1019DOH - Applied Oral Biology (ORAL HISTOLOGY)

hello! this is for the real ones lmao:) please study these unshuffled! they're meant to be in sequence. we got this YEAHH

what is the stomodeum?

the small distance between the heart and head in an embryo of 5-6 weeks in utero

stomodeum alternative name

primitive oral cavity

3 germ layers of the 5-6 WIU embryo (WIU = weeks in utero)

1. ectoderm

2. mesoderm

3. endoderm

ectoderm germ layer of the embryo

• external lining of the body and oral cavity later on

• epithelial in origin

• anything we can touch (eg. eyes, hair, nails, skin) is this

mesoderm germ layer of the embryo

• connective tissue in origin (eg. blood, bone, cartilage, muscle)

• anything inbetween guts and epithelium is this

endoderm germ layer of the embryo

• lines internal organs (guts)

• not supposed to be exposed to the external environment

• anything related to the GI (gastrointestinal) system is this

• starts from pharynx and travels down

all 3 ____ ______ are needed for an embryo to develop

germ layers

what are the different embryonic origins of the tissues found in the oral cavity?

• enamel) external layer, epithelial in origin ∴ ectoderm

• dentin, pulp, cementum, PDL, alveolar bone) connective tissue ∴ mesoderm

• NO ENDODERM (no guts in teeth lol)

what 2 layers form the ectoderm? (think microscopically)

what does the ectoderm lay above?

• a layer of superficial flat cells

• a layer of lower columnar cells

• ectoderm rests on a basement membrane and then a lower layer of connective tissue (mesoderm)

what does it mean if the basement membrane breaks?

• early signs of malignancy

• mixture of ectoderm and mesoderm is very bad (except in some embryological events)

from both ectoderm and mesoderm, the ____1___ (derived from mesoderm) is formed, which all __2___ and ___ ___3______ are formed from.

1. mesenchyme

2. teeth

3. oral structures

the development of ______ ____ ______ is the only development that precedes tooth development

neural crest cells

when do neural crest cells appear….

• during embyrological development?

• physically on the embryo?

• during CNS development (CNS = central nervous system)

• at the top of neural grooves (at the crest of the groove)

neural crest cell properties

(note: neural crest cells will be referred to as NCC’s from now on)

1. ectodermal in origin

2. ability to migrate to other tissues

3. can differentiate into other cell types

4. can send signals to neighbouring tissues and tissues they migrate to

why are NCC’s important?

• act as a leader during embryological development

• have different abilities (eg, ectodermal in origin like nervous tissue and can move and migrate like stem cells)

• when they move, they send signals to migrated to and surrounding tissues

NCC’s migrate from the __1__ surface of the embryo to the ___2____, where they reside in the ____3____ to change it to the _____4______.

1. dorsal

2. stomodeum

3. mesenchyme

4. ectomesenchyme

after NCC’s migrate to the mesenchyme, where will they reside?

below the basement membrane

when NCC’s migrate to the mesenchyme, why does the name change from mesenchyme to ectomesenchyme?

• NCC’s are ectodermal in origin - ‘ecto’ in ectomesenchyme refers to this

• mesoderm (mesenchyme) is pure connective tissue

• but because the NCC’s have travelled there it isn’t pure connective tissue anymore

• this is why the name changes

ectomesenchyme is needed to develop a tooth. why?

• enamel is ectodermal in origin

• PDL, alveolar bone, periodontal ligament, cementum, pulp and dentin are all mesodermal (connective tissue) origin

• ectomesenchyme is both ectoderm and mesoderm which is why it is needed to develop all above structures

once NCC’s reside in the ectomesenchyme, what do they do and what does this affect?

• start sending signals to migrated to and surrounding tissues

• signals therefore affect not only the mesenchyme but the ectoderm on top too

the embryo is flattened ____-_______

dorso-ventrally

what are branchial arches?

• six finger-like projections that extend from the ectoderm of the foregut to the outer ectodermal lining

• lined by ectoderm on outside, endoderm on inside and mesoderm inbetween

• ONLY FIRST BRANCHIAL ARCH LINED BY ECTODERM ON THE INSIDE AND OUTSIDE

• contain all information for developing organs

why is the first branchial arch lined by ectoderm on the inside and outside?

• because the first branchial arch develops into all structures in the head

• endoderm is for guts, and the head doesn’t need it

• this is why its lined by ectoderm on the inside and outside

what do the 2nd, 3rd/4th, 5th/6th branchial arches contain the information for?

• 2nd: neck

• 3rd/4th: thorax

• 5th/6th: anything below (guts/lower limbs)

what are clefts in branchial arches? what are they lined by?

• depressions between branchial arches from the outside

• lined by ectoderm

what are pouches in branchial arches? what are they lined by?

• depressions between branchial arches form the inside

• lined by endoderm (except between 1st and 2nd arches where lined by ectoderm - no need for endoderm in the whole head area including the oral cavity)

flip card over for transverse section of a branchial arch

6 derivatives of the first branchial arch (mandibular arch)

1. mandible and maxilla

2. lower lip, part of upper lip and cheek

3. all teeth

4. all salivary glands

5. all masticatory muscles

6. anterior 2/3rds of the tongue

all structures derived from the same branchial arch will share the same…

nerve and blood supply

how many branchial arches (and therefore, how many nerve supplies) form the tongue?

• 3 branchial arches form the tongue

• therefore, there are 3 nerve supplies

structures from the first branchial arch are supplied by the ____1____ nerve or a branch of this nerve, therefore, the first branchial arch can also be called the _______2_ ____

1. mandibular

2. mandibular arch

the first branchial arch is directly below the…

stomodeum (primitive oral cavity)

the anterior 2/3rds of the tongue is formed from the first branchial arch. which nerve is its sensory nerve supply?

• the lingual nerve (branch of mandibular nerve)

• what is Meckel’s cartilage?

• runs from where to where

• guides development of…

• remnants will leave how may ligaments and bones behind?

• Meckel’s cartilage is the cartilage within the mesodermal part (centre) of the mandibular arch

• runs from the middle ear to the mandible ( ∴ follows shape and structure of mandible)

• guides development of mandible but doesn’t contribute that much to it (can still develop a mandible without Meckel’s cartilage)

• remnants will leave 2 ligaments and 4 bones behind

list the structures derived from Meckel’s cartilage remnants

1. incus and malleus

2. lingula of the mandible (spiky bit on inner side of mandible - guards entrance of mandibular foramen)

3. mental ossicles (at midline, aka genial tubercles, are 4 little bones on lingual side of mandible)

4. sphenomandibular ligament

fill this in:

1st branchial arch (_____1____ arch) → ____2__ nerve (or branch of)

2nd branchial arch (__3___ arch) → __4___ nerve (or branch of)

1. mandibular

2. mandibular

3. hyoid

4. facial

name the stages in tooth development in order

1. dental lamina

2. bud

3. cap

4. bell

5. eruption

describe the formation of dental lamina

(this is step 1 of tooth development)

• note: phrase highlighted in yellow is something you need to include when you start any stage of tooth development (will appear from this card onward when describing tooth formation steps, so keep an eye out 👀)

• under influence of the NCC’s, downward proliferation of ectoderm forms two dental laminae

• dental laminae appear as band-like structures

• NCC’s do this by sitting in the mesenchymal area and signalling to the ectoderm to proliferate downwards

• one lamina carries the upper deciduous teeth

• the other lamina carries the lower deciduous teeth

describe the bud stage

(this is step 2 of tooth development)

• under influence of the NCC’s, 10 rounded projections/buds (enamel organs) appear on the labial surface of the dental lamina (corresponding to 10 deciduous teeth!)

• each bud is surrounded by a mesenchymal condensation (which is the dental papilla and the dental sac)

• each lamina (from step 1) have 10 tooth buds each

the enamel organ in the bud stage of tooth formation is the __1_dermal part, and eventually forms the ___2___

1. ecto

2. enamel ;)

in the bud stage, the dental papilla (mesenchymal condensation) eventually forms what under the influence of the NCC’s?

the dental papilla forms dentin and pulp of the tooth

the dental sac in the bud stage is ________/________ in origin, and forms what 3 structures to connect the tooth to the outside environment?

• mesenchymal/mesodermal

• cementum, PDL and alveolar bone

the tooth germ is formed of three parts. what are they? what are their dermal origins?

• enamel organ (ectoderm in origin)

• dental papilla (mesodermal in origin)

• dental sac (mesodermal in origin)

what is the difference between the enamel organ and the tooth germ?

• the enamel organ is 1 of 3 parts forming the tooth germ

• the tooth germ is all 3 parts

which tissue in the tooth germ (bud stage) is ectodermal in origin?

• the enamel (derived from the enamel organ)

describe the early cap stage

(this is step 3 of tooth development)

• note: there’s 2 parts to this stage and this is the first. the second part of this stage will be called the ‘late cap stage’

• under influence of NCC’s, the enamel organ of the bud stage undergoes unequal growth to change in shape from a bud to a cap

• enamel organ during this stage has its inner surface lined by columnar cells, called inner enamel epithelium or the inner dental epithelium

• the outer surface is lined by cuboidal cells and is called the outer enamel epithelium or the outer dental epithelium

• inner and outer dental epithelium meet at the cervical loop

what structures form above and below the cervical loop?

• above the loop: crown of the tooth

• below the loop: root/s of the tooth

true or false?

the inner and outer epithelium have the same functions as each other (cap stage)

false

• they have different functions

in the cap stage of tooth formation, are the tissues hard or soft?

• all tissues are soft tissue in this stage

what is morphal differentiation? (cap stage)

• the convexity of the outer surface and the overall morphological change to a cap shape

what is histo-differentiation? (cap stage)

• in bud stage, you have 2 layers of ectoderm (flat and columnar cells)

• but in cap stage, there are more layers. the inner surface is lined by different cells than the outer surface

in the early cap stage, which surface grows at a lower rate?

• the inner surface grows at a slightly lower rate than the outer surface

describe the late cap stage

(this is step 3.2 of tooth development)

• note: this is the second and final part of the cap stage

• polyhedral cells in the middle of the enamel organ change into star-shaped cells called stellate reticulum, due to secretion of a hydrophilic substance called glucosaminoglycans (GAG’s) produced by polyhedral cells

• lots of cytoplasm/tissue fluids will be attracted to the polyhedral cells

• with no escape route for the fluid, cells change shape to star-like forms (stellate reticulum cells) and form long processes to occupy space and prevent bursting.

• desmosomes connect stellate reticulum cells and form a tight network between them

• note: the enamel organ is still surrounded by dental papilla and dental sac-like in the bud stage

3 functions of the stellate reticulum cells (cap stage)

1. resist against physical trauma (as filled with tissue fluid/cytoplasm)

2. help enamel mineralisation/maturation

3. distribution of calcium to newly formed enamel

list and describe the 2 transient structures in the cap stage

• note: these structures appear in this stage only!

1. enamel knot and enamel cord

   • rounded cells that connect inner and outer enamel epithelium

   • KNOT) thought to determine the future site of the cusp/incisal ridge

   • CORD) may give rise to another type of cells called stratum intermedium

1. cell free zone

   • thin area between enamel organ and dental papilla

   • separates dental papilla from inner surface of enamel organ

   • helps cells perform their function

   • elongate during early and late bell stage

enamel knot and cord and stratum intermedium cells (late cap stage and transient structures)

• the enamel knot and cord will eventually disappear (as they are only present in the cap stage)

• after the enamel knot and cord pull the inner enamel epithelium to the future site of the cusp/ridge, they will leave remnants

• these remnants are called stratum intermedium cells

• these cells appear rounded and flat

• they appear on top and work with the inner enamel epithelium and help it perform its function

flip card over for a visualiser of the cap stage

describe the early bell stage

(this is step 4 of tooth development)

• note: there’s 2 parts to this stage and this is the first. the second part will be called the ‘late bell stage’

• under influence of NCC’s (by giving signals), the IEE align themselves on the future dentino-enamel junction (or, cervical line)

• this is so the shape and type of the tooth can be determined during this stage

• lingual to the enamel organ of the deciduous tooth, a downward proliferation from the main dental lamina appears, called successional dental lamina, which carries the permanent successor

• all changes that occur here are morphal differentiation

clinical significance of the early bell stage

• sometimes kids will have 2 rows of teeth

• this happens because the permanent tooth has erupted and the deciduous one hasn’t been shed yet

• as a clinician, you should remove the dec. tooth and keep the permanent one

• the succession of the dental lamina will always be LINGUAL to the deciduous one.

• if there are 2 rows of teeth, which should you pull, the labial or the lingual one?

  • always pull the labial one

  • the permanent erupts lingual to the deciduous (dec in front)

  • you can also look for mamelon presence, do an x-ray or look for size difference to tell as well

• this phenomenon occurs more often on the lower jaw around the incisor area

where does histo-differentiation (histological change) take place in during the early bell stage?

• in the cells of the enamel organ

describe the histological changes that take place during the early bell stage

1. the inner enamel epithelium becomes longer

   • rearrangement of the organelles take place - called repolarisation

   • these cells are now called preameloblasts (which are ready for enamel matrix formation)

   • the IEE elongate toward the cell-free zone, leading to its disappearance, so…

   • preameloblasts come into contact with the peripheral layer of the dental papilla (lots of signalling molecules as ectomesenchyme has inducive properties), leading to the induction of odontoblasts

2. the odontoblasts start forming dentin matrix

   • once they create the first few layers, they realise that the preameloblasts haven’t done anything (no enamel matrix is formed)

   • so they start reciprocal induction to ‘wake up’ the preameloblasts again

3. stratum intermedium

   • present 2-3 layers above the preameloblasts

   • arise from enamel knot and cord (remnants of transient structures)

4. preameloblasts

   • rich in alkaline phosphatase

     • this is an enzyme that helps preserve mineralised tissue and stop reabsorption

_______ formation is always ahead of enamel formation

dentin

what is the reverse of functional polarity?

• also known as repolarisation

• certain organelles move toward the upper pole/end of the cell while others move downwards

• the nucleus and mitochondria move to the top of the cell

• while the other organelles move to the bottom of the cell, close to the basement membrane

a '-blast’ cell is a _______ cell

a ‘-clast’ cell is a _______ cell

• formative

• destructive

describe the physical characteristics of the cell types in the early bell stage (OEE, stellate, DP and DS)

• outer enamel epithelium - cuboidal cells (like in cap stage) but shorter

• stellate reticulum - same as cap stage but longer processes and smaller cell body

• dental papilla - same as cap stage but cell-free zone disappears after elongation of IEE

• dental sac - same as cap stage

which two cell types are important for secreting alkaline phosphatase, which is necessary for enamel maturation/calcification?

1. stratum intermedium

2. stellate reticulum

what are the 3 differences between the early and late bell stage?

1. original/main dental lamina has degenerated in the late bell stage

2. there are 2 types of cells in the dental lamina in the early bell stage. in the late bell stage, epithelial rests of serres or serres pearls appear

   • they mostly do nothing

   • in some cases under certain stimulation they multiply and form an eruption cyst

   • the epithelial rests are remnants of the dental lamina

3. teeth start developing at the cusps/incisal edges and go down. in the late bell stage, you can see the first few drops of the dentin matrix.

   • even one drop of dentin matrix changes the early bell stage into the late bell stage

what is the clinical significance of dentin formation being ahead of enamel formation?

• if there is a developmental abnormality (eg dentinogenesis imperfecta), then there is a high chance that the enamel will be affected to (amelogenesis imperfecta)

• but not vice versa! the stimulus that caused the imperfection may have started during enamel formation period and not dentin, so therefore will ONLY affect the enamel

describe what happens in the late bell stage

(this is step 4 of tooth development)

• note: this is the second and last step of the bell stage

• the deposition of the 1st layer of the hard dental surface occurs - dentin

• the enamel organ loses its connection to the dental lamina

• the remnants of the dental lamina are called epithelial rests of serres, or serres pearls

• inner enamel epithelium - preameloblasts elongate more to form ameloblasts through reciprocal induction

• the stratum intermedium is the same as in the early bell stage

• the outer enamel epithelium becomes flat and its plasma membrane becomes folded

  • this is to increase the surface area available for obtaining nutrition from the dental sac

  • nutrition from the dental papilla cannot be obtained after deposition of the hard dental structures (enamel + dentin)

  • as the OEE is epithelial in origin (avascular), it relies on underlying connective tissue for nutrients (ectomesenchyme/mesoderm)

  • some small blood vessels/capillaries are present here

• dental papilla + dental sac are the same as in the early bell stage

what is the clinical significance of serres pearls?

they sometimes can produce a pathology/cyst called an eruption cyst

where and when does root formation start?

• starts after complete formation of the crown

  • i.e) when enamel + dentin formation reaches the future CEJ

• starts at the cervical loop

what is the epithelial diaphragm?

• a horizontal proliferation formed of IEE and OEE

• leads to a primary close of the apex (no calcification yet)

• secondary closure takes place through deposition of dentin and cementum at the apex

what is the epithelial root sheath of Hertwig (ERSH)?

• a vertical proliferation formed of IEE and OEE that forms the length and shape of the root

• a proliferating zone of dental papilla occurs with epithelial diaphragm and ERSH

the peripheral layer of dental papilla is differentiated into ________. these start formation of dentin, and when this occurs, ____ is degenerated.

• odontoblasts

• ERSH

what are epithelial rests of malassez?

• the remnants of ERSH after dentin formation occurs

what happens after the ERSH is degenerated?

• dentin is exposed to the dental sac, so UMC’s (undifferentiated mesenchymal cells) differentiate into…

  • cementoblasts (lay down cementum matrix)

  • fibroblasts (form periodontal ligaments)

  • osteoblasts (form alveolar bone matrix)

in most cases the epithelial rests of malassez will do nothing, but under certain circumstances they form…

a pathology/cyst called a periodontal cyst

in case of bi-rooted teeth, _ tongue-like projections develop at the level of bifurcation

in case of tri-rooted teeth, _ tongue-like projections develop at the level of bifurcation

• 2

• 3

what are accessory root canals?

• caused by early degeneration of ERSH before odontoblasts differentiate

• can also form as a result of:

  • blood vessels interrupting the course of ERSH

  • failure of fusion of tongue-like projections in multi-rooted teeth (more common)

what are enamel pearls?

• caused by late degeneration of ERSH after complete formation of the whole thickness of dentin

• leads to enamel formation on the root - an enamel pearl

• can also occur in localised places - doesn’t have to be the whole root

• IEE will be in contact with dentin for a very long time and will pursue the process of crown formation. why?

  • IEE will become preameloblasts, then ameloblasts, then enamel along that area

• very common on cervical 1/3 of root and around bi/trifurcation area

• will appear glowing on an x-ray

• when scaling may initially seem like some stubborn calculus under the gingival margin

what is intermediate cementum?

• a tissue that has intermediate properties between dentin and cementum

• occurs due to degeneration of ERSH after odontoblasts differentiate and before complete thickness of dentin is formed.

  • (should degenerate after dentin is formed not before)

• dentin and cementum are not meant to be forming at the same time during root formation

• seen as an anatomical variation

what is dilaceration?

• describes teeth with severe curvature of the root

• occurs due to trauma of a deciduous tooth pushing the developing tooth germ and bending the ERSH

• can happen after the tooth has erupted because roots aren’t fully developed yet

• if extracting, will need to break tooth and remove parts separately

• root canal - instruments don’t bend severely so will be difficult to do

• will need x-rays before procedure

what are the physical properties of enamel?

1. colour

2. thickness

3. hardness

4. brittleness

5. permeability

describe physical property 1 of enamel (colour)

• yellowish/greyish white (not supposed to be stark white)

• canines being darker than incisors is normal

• in some teeth, the cervical 1/3 of the tooth is darker than the incisal 1/3 which is more translucent

• the darker the enamel the stronger they are (more calcium)

• whitening teeth weakens enamel

describe physical property 2 of enamel (thickness)

• 2-2.5mm at cusp tips/incisal edges (stress bearing areas) - relatively thin

• becomes thinner to end as a feather/knife edge on the cervical line

• enamel is the highest mineralised/calcified tissue in the human body

• structure of enamel based on mineral content and complexity of enamel structure, not thickness

describe physical property 3 of enamel (hardness)

• enamel is the hardest calcified tissue in the body due to:

  • high mineral content (30% more than any other tissue)

  • complex crystal orientation

• note: enamel of permanent teeth is stronger than enamel of deciduous teeth

describe physical property 4 of enamel (brittleness)

• very brittle due to large amount of minerals

• most hard things are brittle (but doesn’t chip easily because of dentin

• dentin, which is less mineralised and brittle compensates for enamel and is necessary as a support for enamel (more elastic, flexible) - acts as a shock absorber

• part of the longevity of enamel depends on health/preservation of dentin layers

describe physical property 5 of enamel (permeability)

• acts as a semi-permeable membrane for certain ions (especially fluoride ions in saliva)

• bigger molecular weight substances won’t pass through the enamel but things with smaller weight can

• this property is both good and bad

  • fluoride ions in water and saliva can absorb through enamel rods/crystals and strengthen the tooth (GOOD)

  • but sugars - especially if in liquid form - can also penetrate through the enamel (BAD)

describe the chemical composition of enamel

• 96% inorganic

• mainly hydroxyapatite crystals (basic structure of any mineralised tissue as well)

• loose hydroxyl group present at the end (both good and bad thing)

  • good - replaceable with fluoride and end up with fluorapatite - more resistant to decay and doesn’t dissolve in acids that easily (inc. resistance by 30-40%)

  • bad - hydroxyl group can be easily broken by acids/sugars/toxins and then the hydroxyapatite crystals get broken

what are the 2 types of histological preparations for examination of hard tissues?

• describe them

• what about tissues that are nearly equal in organic - soft - (35%) and inorganic - hard - (65%) content, like dentin?

DECALCIFIED SECTIONS

• best for soft tissues

• removes calcium from the tooth structure

GROUND SECTIONS

• tooth is ground until very thin layer is made

• the grinding creates lots of friction and heat, so that tissues with less minerals become more burnt than others, creating a dark appearance

• darker = softer, lighter = harder

• this is put on a slide without any staining and examined

DENTIN

• can use both methods to examine dentin as the amount of inorganic and organic material is significant enough to be seen in either section

describe the enamel rod

• basic structural unit of enamel

• may be hexagonal or prism-like in cross-section

• rods don’t have a fixed regular geometric outline

• both ‘enamel rod’ and ‘enamel prism’ terms can be used interchangeably

• smallest tooth (lower perm. central incisor) has ≃ 5 million rods

• largest tooth (upper perm. first molar) has ≃ 12 million rods

• each enamel rod contains hundreds of thousands of hydroxyapatite crystals

what’s the difference between ‘direction’ and ‘course’?

direction - describes going from point A to point B

• for example, going from the gold coast to brisbane, the direction is to brisbane

course - describes the ways to get there

• could go through the suburbs, go through motorway, fly there etc

describe the direction of permanent enamel

• enamel arrangement is better than deciduous, because its supposed to last longer - in cervical 1/3rd, follows the shape of the tooth and ends up as a knife edge, blends in perfectly with the CEJ

describe the course of enamel

• wavy course except at the incisal edges/cusp tips where enamel rods have a complex course (gnarled enamel)

  • this is to withstand masticatory forces

  • enamel rods twist around each other, making the course more complex - and more strong

what are the two patterns that appear in a cross section of enamel?

1. keyhole pattern

2. fish scale pattern

what is the difference between the enamel rods/prisms and the interrod/interprismatic substance?

• the angulation of the crystals

  • eg if rods were 45 degrees, the interrod substance would be 90 degrees (different angulation)

what are the 3 tissues the body can’t regenerate

(i actually don’t know if we need to know this but bakr mentioned it, so…)

1. enamel

2. pericardium (heart muscle)

3. brain cells

why do ameloblasts have a life history and not a life cycle?

• the ameloblasts serve a purpose before the tooth erupts and degenerates when the tooth is erupting

• the process of enamel formation stops so there’s no need for ameloblasts anymore

what are the 6 different stages the ameloblasts go through?

1. differentiating (presecretory) stage

2. secretory stage

3. transitional stage

4. maturative stage

5. protective stage

6. desmolytic stage

give details about stage 1 of ameloblasts (presecretory/differentiation stage)

• ameloblasts differentiate from IEE

• in early bell stage, IEE is separated from DP by cell-free zone

• IEE cells elongate at the expense of cell-free zone - comes into contact with the DP

• now they are preameloblasts

• preamel. induce the formation of odontoblasts by a process called induction (from the peripheral layer of dental papilla)

• the cells are columnar, but shorter in comparison to what the secretory/mature ameloblasts will look like