L3 Flashcards
22 pairs of
autosomes
Germ cells (gametes) =
spermatozoa and ova
Osteogenesis imperfecta (OI) =
Hereditary disease affecting bones and teeth
Mutations in COL1A1 (type I collagen)
Autosomal dominant = 1 mutant allele is enough to cause OI phenotype
AD: Usually shows up in every generation, 50% chance to show up in offspring
Supernumerary incisor
Autosomal recessive = Need 2 mutant alleles to change phenotype
AR: Two “carriers” necessary to contribute 2 mutant alleles to offspring
AR: Does not show up every generation, beware of consanguineous unions
Amelogenesis imperfecta (AI)
Mutations in AMELX gene on X chromosome
X-linked
Induction: The process that initiates
differentiation.
An inducer is an agent that provides cells with a signal to differentiate
BUT, cell must be competent to receive the signal
Competence: Ability of cell to receive and respond to a
molecular signal
Receptors
Internal machinery
Signaling affects…
Cell differentiation
Cell proliferation: Dividing into more cells
Cell migration: Moving to a new location
Transcription factors (TFs):
Controlling gene expression
Proteins that control whether genes will be transcribed into mRNA (to be translated into proteins)
One TF can control expression of dozens, hundreds of genes
TFs can activate or repress expression of target genes
DLX3: Distal-less Homeobox 3
214 amino acid TF in DLX family
Mutations in DLX3 lead to TDO syndrome
Tricho-dento-osseous (TDO) syndrome
(Hair-teeth-bone)
TDO syndrome
DLX3 regulates…
Hair follicle differentiation
BMP signaling
Enamel genes
Amelogenin
Enamelin
Kallikrein 4
Bone
Formation
Resorption
Homeostasis
Fetus
8-40 weeks
0 to 4 weeks Mostly
proliferation and migration, some differentiation
4-8 weeks
Cell differentiation, formation of major external and internal structures (morphogenesis)
Blastocyst stage
Embryoblast= Inner cell mass (ICM)
ICM forms ALL tissues of the embryo = embryonic stem cells
Other cells = trophoblast layer
Bilaminar (2 layer) embryo
2 germ layers: primary cell layers
Ectoderm dorsal
Endoderm ventral
Ectodermal cells converge toward the
midline to form the primitive streak
Ectodermal cells migrate through the
streak between ectoderm and endoderm
New mesoderm layer
Gastrulation: Conversion to
trilaminar embryo (3rd week)
Gastrulation
New
mesoderm layer separates ectoderm and endoderm (3rd germ layer)
Cephalic (more rostral) migrating cells form
notochord to support the embryo
Buccopharyngeal membrane remains
bilayer of ectoderm and endoderm – NO mesoderm
Weeks 4-8
Differentiation
Major structures
Rostro-caudal (front-back) folding –
Happening during week 4
One consequence of rostral-caudal folding – a !
MOUTH
Stomatodeum =
primitive oral cavity
Buccopharyngeal membrane defines most
rostral boundary of primitive gut
Buccopharyngeal membrane =
Bilayer of ectoderm and endoderm
Breaks down to allow stomatodeum to communicate with foregut
Cranial neural crest cells (NCCs)
Ectoderm in origin
Cranial neural crest cells (NCCs): Adjacent to
neural tube – separate from neural plate when neural tube closes (~ day 22/end of 3rd week)
Cranial neural crest cells (NCCs): Capacity to
migrate and differentiate extensively
Cranial neural crest cells (NCCs): Induced to undergo
epithelial-mesenchymal transformation- they are referred to as ectomesenchymal cells
Cranial neural crest cells (NCCs): Acting as
mesenchyme, NCC ectomesenchymal cells form most of the connective tissues of the head, including teeth and their supportive tissues
Some craniofacial bones come from
ectomesenchyme
During brain development, the neural tube expands as
forebrain, midbrain, hindbrain
Hindbrain forms
8 rhombomeres = bulges
Rhombomeres define the origins of
distinct populations of NCCs
NCCs from midbrain and rhombomeres 1, 2 contribute to
branchial arch 1
Contribute to embryonic connective tissue for craniofacial development
First stream = Face
Second stream = First branchial arch
NCCs from rhombomeres ≥ 3 express
Hox TFs
Hox TF genes =
ancient rostral-caudal patterning genes that define body segments
Craniofacial region is a more
recent evolutionary structure- developed new set of TFs (examples in figure)
NCCs that migrate to the face and first branchial arch are
Hox-free
All transcription factors include a
DNA-binding domain that allows them to interact with genes
Homeobox TFs include a specific
~180 base pair “homeobox” with specific sequence
Hox TFs are a subset of
Homeobox genes that were discovered to be very ancient body patterning directors
HOM-C Homeobox gene
Controls formation of
legs during development
Loss-of-function: Legs → Antennae (loss of body patterning)
Gain-of-function (ectopic expression) : Antennae → Legs
Similar to legs exchanging with arms or molars with incisors
What patterning TFs direct craniofacial development?
Otx2: Orthodenticle homeobox 2
Msx: Muscle segment homeobox (MSX1-3)
Dlx: Distal-less homeobox (DLX1-7)
Barx: BarH-like homeobox (BARX1-2)
NCCs contribute to the
mesoderm in the branchial arches
Arch =
External “bumps” (internally, a mix of NCC and mesoderm)
Groove/cleft = Separate arches externally
Pouch = Internal depression
(recall stomatodeum was bilayer of
ectoderm and endoderm– as a result oral cavity is lined with ectoderm while rest of digestive tract is lined with endoderm)
Dlx homeobox TFs
Family Dlx1-7 in humans How to understand functions? Dlx1/2 mutant mice show altered craniofacial morphology lack of maxillary molars Dlx1/2 affects proximal BA1 (maxillary process)
Treacher-Collins syndrome aka Mandibulofacial dysostosis
Underdevelopment of craniofacial region and mandible
Failure/impairment of NCC migration to the facial region
Mutations in TCOF1, POLR1C, or POLR1D (functions still unknown)
Epithelial-mesenchymal interactions drive
tooth formation
Conserved mechanism for many ectodermal organs
For example, hair, mammary gland, feather
Placode – bud – morphogenesis
Both adjacent tissues are essential for the organ to form properly
Molecular signaling during crown development:
Epithelial-mesenchymal (E-M) signaling
Reciprocal: E-M signals must continue back and forth to push tooth formation forward
Reiterative: Cells use same signaling pathways again and again at different stages (BMP, FGF, SHH, WNT, TNF)
Sequential: Orderly sequence of events determine whether cells are ready to receive the signal and how they respond
Primary epithelial band – dental lamina
Proliferation of epithelial cells
Altered orientation of axis of division
Formation of dental lamina (=odontogenic bands) and 20 dental placodes (= local thickening of ectoderm for primary teeth)
Which tissue holds the odontogenic potential?
Both actually…
** First epithelium, then mesenchyme
If you combine odontogenic epithelium with 2nd BA mesenchyme, together they
still form a tooth
If you combine molar mesenchyme with incisor epithelium, they form a
molariform tooth
Tooth type determination (crown pattern)
Still not well understood
Epithelial signals (BMP4, FGF8) induce mesenchymal expression of homeobox TFs (**again, epithelium is first to initiate)
Mesenchymal homeobox TF expression in overlapping domains that direct tooth shape formation
E.g. Incisors directed by Msx1, Msx2, and Alx3 and molars guided by Barx1, Dlx1 and 2
Down growth of dental lamina into a
bud
Up growth of mesenchyme packing into condensation
Cap stage
Epithelial growth forms concave, cap-like structure
Condensed ectomesenchyme = dental papilla
Surrounding ectomesenchyme = dental follicle
Further separated in next stages…
Signaling in the Cap stage: (
Primary) Enamel Knot
Primary enamel knot
Non-dividing enamel organ cells in cap stage
Expresses numerous signaling molecules (FGF4 above, right)
Directs proliferation of surrounding epithelial cells
Essential for bud to cap transition by regulating cap morphology
Disappears by apoptosis (directed cell death)
Transitioning to Bell stage
Enamel organ has 4 components Outer enamel epithelium (OEE) Inner enamel epithelium (IEE) Stratum intermedium (SI) Stellate reticulum (SR)
Late Bell stage
Signaling in the Bell stage:
(Secondary) Enamel Knot(s)
Secondary enamel knots
Non-dividing enamel organ cells in bell stage, appearing at sites of cusps (NOT in incisors)
Express signaling molecules (FGF4 above, right)
Direct proliferation of surrounding epithelial cells → IEE completes folding, determines number and location of cusps
Stimulates terminal differentiation of odontoblasts to begin dentinogenesis (always begins at cusp tips)
Late Bell stage
Zone of maturation occurs
Dental papilla → → → Odontoblasts
IEE → → → Ameloblasts
Crown patterning
Anodontia:
Absence of all primary or secondary teeth; tooth agenesis
Oliogodontia:
6 or more missing teeth
Hypodontia:
1-5 missing teeth
Hyperdontia:
More than the normal numbers of teeth (supernumerary teeth)
Oligodontia: PAX9
Autosomal dominant mutations in PAX9 transcription factor
PAX9 expressed in dental mesenchyme early in development
Most affected individuals missing maxillary and mandibular 2nd and 3rd molars
Incisors much less affected
Hyperdontia: RUNX2
Autosomal dominant mutations in RUNX2 transcription factor
RUNX2 expressed in dental mesenchyme
Supernumerary primary teeth
Why extra teeth? RUNX2 probably negatively regulates tooth-initiating signal like Wnt…
Wnt signaling and tooth initiation
Constitutively active Wnt signaling in mouse
Supernumerary teeth
Ectopic enamel knots
Numerous and malformed teeth
Pluripotent: Ability to differentiate to all
3 germ layers
Divide indefinitely
Pros: Ability to make any tissue, repair any defect
Cons: How to control induction? Teratoma tumor formation, ethical concerns
Adult (postnatal) stem cells
Multipotent: Ability to differentiate within limits
Divide asymmetrically (not indefinitely)
Pros: Ability to make several types of tissues, easier to control induction
Cons: Limited differentiation and division, quality/quantity decrease with age
Induced pluripotent stem (iPS) cells
Reprogram adult somatic (body) cells or postnatal stem cells
Provide signals/induction factors
Pros: Pluripotent cells made from adults, patient-matched ES cells, combine the best of embryonic and postnatal stem cells
Cons: Reprogramming slow, inefficient, and expensive, controlling induction to specific tissues may be challenging
Dental stem cells?
Several populations of adult/postnatal stem cells
Dental pulp, Periodontal ligament (PDL), dental follicle, apical papilla
Potential for tooth repair or engineering (or other tissues)
Successes with dental stem cells
Creating tooth buds or repairing tooth structure in animal models
Treating other conditions like multiple sclerosis (MS) or spinal cord injury
Limitations with dental stem cells
Size and shape of engineered teeth poorly controlled
How to integrate an engineered tooth into the jaw?
Human teeth take months-years to form
A mutation in gene FAM83H causes a hereditary condition where enamel is brown soft and rough. 3 gen of families display this in both male and females
Genotype, phenotype, heritable, recording a pedigree can help determine what this is genetically.
When cell signaling is targeted to nearby adjacent cells, it is called
paracrine
What are the proteins that regulate mRNA expression of other genes
transcription factors
TDO syndrom features defects in hair, tooth, bone dev, because mutations deactivate DLX3, a TF that has multiple functions in these organs.
T
During 4th week of dev, rostral-caudal folding of the trilaminar embryo is an essential process in…
formation of the stomodeum
Transition from bilaminar to trilaminar embryo in week 3 of embryo is called
gastrulation
Which statement about the oral cavity is false
Buccopharyngeal membrane is a trilayer
Neural crest cells are derived from
Ectoderm
Which one of these statements about the midbrain is true
Neural crest cells from this region will migrate to branchial arch 1.
Choose the answer to the correct chronological sequence of events (earliest to latest)
Blastocyst, gastrulation, cells migrate to branchial arch 1
Hox genes are expressed by neural crest cells that migrate to
branchial arch 2
What types of cells signal back and forth to direct tooth formation
mesenchymal and epithelial
Odontogenic potential lies first with the —– layer, but is later transferred to the —– layer
epithelial, mesenchymal
signaling cell centers in the bell stage that control cusp formation is/are called
secondary enamel knots
During week 4 of embryo, rostral caudal folding is an essential process in
stomodeum formation
Dental stem cells are
multipotent
