Oral Histology - Enamel and Dentin

what describes enamel?

avascular and has no nerve supply

enamel is not what?

renewable

what is enamel composed of?

interlocking rods that resist masticatory forces

enamel is variable in what?

its density and hardness

what percentage of enamel is inorganic mineral in the form of hydroxyapatite?

95%

what percentage of enamel is water and organic matter?

5%

what is the organic component of mature enamel?

the protein enamelin

enamel ranges in thickness from what to what?

a knifelike edge at cervical margin to about 2.5 mm over the occlusal or incisal surface

enamel is composed of rods that extend from what?

the dentinoenamel junction to the enamel outer surface

one enamel rod is formed by what?

four ameloblasts

an enamel rod is composed of what parts?

a head, a neck, and a tail

the “keyhold” or “fish” shape of the enamel rods helps do what?

maximize the mineralized area of the enamel and minimizes the organic portion

one enamel rod is about the same size as what?

RBC

crystals in the head of the enamel rod follow what?

the long axis of the rod

crystals in the tail of the enamel rod are what?

perpendicular to the crystals in the head

what describes the orientation of enamel rods?

perpendicular at the dentinoenamel junction and curve slightly toward the cusp tip

enamel layers include what?

a sheath and a core

does the rod sheath or core contain more organic matter?

rod sheath

what does the interrod area provide?

structural support and continuity between the rods

what is the interrod area?

crystal in between the rods that are oriented in a different direction

where does the rod sheath lie?

between the rods and the interrod enamel

rods run almost perpendicular to the enamel surface at the cervical region but are what?

gnarled and intertwined at the cusps tips

what happens when light is projected at the surface of a thin slab of enamel?

dark and light bands appear (known as Hunter-Schreger bands phenomenon)

the Hunter-Schreger bands extends through what?

½ to 2/3 of thickness of enamel

why does the Hunter-Schreger bands phenomenon occur?

since the rods interdigitate, light transmits along the long axis of one group but not along the adjacent rods, which lie at a right angle

one feature of outer enamel near the surface is what?

prismless enamel

in prismless enamel, enamel rods are almost what?

completely perpendicular to the surface

what describes features of prismless enamel compared to other enamel?

more mineralized and less porous

incremental lines in enamel are the result of what?

rhythmic recurrent deposition of enamel 

in between rhythmic recurrent deposition of enamel, there are what?

entrapped air molecules which accentuates the developmental line called Lines of Retzius

a notable line of Retzius is seen associated with what?

time of birth (called the neonatal line)

why does the line of Retzius called the neonatal line occur?

because of the change in nutrition and environment at the time of birth

prenatal enamel has ________ defects than postnatal enamel.

fewer

postnatal enamel has what?

numerous minute spaces that are stained with pigment

what are enamel lamallae?

cracks in the surface of enamel that are visible to the naked eye

enamel lamallea extend form where?

surface to the DEJ

enamel lamallae may be caused by what?

stress cracks that occur because of impact or temperature changes between rods

existing lamellae can also be made larger and exacerbated by what?

carious involvement

where are enamel tufts located?

at the DEJ; perpendicular to the DEJ

enamel tufts extend how much of the way up toward the enamel?

10-20%

enamel tufts form between what?

groups of enamel rods, which are oriented at different directions at the DEJ

interface between dentin and enamel is scalloped, so what form?

enamel tufts

enamel spindles are extensions of what?

dentinal tubules

enamel spindles are shorter and thinner than what?

enamel tufts

what are ridges that result from the termination of the striae of Retzius or ends of rod groups on the surface called?

perikymata or imbrication lines

where are perikymata (imbrication lines) most prominent?

on the facial surface in the cervical region

what is an important consideration in clinical therapy?

passage of fluid, bacteria, and bacterial products through enamel

permeability of enamel is caused by several factors such as?

• through leakage around restorations

• decomposition of the tooth through caries

• lamellae, cracks, tufts, and spindles

• minute spaces between or around enamel rods and through crystal spaces within rods (microlamellae)

• surface irregularities such as central fissures and near cervical region

what are microlamellae?

minute spaces between or around enamel rods and through crystal spaces within rods

crystals in enamel are about what?

30x larger than those in dentin (factor in extreme hardness of enamel in contrast to dentin)

etching with dilute acids, such as citric acid, may do what?

alter the surface of enamel

etching does what?

• selectively etches the end of the enamel rods

• provides adherence to the surface of enamel

the peripheral rod enamel resists what?

demineralization to a greater extent than the rod core

what is attrition caused by?

tooth-tooth contact

what is adhesion caused by?

action of an external agent on a tooth

what is erosion caused by?

gradual loss of structure by nonbacterial chemical process (acid from foods, GERD, bulimia, perimolysis)

what is permiolysis?

erosion from dental exposure to gastric secretions

what is abfraction?

loss of tooth structure from occlusal stresses that create tooth flexture at a location away from point of loading; usually occurs in cervical area of tooth

root dentin is covered by what?

cementum

coronal dentin is covered by what?

enamel

dentin is composed of what?

primarily of an organic matrix of collagen fibers and the mineral hydroxyapatite

what percentage of dentin is hydroxyapatite crystals?

70%

what percentage of dentin is organic collage fibers?

20%

what percentage of dentin is water?

10%

dentin is still slightly harder than what?

bone or cementum

dentin allows what?

the impact of mastication to occur without fracturing the overlying enamel

as odontoblasts continue to migrate toward the pulp, what forms within the dentin?

a tubule

what lies within a dentin tubule?

the odontoblast processes, dentinal fluid, and extracellular matrix substances

what is predentin?

a band of newly formed, unmineralized matrix of dentin at the pulpal border

what are the two stages that dentin forms in?

1. organic matrix is deposited

2. mineral substance is added

where does mineralization occur?

at the predentin-dentin junction

what makes up the bulk of dentin in a tooth?

primary dentin

dentinal tubules, through both primary and secondary dentin, form what?

an “S” curve

what forms the bulk of primary dentin?

circumpulpal dentin

what is the first primary dentin formed?

mantle dentin

what kind of primary dentin is nearly free of developmental defects and serves as covering over the rest of the dentin?

mantle dentin

what type of primary dentin is found beneath mantle dentin?

globular dentin (aka interglobular dentin)

globular dentin may only exist where?

in the crown 

globular dentin contains what?

hypomineralized areas called interglobular space

circumpulpal dentin is found where?

beneath globular dentin

what describes circumpulpal dentin?

thicker in the crown, thinner near the roots

as odontoblasts elongate, they branch at right angles and these secondary processes are what?

contained within canaliculi located perpendicular to the dentinal tubule

the secondary processes give dentin what?

vitality

the surface area of dentin at the DEJ is what?

much larger than that of dentin at the pulp interface

dentinal tubules are much closer together where?

at the pulp surface than at the DEJ

the opening of the tubules are what?

much smaller at the DEJ than at the pulpal border

the dentinal matrix that immediately surrounds the dentinal tubule is termed what?

peritubular dentin

between tubules is what?

intertubular dentin

while intertubular dentin is highly mineralized, peritubular is what?

more highly mineralized than intertubular dentin

secondary dentin forms?

internally to primary dentin

secondary dentin happens what?

naturally with age

secondary dentin forms after what?

crown has come into occlusal function and roots are nearly completed

secondary dentin deposited how?

more slowly than primary dentin

more secondary dentin may be deposited where?

above pulp horns as occlusal function occurs

tertiary or reparative dentin results from what?

pulpal stimulation

tertiary dentin forms at the site of what?

odontoblastic activation

tertiary dentin may be the result of what?

attrition, abrasion, caries, or restorative procedures

tertiary dentin is deposited only where?

at underlying stimulated areas and can be deposited rapidly

tertiary dentin appears how?

irregular with sparse and twisted tubules

odontoblasts, fibroblasts, and bloods cells have been found in what type of dentin?

tertiary dentin