domains especially mandible.
Continued site specific enlargement of crest ectomesenchyme gives rise to the individual tooth buds.
Dental lamina proliferates to form general lamina which give rise to:-
Lateral lamina: forms germs of primary teeth.
Successional lamina: forms germs of permanent premolar and anterior teeth.
Mother (parent) lamina: forms germs of permanent molars.
Rudimentary lamina: it is the remnants of epithelial cells at the connective tissue around as a result of atrophy and disintegration of the dental lamina after does its work.
These epithelial cell remnants are called epithelial pearls or gland of serres which may cause supernumerary teeth or cysts or tumors.
Clinical application:-
Absence of teeth (partial or total) "anodontia" due to factors that disrupt tooth development at the initial stage (dental lamina or bud stage).
The most common missing teeth are maxillary and mandibular third molars.
Canine are least likely to be missing.
Factors affecting toothy development:-
Role of Homeobox genes:-
Homeobox genes are widely expressed in embryonic craniofacial tissues.
Once the tooth buds are formed, the homeobox genes are activated in a more localized pattern.
It was suggested that homeobox genes take part in regulating the overall growth of the tooth development.
Growth factors:-
Various growth factors are excharged between the epithelium and ectomesenchyme during the enamel organ development.
Proteins:-
Excharged between the epithelium and ectomesenchyme during the enamel organ development.
Vitamin A and its metabolic derivatives:-
Essential regulators of epithelial cells proliferation and differentiation.
Have special impact on tooth development.
Stages of tooth development:-
Teeth are formed from:-
Oral epithelium (dental lamina) which forms the enamel.
Ectomesenchyme (neural crest) which form the dentin, pulp, cementum and periodontal ligament.
At 6 weeks, a condensation of connective tissues occurs to form the primary epithelial band (horse shoe shape) that form upper and lower jaws.
At 35 days of development, epithelial tissues form ectomesenchymal tissues.
Note:-
Ectoderm is epithelial tissues.
Mesoderm is connective tissues.
Ectomesenchymal cell is ectoderm in origin and mesoderm in its behavior.
The role of neural crest:-
The neural tube formed by invagination of the overlying cells.
The neural crest cells migrate from the dorsal midline region of the neural tube.
In exiting from neural crest cells invade the developing branchial arches.
In a series of reciprocal inductive interactions with early epithelium, form tooth promodia (undeveloped tooth).
Dental ectomesenchyme formed from neural crest:-
From: the posterior midbrain.
To: lesser extent from the anterior hindbrain.
Failure of neural crest ectomesenchymal cells to migrate normally to their sites during craniofacial development leads to serious developmental defects, including the absence of teeth (anodontia) and underdeveloped jaw bones (micrognathia).
Subsets of cranial neural cells give rise to:-
Chndrocytes which give condyles.
Esteoblasts which give bones.
Periodontal ligament fibroblasts which give periodontal ligaments.
Cementoblasts which give cementum.
Odontoblasts which give dentin.
Final phenotype differentiation regulated by interaction of the ectomesenchymal cells with extrinsic factors (growth factors and substrate adhesion molecules in the oral microenvironment.
The molecular code for each tooth type appears to reside in specific sets of homeobox genes.
Ectomesenchyme may be forth germ layer, merging of ectoderm and mesoderm cells.
Primary epithelial band:-
At 6 week intrauterine (i.u), embryo stomodeum lined by primitive two or three cells thick layers of epithelium covering embryonic connective tissue.
It has been suggested that the zone of epithelial thickening contains the genetic determinants for the initiating signals that regulates the number and position of the future tooth bud.
Primary epithelial band proliferates to:-
Vestibular lamina.
Dental lamina.
Vestibular lamina:-
It is a wedge of epithelial cells facial to dental lamina.
It proliferates into ectomesenchyme. Then degenerates at the center to form cleft which becomes the oral vestibules.
Dental lamina:-
Formation:-
After later stage of development and in the region of future alveolar process of each jaw, the epithelium of the primary epithelial band invaginated into the underlying mesenchyme to form dental lamina.
It is also horse shoe shaped band that transverse the circumference of the lower and upper jaws.
This process begins in the distal (molar) region and lateral in the midline.
Then the dental lamina shows signs of additional differential growth reflecting the determination of incisor, canine and molar domains.
Deep notches in the dental lamina are present between the incisor and canine domains especially mandible.
Continued site specific enlargement of crest ectomesenchyme gives rise to the individual tooth buds.
Dental lamina proliferates to form general lamina which give rise to:-
Lateral lamina: forms germs of primary teeth.
Successional lamina: forms germs of permanent premolar and anterior teeth.
Mother (parent) lamina: forms germs of permanent molars.
Rudimentary lamina: it is the remnants of epithelial cells at the connective tissue around as a result of atrophy and disintegration of the dental lamina after does its work.
These epithelial cell remnants are called epithelial pearls or gland of serres which may cause supernumerary teeth or cysts or tumors.
Clinical application:-
Absence of teeth (partial or total) "anodontia" due to factors that disrupt tooth development at the initial stage (dental lamina or bud stage).
The most common missing teeth are maxillary and mandibular third molars.
Canine are least likely to be missing.
Factors affecting toothy development:-
Role of Homeobox genes:-
Homeobox genes are widely expressed in embryonic craniofacial tissues.
Once the tooth buds are formed, the homeobox genes are activated in a more localized pattern.
It was suggested that homeobox genes take part in regulating the overall growth of the tooth development.
Growth factors:-
Various growth factors are excharged between the epithelium and ectomesenchyme during the enamel organ development.
Proteins:-
Excharged between the epithelium and ectomesenchyme during the enamel organ development.
Vitamin A and its metabolic derivatives:-
Essential regulators of epithelial cells proliferation and differentiation.
Have special impact on tooth development.
Stages of tooth development:-
Bud stage.
Cap stage (early, late).
Bell stage (early, late).
Cap stage (early, late).
Bell stage (early, late).
Bud stage (proliferated stage):-
At the region of future tooth, ectomesenchymal tissue group collected around localized growth of the epithelial cells of dental lamina formed infront mesenchymal group (20 groups refer to primary teeth).
The bud is formed of embryonic cells to compensate the condensation occurred to ectomesenchymal tissues.
Cap stage:-
Early cap stage:-
The inner most cells of bud produce acid mucopolysaccharide into intercellular space.
This will make water putted into intercellular space.
The cells become star shaped because of the pressure of water between cells on their walls and stellate reticulum cells are formed.
This increase pressure on the periphery of the bud and becomes like cap.
The inner cell become low columnar called inner epithelium or inner dental epithelium.
The outer cells become flat and called outer dental epithelium or outer enamel epithelium.
The ectomesenchymal tissue becomes condensed in the concave surface and called dental papilla (much cells, low fibers).
Some ectomesenchymal fibers surround the cap called dental follicle (sac) (low cells, much fibers).
Enamel knot may be formed due to localized thickening at the inner dental epithelium at center.
Strands of epithelial cells also may be formed and called enamel cord or enamel septum.
Enamel knot:-
Enamel knot cells produce fibroblast growth factor 4, which is a potent stimulator of epithelial and mesenchymal cell proliferation.
It is a potential regulatory center of cell proliferation involved in cusp formation.
In multicusped teeth, secondary enamel knots are formed over the tips of the future cusps.
Late cap stage:-
A fourth layer of epithelial cells is formed between the inner dental epithelium and stellate reticulum called stratum intermedium.
It is formed of two layers of rounded cells which have high activity of alkaline phosphatase enzyme that responsible for enamel formation.
Cervical loop:-
It is the most peripheral areas in both sides of cap (the proliferation zone) the outer and inner enamel epithelial cells come together with minimal inclusion of the stratum intermedium and stellate reticulum.
After crown formation cervical loop cells proliferate to form bilayer of cells (outer and inner enamel epithelium) called root sheath of Hertwig's.
Bell stage:-
Early bell stage:-
The inner dental epithelium cells elongate to form ameloblasts which induce the formation of Odontoblasts.
Stratum intermedium cells become spindle shape connected to ameloblasts by desmosomes and gap junctions. They help in mineralization of enamel.
Stellate reticulum is star shape with desmosomes attachment and large extracellular space in between filled with proteoglycanes.
Function of proteoglycanes:-
Provide a protective cushion for developing crown.
Provide space for the movement of ameloblasts during enamel formation.
Late bell stage:-
Odontoblasts form the first layer of dentine. That lead to cut off the source of nutrition from dental papilla to ameloblasts. Ameloblasts and other cells get their need from nutrition and oxygen from capillary plexus of the dental sac.
Ameloblasts form the first layer of enamel from cusp area then spread laterally till the cervical area.
The dental papilla becomes pulp organ (fibroblasts, delicate fibers, scattered collagen fibers, undifferentiated mesenchyme, nerves, few large blood vessels at center and smaller in periphery).
رابط تحميل شرائح الـTooth Development
حمل من هنا
http://http://www.filefactory.com/file/a18bh79/n/Tooth_Development.rar
ليست هناك تعليقات:
إرسال تعليق