DEVELOPMENT OF TOOTH AND TONGUE

ODONTOGENESIS INTRODUCTION PRIMARY EPITHELIAL BAND FORMATION DENTAL AND VESTIBULAR LAMINA FORMATION MORPHOLOGIC STAGING BUD STAGE CAP STAGE BELL STAGE ROOT FORMATION HISTOPHYSIOLOGICAL AND CLINICAL CONSIDERATIONS

DEVELOPMENT OF TONGUEINTRODUCTION BODY OF TONGUEBASE OF TONGUEMUSCULATURE AND CONNECTIVE TISSUEPAPILLAE AND TASTE BUDNERVE SUPPLYBLOOD SUPPLYLYMPHATICSANOMALIES AFFECTING TONGUE

INTRODUCTION

• In Greek , ODONTO means TOOTH and GENESIS means PRODUCTION i.e., the process of development of the teeth.

•Humans have 2 generations of teeth, Deciduous and Permanent.

•Complete deciduous dentition has 20 teeth, 10 in each jaw and permanent has 32 i.e; 16 in each jaw.

• Tooth development is a continuous process that occurs as a result of complex interaction between EPITHELIAL and MESENCHYMAL tissues.

Tooth development is a continuous process where each and every tooth pass though similar stages which has been divided as MORPHOLOGIC and PHYSIOLOGIC stages.

• MORPHOLOGIC STAGES Bud stage Cap stage Bell stage • PHYSIOLOGIC STAGES Initiation Proliferation Histodifferentiation Morphodifferentiation Apposition

•When embryo is 3 weeks old , at anterior end of embryo ectoderm

invaginates to meet endoderm forming buccopharyngeal membrane

which forms primitive mouth which is lined by stratified squamous

epithelium beneath which ectomesenchymal cells also called neural crest

cells are present. The interaction between these layers of cells leads to

tooth development.

• At about 27th day of gestation, primitive oral cavity (stomatodeum) is lined by stratified squamous epithelium (oral ectoderm).

• Separated from gut by a thin membrane known as buccopharyngeal membrane.

Tooth Development

• At about this time primitive oral cavity establishes communication with foregut.

• Hence the oral ectoderm , foregut endoderm and nural crest mesenchyme plays the role in tooth initiation

• The primitive oral cavity or stomatodeum at this stage consist of :

Two or three layered epithelium covering the embryonic connective tissue that is of neural crest origin and which has migrated to the jaws from the margins of the neural crest.

Primary epithelial band formation:• After about 37th days of gestation, continuous horse shoe

shaped bands are formed around mouth in presumptive upper & lower jaw by thickening and invagination of oral epithelium into mesenchyme known as primary epithelial band.

• The formation of these thickened epithelial bands is result not so much of increased proliferation activity within epithelium as of a change in orientation of mitotic spindle & cleavage plane of dividing cells.

Dental lamina & Vestibular lamina formation:• Primary epithelial band quickly divides into: Vestibular lamina or lip furrow band Dental lamina

Vestibular lamina:

• It proliferates into underlying mesenchyme.• The cells enlarge ,then degenerate centrally to form a cleft that

becomes vestibule between cheek & tooth bearing area.

• Dental lamina : • Seen lingual to the vestibular

lamina.• Rapidly forms a horse shoe

shaped band which proliferates in 10 different areas to initiate tooth germ formation

• Serves as primodium for ectodermal component of deciduous tooth.

Fate of dental lamina:

• Total activity of dental lamina extends over a period of 5 yrs. After which it begins to degenerate .

• Remnants of dental lamina

persist as epithelial pearls or islands within the jaw as well as in gingiva (Epithelial rests of serre).

STAGES OF DEVELOPMENTAs cell proliferation continues, each enamel organ increases in size & change in shape. On the basis of change in shape tooth development divided into following stages: 1. Bud Stage 2. Cap stage 3. Bell stage

STAGES OF DEVELOPMENT

BUD STAGE• Enamel organ differentiate

into round or ovoid swelling called tooth bud.

• Enamel organ at this stage consists of:

1.Peripherally located low columnar cells.

2.Centrally located polygonal cells

• Epithelium of dental lamina is separated from underlying mesenchyme by a basement membrane.

• Ectomesnchymal condensation occurs in relation to enamel organ.

• Ectomesnchymal condensation just below enamel organ is known as dental papilla. It forms future dentin & pulp.

• Ectomesnchymal condensation that surrounds tooth bud & dental papilla is known as dental sac. It forms future cementum & periodontal ligament.

Dental papilla & dental sac are not well defined in this stage.

Molecular Basis:• Msx gene expression induces ectomesnchymal condensation .• BMP4 Induces ectomesnchymal condensation . Induces Msx. Induces BMP2 & Shh which helps in transition from Bud to Cap stage.

CAP STAGE• As the tooth bud continues to proliferates, it does not expand uniformly into a large sphere. Instead tooth bud leads to the cap shape which is characterised by shallow invagination on deeper surface of the bud.

• At this stage , tooth germ consists of:• Outer Enamel epithelium• Inner enamel epithelium• Stellate Reticulum• Dental papilla • Dental sac

• Outer Enamel epithelium cover convexity of cap. These cells are cuboidal in shape. They are separated from dental sac & inner enamel epithelium from dental papilla by a delicate basement membrane.

• Inner enamel epithelium covers concavity & are columnar in shape.

• Stellate reticulum consists of polygonal cells which separate from one another as more & more intracellular fluid accumulates

• They give a cushioning consistency that may support & protect delicate enamel forming cells.

Enamel knot, Enamel cord & Enamel Niche:

• Late cap stage & very early bell stage.• Transient structures.• Not seen in every tooth germ.

Enamel knot & chord:

• The cells in the center of enamel organ are closely packed & form enamel knot.

• This knot projects in part towards underlying papilla .

• A vertical enamel knot called enamel chord.

• These structures (enamel knot & chord) represent organisation center which orchestrates cuspal morphogenesis. Also they may act as a reservoir of dividing cells for growing enamel organ.

Enamel Niche: • Apparent structure created during histological preparation due to the sheet like structure of dental lamina.

•Appears like a concavity filled with connective tissue and gives a impression of that the tooth-germ has a double attachment to the oral epithelium

Dental papilla:• Under the influence of proliferating

epithelium of enamel organ, ectomenchyme gets enclosed by invaginated portion of inner enamel epithelium & condense to form dental papilla.

• The papilla shows active budding of capillaries & mitotic figures.

Dental sac :• Formed by ectomesnchymal

condensation surrounding enamel organ & dental papilla.

• Gradually this zone becomes dense & more fibrous.

BELL STAGE• As the invagination of epithelium deepens & its margins

continue to grow ,enamel organ assumes bell shape .

• Bell stage can be further divided into

Early Bell Stage Advanced Bell Stage

• Inner enamel epithelium• Outer enamel epithelium• Stratum Intermedium• Stellate reticulum• Cervical loop or zone of reflexion• Dental Papilla• Dental Sac

Early Bell Stage:

Inner enamel epithelium:

• Consists tall columnar cells about 4 to 5 microns in diameter & about 40 microns .

• Contains nucleus away from basement membrane. Nucleus/cytoplasmic ratio is high.

• Characterised by high glycogen content.

• Attached to one another by junctional complex & to stratum intermedium by desmosomes.

• Separated from dental papilla by basement membrane. Inner enamel epithelium

Stratum Intermedium:• A few layers of sqamous cells

form stratum intermedium between inner enamel epithelium & stellate reticulum. These cells are closely attached by desmosomes & gap junctions.

• The well developed cytoplasmic organelles, acid mucopolysacharides,alkaline phosphatase & glycogen deposits indicate a high degree of metabolic activity.

• This layer seems to be essential for enamel formation.

Stratum Intermedium

Stellate reticulum

Inner enamel epithelium

Dental Papilla

Stellate reticulum :• Expand further mainly by an

increase in amount of intracellular fluid.

• Cells are star shaped& attached to one another & to outer enamel epithelium and stratum intermedium by desmosomes.

• Contains sparsely distributed organelle in cytoplasm.

Outer enamel epithelium:• Consists of low cuboidal

epithelial cells.• Supported by basement

membrane around its periphery.

• Rich glycogen and cytoplasmic organelle.

• High nuclear cytoplasmic ratio.

• Join to adjacent cells by means of Junctional complexes & to stratum reticulum by desmosmes.

Dental Papilla:• Dental papilla consists of undifferentiated mesenchymal

cells & fine scattered collagen fibrils scattered throughout extracellular space.

• Ramifying nerves & vessels are also seen.• It is separated from dental organ by a basement

membrane.

Dental Sac:• Consists of undifferentiated mesenchymal cells &

circularly arranged collagen fibrils around enamel organ & dental papilla.

• Collagen fibrils are more in dental sac than dental papilla.

• Ramifying nerves & vessels are also seen.

Cervical loop or zone of reflexion :• Consists of only outer & inner enamel epithelium.• This is the point where cells continue to divide until

tooth attains its full size & which after crown formation gives rise to epithelial component of root formation.

Advanced Bell Stage:• Separation of tooth germ from Dental Lamina.

• Hard tissue formation.

• Morphogenesis of crown.

Separation of tooth germ from Dental Lamina:

• Dental lamina joining tooth germ to oral epithelium breaks into discrete islands of epithelial cells, thus separating developing tooth germ from oral epithelium.

Hard tissue formation: • At the site of future

cusp tips, where a layer of dentin first appears, mitotic activity ceases & short columnar cells of inner enamel epithelium elongate &reverse their polarity, becoming taller with their nuclei adjacent to stratum intermedium and golgi complex facing dental papilla.

• Under organizing effect of inner enamel epithelium undifferentiated ectomesenchyme increases in size & ultimately differentiate into odontoblasts, dentin forming cells which eliminate acellular zone between dental papilla & dental epithelium. First they assume cuboidal form, later they assume a columnar form & acquire the specific potential to produce dentin matrix which ultimately mineralizes.

• Once mineralized dental papilla is known as dental pulp. As organic matrix is deposited ,the odontoblasts move towards center of papilla leaving a cytoplasmic extension around which dentin is formed.

• Shortly after dentin formation initiates, inner enamel epithelium differentiates into ameloblasts that produce organic matrix against newly formed dentinal surface which ultimately undergoes mineralization.

Before first dentin formation, enamel organ receives nourishment from two sources: 1.Blood vessels located in dental papilla 2. Vessels located along the periphery of outer enamel epithelium.

When dentin is formed there is drastic reduction in nutrients reaching enamel organ.. The demand is fulfilled by collapse of stellate reticulum & invagination of outer enamel epithelium by blood vessels outside.

Once enamel formation completes, ameloblast layer and rest of the enamel organ regresses and together constitute the reduced enamel epithelium.

Morphogenesis of crown: When tooth germ is growing rapidly during cap to bell stage ,cell division occurs throughout inner enamel epithelium. As division continues, division ceases at a particular point because cells are beginning to differentiate & assume their eventual functioning of producing enamel. The point at which inner enamel epithelium differentiation occurs first represent the site of future cusp or growth center.Because inner enamel epithelium is constrained between cervical loop & cusp tips ,continued proliferation causes the inner dental epithelium to buckle & form cuspal outline. Thus future cusp is pushed towards outer dental epithelium.

Thus inner dental epithelium completes its foldings making it possible to recognise shape of future crown pattern of tooth.Eventually differentiation of inner enamel epithelium & dental papilla seeps down & is followed by dentin & enamel formation.

• It begins after enamel & dentin formation has reached cemento enamel junction.

• The enamel organ plays important role by forming Hertwig’s epithelial root sheath.

• It is formed by proliferation of cervical loop cells .

• It consists of only inner & outer enamel epithelium.

• It molds the shape of root & initiate radicular dentin formation.

Root Formation

• When dentin is formed ,it looses its structural integrity & its close relation with root surface.

• This loss of structural integrity is as a result of invasion of surrounding connective tissue of dental sac.

• The epithelium is moved away from surface of dentin so that connective tissue cells come into contact with outer surface dentin & differentiate into cementblasts that deposite a layer of cementum onto surface of dentin.

• Remnants of Hertwig’s epithelial root sheath are found in periodontal ligament & are called rests of Malassez .

• Prior to the beginning of root formation ,epithelial root sheath forms epithelial diaphragm by bending at future cemento enamel junction into horizontal plane ,narrowing the wide cervical opening of tooth.

• Proliferation of cells of epithelial diaphragm is accompanied by ectomesenchymal cell proliferation ,adjacent to diaphragm.

• In last stages of root development ,the proliferation of epithelium in diaphragm lags behind that of pulpal connective tissue.

• Thus wide apical foramen is first reduced to width of diaphragmatic opening itself , later by apposition of dentin & cementum at the apex of root.

• In case of multirooted teeth, there is differential growth of epithelial diaphragm in the form of tongue like extensions which grow towards each other & fuse causing division of trunk into two or three roots.

Histophysiological & Clinical Considerations

1. Initiation 2. Proliferation 3. Histo differentiation 4. Morpho differentiation 5. Apposition

Initiation:• A lack of initiation results in absence of either single tooth

or multiple teeth.• Most frequently the permanent upper lateral incisor,

third molar, and lower second premolars.• Abnormal initiation may result in development of single

or multiple supernumerary teeth.

Histodifferentiation:• This phase reaches its peak in the Bell stage, just before

hard tissue formation.• In vitamin deficiency ameloblasts fail to differentiate ,as a

result of which adjacent mesenchyme fails to differentiate & a atypical dentin known as osteodentin is formed

Morphodifferentiation:• Disturbance in this phase may result in supernumerary cusps or roots or suppression of parts may be there( loss of cusps or roots) or may result in peg or malformed teeth( e.g., Hutchinson’s incisors) with normal enamel & dentin.

Apposition:• Genetic & environmental factors may disturb the normal

synthesis & secretion of organic matrix of enamel leading to condition called enamel hypoplasia.

• If organic matter is defective, then enamel or dentin is said to be hypocalcified or hypomineralised.

Histophysiological & Clinical Considerations related root formation• If the epithelial root sheath remains adherent to the dentin

surface ,they may differentiate into ameloblasts and produce enamel. Such droplets of enamel called enamel pearls are seen sometimes in furcation area of permanent molars.

• If continuity of root sheath is broken prior to dentin formation, a defect in dentinal wall results. Such defects are commonly found in furcation area. This accounts for the development of accessory root canals opening on the periodontal surface of root.

DEVELOPMENT OF TONGUE

DEVELOPMENT OF TONGUE

• Tongue arises from the ventromedial wall of the primitive oropharynx from the inner lining of the first four branchial arches.

• Development of tongue can be studied under the following headings

• Anterior 2/3rd (or the body of tongue) • Posterior 1/3rd(base of the tongue) • Musculature and connective tissue • Papillae and taste buds. • The nerve supply

Anterior 2/3rd of the tongue

• During the 4th week of IUL, paired lateral swellings appear on the internal aspect of the 1st brachial arches

• Tuberculum impar is a median eminence whose caudal end has a pit called the foremen caecum

• These lateral swellings rapidly enlarge, merge with each other and overgrow the tuberculum impar to form the body of the tongue

• A U shaped sulcus, the linguogingival groove develops in front of the oral part, which allows it to be free and highly mobile except at the region of the lingual frenum where its attached to the floor of the mouth

Posterior 1/3rd of the tongue

• The base of the tongue is initially indicated by two midline elevations that develop caudal to the foremen caecum

The copula The hypo brachial eminence• As the tongue develops, the hypobranchial eminence

overgrows the second branchial arch to become continous with the body of the tongue. The site of union b/w the the body and base of the tongue is delineated by a V shaped groove, called the SULCUS TERMINALIS

Musculature and connective tissue

• Branchial arch mesenchyme forms the connective tissue and the lymphatic and the blood vessels of the tongue

Tongue musculature• During the 5th to 7th

week of the IUL, 3-4 occipital myotomes, migrate anteriorly to form the musculature of the tongue.

Papillae of the tonguePapillae are projections of mucous membrane on the dorsum of the tongue. Circumvalate papillae Fungiform papillae Filliform papillae Foliate papillae

Taste buds• Taste buds are located on the dorsum of the tongue and are

associated with the papillae • Taste buds appear around 8th week of IUL. • Taste buds arise by inductive interaction b/w the epithelial cells and

the invading gustatory nerve cells

Nerve supply of the tongue• Development of tongue from the branchial arch explains its nerve supply

Sensory supply • Ant 2/3rd-Lingual nerve for general sensation.• chorda tympani for the special sensation • Post 1/3r Glossopharyngeal nerve • Posterior most part - vagus nerve Motor supply • All the intrinsic and extrinsic muscles except the palatoglossus are supplied by the

hypoglossal nerve. • Palatoglossus –pharyngeal plexus

Arterial supply

It is chiefly derived from the lingual artery a branch of external carotid artery. The root of the tongue is also supplied by tonsillar and ascending pharyngeal artery.

Venous drainage:

The deep lingual vein is the principal vein of the tongue.

Lymphatic drainage:

The tip of the tongue drains into submental nodes. The remaining part of the anterior 2/3rd drains into the submandibular nodes. The posterior 1/3rd of the tongue drains into jugulo-omohyoid nodes.

LYMPHATIC DRAINAGE

Structure of the tongue• The bulk of the tongue is

made of muscles.• Intrinsic muscles• Extrinsic muscles

• The ORAL PART of dorsum of the tongue has numerous papillae associated with the taste buds.

• The PHARYNGEAL PART of the dorsum of the tongue is rich in lymphoid follicles that is collectively called the lingual tonsil. Mucous glands are also present.

• The inferior surface of the tongue shows the frenulum linguae,deep lingual veins and the plica fimbriata.

• The entire tongue is in the mouth at birth .Its post. 1/3rd desends into the pharynx by the age of 4yrs. The tongue normally doubles in length,breadth and thickness from birth to adolesence reaching maximal size at about 8yrs.

ANOMILIES OF THE TONGUE• Aglossia• Microglossia • Macroglossia • Bifid tongue• Ankyloglossia• Lingual thyroid• Fissured tongue • Median rhomboid glossitis

ANKYLOGLOSSIA

LINGUAL THYROID

FISSURED TONGUE

MEDIAN RHOMBOID GLOSSITIS

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