last few

Microdontia

Unusually small teeth; more common in females, associated with hypodontia

Relative microdontia

Normal-sized teeth that appear small because the jaw/maxilla is larger than normal

Diffuse true microdontia

All teeth affected; seen in Down syndrome, pituitary dwarfism, rare hereditary disorders

Most common tooth for isolated microdontia

Maxillary lateral incisor (peg-shaped crown, normal root)

Isolated microdontia inheritance pattern

Autosomal dominant with incomplete penetrance

Macrodontia

Teeth larger than normal

Relative macrodontia

Normal-sized teeth in a small jaw; can cause crowding and impaction

Diffuse macrodontia causes

Pituitary gigantism, pineal hyperplasia with hyperinsulinism, hemifacial hypertrophy

Gemination

A single enlarged/joined tooth where tooth count is NORMAL when the anomalous tooth is counted as one

Gemination location

Anterior maxilla most common; bilateral is less frequent

Fusion

A single enlarged/joined tooth where tooth count reveals a MISSING tooth when the anomalous tooth is counted as one

Fusion location

Anterior maxilla most common; bilateral less frequent

Concrescence

Union of two adjacent teeth by cementum alone, WITHOUT confluence of underlying dentin

Concrescence common cause

Post-inflammatory, usually in large carious molars with periapical pathosis (posterior maxilla most common)

Turner's hypoplasia

Enamel defects on a permanent tooth caused by periapical inflammatory disease of the overlying deciduous tooth

Turner's tooth

The affected permanent tooth resulting from Turner's hypoplasia

Determining factors of Turner's hypoplasia severity

Stage of tooth development, length of time infection remains untreated, virulence of organism, host resistance, traumatic injury

Turner's hypoplasia most commonly affects

Permanent bicuspids (premolars), since they sit next to primary molars that get infected

Syphilitic hypoplasia

Dental changes due to congenital syphilis; currently very rare; causes enamel hypoplasia

Hutchinson's teeth

Anterior teeth shaped like straight-edge screwdrivers with a central notch on the incisal edge (from congenital syphilis)

Mulberry molars

Molars with constricted occlusal table and disorganized surface anatomy resembling a mulberry (from congenital syphilis)

Talon cusp

Well-delineated additional cusp on an anterior tooth extending at least half the distance from the CEJ to the incisal edge

Talon cusp most common location

Maxillary lateral incisor

Talon cusp genetic influence

Documented in twins

Talon cusp permanent teeth distribution

55% maxillary lateral, 33% maxillary central, 6% mandibular incisors, 4% maxillary canine

Talon cusp associated with

Supernumerary teeth, odontoma, impacted teeth, peg-shaped lateral incisors, syndromes (Mohr, Rubinstein-Taybi, Sturge-Weber)

Ankylosis

Cessation of eruption after emergence due to anatomic fusion of cementum with alveolar bone

Ankylosis cause

Unknown; may be trauma, injury, chemical/thermal irritation

Ankylosis other terms

Infraocclusion, secondary retention, submergence, reimpaction, reinclusion

Ankylosis etiology factors

Disturbed local metabolism, trauma, genetically decreased PDL gap, disturbed root resorption/repair, genetic predisposition

Ankylosis most common tooth

Primary first molar, mandible

Ankylosis X-ray finding

Absence of periodontal ligament space

Dens invaginatus (dens in dente)

Deep surface invagination of the crown or root lined by enamel; two types: coronal and radicular

Coronal dens invaginatus most common tooth

Maxillary lateral incisor (also central incisors); maxillary predominance

Dens invaginatus Type I

Confined to the crown

Dens invaginatus Type II

Extends below CEJ and ends in a blind sac

Dens invaginatus Type III

Extends through the root and perforates the apical or lateral radicular area

Radicular dens invaginatus

Rare; due to inappropriate invagination of Hertwig's epithelial root sheath with a strip of enamel along the root surface

Taurodontism

Enlargement of body/pulp chamber of a multirooted tooth with apical displacement of pulpal floor and root bifurcation ("bull teeth")

Hypotaurodontism

Mild degree of taurodontism

Mesotaurodontism

Moderate degree of taurodontism

Hypertaurodontism

Severe degree of taurodontism

Taurodontism field effect

First molar least affected, increasing severity toward second and third molars

Hypercementosis

Non-neoplastic deposition of excessive cementum continuous with normal radicular cementum

Hypercementosis common tooth

Premolars; incidence increases with age

Hypercementosis X-ray finding

Thickening/blunting of root; PDL space is maintained around area of proliferation

Dilaceration

Abnormal angulation or bend in the root (or less often the crown) of a tooth

Dilaceration etiology

Trauma to tooth germ causing displacement/formation at abnormal angle; can also be secondary to cyst, odontoma, or supernumerary tooth

Dilaceration most affected teeth

Permanent maxillary and mandibular incisors

Cell fate determinants

Cell fate depends on origin, temporal-spatial history, and is determined by genes/environment (autonomous/intrinsic and non-autonomous/extrinsic factors)

Neural crest alone (before day 12)

When cultured alone, forms cartilage and neural crest tissue but NOT teeth or bone

Mandibular epithelium alone (before day 12)

When cultured alone, forms none of teeth, bone, cartilage, or neural crest -- cannot act alone

Neural crest + mandibular (1st arch) epithelium

Recombination that produces teeth, bone, cartilage, AND neural crest -- shows epithelium is instructive

Neural crest + limb (or 2nd arch) epithelium

Produces bone, cartilage, and neural crest but NOT teeth -- shows only first arch epithelium is odontogenic

Odontogenic epithelium + skin mesenchyme (after day 12)

Forms skin, not teeth -- after day 12 epithelium alone can't direct fate

Skin epithelium + odontogenic ectomesenchyme (after day 12)

Forms a tooth -- after day 12 the ectomesenchyme directs tooth development

Key conclusion of recombination experiments

Before E12, epithelium initiates odontogenesis; after E12, epithelium loses odontogenic potential and ectomesenchyme takes over directing tooth development

Three stages of tooth morphogenesis

Initiation (dental lamina) -> Morphogenesis (bud, cap) -> Differentiation (bell)

Hox genes

Family of homeobox-containing genes first discovered in Drosophila (antennapedia); encode homeodomain transcription factors that control segment identity and specialization

Antennapedia gene

Drosophila Hox gene; mutation causes a leg to grow in place of an antenna, showing Hox genes control segment/appendage identity

Homeodomain

~60 amino acid region of a ~400 amino acid transcription factor protein; binds specific DNA sequences to turn genes on/off, can regulate hundreds of genes

Homeobox

The DNA sequence within a Hox/homeobox gene that, when translated, produces the homeodomain protein region

Synteny

The conserved co-linear order of Hox genes across species during evolution

Co-linearity of Hox genes

The spatial order of Hox genes on the chromosome matches the anterior-to-posterior order of their expression in the embryo

HOX gene clusters in mammals

HOXA, HOXB, HOXC, HOXD -- mammalian equivalent of Drosophila Hox complex, arranged in chromosomal clusters

Msx

Homeobox gene family; originally identified in muscle development in Drosophila; in mammals, expressed in incisor-forming ectomesenchyme

Dlx

Homeobox gene family; originally identified in limb development in Drosophila; combines with Msx and Barx to pattern tooth type

Barx

Homeobox gene family; originally identified in retina development in Drosophila; expressed in molar-forming ectomesenchyme

Genes required to form the dental lamina

Msx1, Msx2, Dlx2, and Barx1 expressed in the overlap region of ectomesenchyme

Incisor identity gene code

Msx-1 and Msx-2 expression in ectomesenchyme

Canine identity gene code

Msx-1, Msx-2, and Dlx-2 expression (combination of anterior and posterior signals)

Molar identity gene code

Barx-1 and Dlx-2 expression

Odontogenic homeobox code model

Concept that combinatorial (overlapping) expression of homeobox genes (Barx1, Dlx1/2, Msx1, Msx2, Alx3) in ectomesenchyme before tooth germ initiation determines tooth type and pattern

FGF8

Growth factor from epithelium that induces/promotes tooth germ formation at future tooth positions

BMP4 (epithelial role)

Growth factor from epithelium that inhibits/restricts where tooth germs can form, helping mark tooth position

Pax9

Transcription factor expressed in ectomesenchyme; activated by FGF8, inhibited by BMP4; marks position of each tooth; ectomesenchyme depends on ligands for Pax9 early but becomes independent by bud stage

SHH (Sonic Hedgehog)

Secreted growth factor from dental epithelium; SHH-soaked beads applied to oral epithelium are sufficient to cause invaginations resembling tooth buds; involved throughout development with multiple roles including a concentration gradient later on

Reciprocal signaling stages

Initiation stage = epithelial signaling; Bud stage = mesenchymal signaling; Cap stage = enamel knot signaling

Epithelial signaling (initiation stage) genes

SHH, BMP4, FGF8 acting on dental mesenchyme; activate Msx1, Pax9, Ptc, Gli1, ActbA

BMP4 expression shift

BMP4 is expressed in dental lamina epithelium initially, then switches to the ectomesenchyme during the bud stage -- this switch indicates the shift in developmental control from epithelium to mesenchyme

Enamel knot

Specific inner enamel epithelium (IEE) signaling center at the cap stage; non-dividing epithelial cells at cusp tips that express p21, organize crown/cusp morphogenesis

Ectomesenchymal BMP4 (cap stage)

Required to induce formation of the enamel knot; enamel knot cells then express BMP4 and FGF4

p21

Cell cycle regulator expressed in enamel knot cytoplasm; causes primary and secondary enamel knot cells to stop dividing (non-dividing signaling center)

Primary enamel knot

First enamel knot to form at the cap stage; induces formation of secondary enamel knots and determines cusp pattern

Secondary enamel knots

Form later (bell stage), induced by the primary enamel knot; position determines multi-cusped tooth pattern; regulate dental papilla cells and laterally regulate IEE cells

Single-cusped vs multi-cusped teeth

Determined by whether only a primary enamel knot forms (single cusp) or primary knot induces secondary knots (multi-cusp)

Enamel knot signaling factors

BMP, FGF, SHH -- knot cells signal laterally to tell neighboring IEE cells not to become knot cells and to keep dividing

Cervical loop

Region of the enamel organ at the bell/late bell stage that becomes the root

Dental lamina

Initial thickening of oral epithelium that marks where teeth will form in each jaw

Dental placode

Localized thickening of epithelium marking the start of tooth initiation, associated with SHH expression

Odontogenic ectomesenchyme

Neural crest-derived mesenchyme beneath the epithelium that becomes competent to direct tooth development

Order of tooth structure formation (late stages)

Bell stage -> late bell stage -> root formation and eruption; produces enamel, dentin, pulp, root, and surrounding jaw bone

MSX gene function/product

Msh-like gene; transcription factor; located inside the nucleus

DLX gene function/product

Distaless homologue; transcription factor; located inside the nucleus

BARX gene function/product

BarH1 homologue; transcription factor; located inside the nucleus

BMP4 function/product

Bone morphogenetic protein; growth factor; located outside the cell

SHH function/product

Sonic hedgehog; growth factor; located outside the cell

FGF function/product

Fibroblast growth factor; growth factor; located outside the cell

Number of genes identified in tooth development

More than 90 genes encoding transcription factors and growth factors (signaling molecules)

Membrane thickness

5-8 nm thick