CHAPTER 12: STRUCTURE & FUNCTIONS OF DENTIN-PULP COMPLEX

from book + summer endo basic + ppt/discussion

morphologic zones of the pulp

pulp-dentin complex

odontoblast layer

cell-poor zone

cell-rich zone

pulp proper

pulp-dentin complex

function as a single biologic unit

if there is an impact on dentin may affect the pulpal components, and that disturbances in the dental pulp will in turn affect the quantity and quality of the dentin produced.

any stimulus to dentin (caries, trauma, heat) can affect the pulp, and pulpal disease alters dentin formation (reactionary/reparative dentin)

• dentin depends on pulp for formation and nourishment

• pulp depends on dentin and enamel for protection

odontoblast layer / zone

consists of odontoblast cell bodies

located immediately beneath predentin

outermost cellular layer of the dental pulp

thickness varies with age and tooth type and location

cell density: coronal pulp > radicular pulp; decreases with aging.

• odontoblastic processes extend:

  • through predentin

  • into the inner dentin (dentinal tubules)

• also contains:

  • capillaries

  • nerve fibers

  • dendritic (immune) cells

replaced by stem-like cells from the cell- rich zone that differentiate into new odontoblasts.

root dentin

has fewer dentinal tubules, so odontoblasts are:

• less crowded

• spread laterally

tall, columnar

height of odontoblast layer in the coronal pulp (young, active teeth)

shorter

height of odontoblast layer in the radicular pulp

• mid-root → cuboidal

• near apical foramen → flattened (squamous)

function of odontoblast layer

defense and signaling

regulation of dentin permeability

increase dentin formation to protect pulp

dentin formation throughout life (primary, secondary, tertiary)

sensory transmission via odontoblast processes and nerve endings

palisade arrangement

tall and column-shaped

appearance of odontoblast layer in young teeth

appears 3–5 cells thick, but is actually only one layer

with aging:

• pulp chamber narrows

• apoptosis helps regulate cell number

• odontoblast crowding increases (especially coronally)

—in older teeth, this layer may change: odontoblasts can become smaller or die

30–40 nm wide

narrow spaces between odontoblasts

allow passage of fluids and small molecules

gap junctions

aka: nexuses

formed by connexin proteins

most numerous during primary dentin formation

• allow direct passage of:

  • ions, small signaling molecules

  • electrical & chemical signals

• function:

  • synchronize odontoblast activity

  • produce uniform predentin

• also connect odontoblasts with:

  • fibroblast processes in subodontoblastic zone

adherens and desmosomes

aka: zonula adherens

help maintain structural integrity

located mainly in apical portions

provide mechanical attachment

tight junctions

aka: zonula occludens

found mainly in young teeth, located in apical part of odontoblasts

• regulate:

  • permeability between pulp and predentin

  • movement of substances and keep the pulp environment sealed

• restrict passage of:

  • molecules

  • ions, fluids

• important when dentin is covered by:

  • enamel & cementum

cell-poor zone

aka: cell-free zone / zone of Weil

this zone doesn’t always appear in young pulps

sensory transmission, lies beneath the odontoblastic layer

may be absent or poorly defined:

• in young pulps → rapid dentin formation

• in older pulps → reparative dentin formation

prominent only when the pulp is in a relatively stable functional state

cell-poor zone contains:

few cells, many nerves

blood capillaries plexus

cytoplasmic processes of fibroblasts

unmyelinated nerve fibers (Raschkow’s plexus)

approximate width of cell-poor zone

40 μm

function of cell-poor zone

helps to transport nutrients and signals

acts as a transitional area between odontoblasts and deeper pulp

• important zone for:

  • nerve plexus (Raschkow’s plexus)

  • sensory transmission

nerve plexus of Raschkow

specific structure found in cell-poor zone

cell-rich zone

repair & regeneration

found beneath the cell-poor zone

begins around the time of tooth eruption

forms due to peripheral migration of cells from the pulp core

immune cells migrate in and out in response to antigenic challenge

can be easily recognized because it has more fibroblasts than the pulp proper

function of cell-rich zone

defense and repair

cell replacement / reservoir

source of new odontoblast-like cells after injury

it’s like the backup team that keeps the pulp healthy and functioning

cell-rich zone contains:

immune cells:

• stem cells

• macrophages

• dendritic cells

• fibroblasts → most abundant

• undifferentiated mesenchymal cells

pulp proper

aka: pulp core / central pulp

vessels & nerves

the central mass of the pulp

explains sensitivity, pulpitis, healing capacity of pulp

is like the heart of the pulp — keeping the tooth nourished, protected, and alive.

pulp proper contains:

loose connective tissue

fibroblasts (most prominent)

ground substance and fibers

larger blood vessels, major nerve trunks

functions of pulp proper

innervation

immune response

responds to injury / defense

support of peripheral pulp zones

supplies nutrients & supports tooth vitality

mild irritation

reactionary dentin

severe injury

reparative dentin

cells of the pulp

mast cell

odontoblast

macrophage

dendritic cell

lymphocyte

odontoblast process

relationship of odontoblast structure to secretory function pulp fibroblast

odontoblast

most characteristic & specialized cells

their processes extend into dentinal tubules

the key link between the pulp-dentin complex

fixed & post-mitotic (do not divide after differentiation)

cell body remains outside (periphery) mineralized tissue

presence of odontoblastic processes in tubules makes dentin a living, responsive tissue

• responsible for dentinogenesis:

  • during tooth development

  • throughout aging

• form:

  • dentin matrix

  • dentinal tubules

odontoblasts, osteoblasts, cementoblasts similarities

are matrix-forming cells, capable of mineralization

• rich in RNA

• nuclei with prominent nucleoli → typical features of protein-secreting cells

• produce:

  • collagen fibrils

  • noncollagenous proteins

  • proteoglycans

• share ultrastructural features:

  • well-developed rough endoplasmic reticulum (RER)

  • prominent Golgi apparatus

  • numerous mitochondria

  • secretory granules

ultrastructural features of active odontoblasts:

nucleus

• large, up to four nucleoli

• located at the basal end, enclosed by a nuclear envelope

cytoplasmic organelles

• golgi complex

  • well developed, in supranuclear region

  • consists of smooth-walled vesicles and cisternae

• rough endoplasmic reticulum (RER)

  • highly prominent, closely stacked cisternae in parallel arrays

  • numerous ribosomes → protein synthesis

  • filamentous material → newly synthesized protein

• mitochondria

  • numerous, evenly distributed throughout cytoplasm

morphologic distinctions

odontoblasts:

• tall columnar cells in coronal pulp

• leaves behind a process

• cell body remains outside, interconnect via canaliculi

osteoblasts and cementoblasts:

• polygonal to cuboidal

• become osterocytes/cementocytes

• cells may become entrapped

secretory products of odontoblasts

mainly type I collagen

small amounts of type V collagen

noncollagenous proteins:

• dentin sialoprotein

• proteoglycans

• phosphophoryn

phosphophoryn

unique to dentin

essential for mineralization

not found in other mesenchymal cells

highly phosphorylated phosphoprotein

enzymes of odontoblast

alkaline phosphatase

acid phosphatase

alkaline phosphatase

closely associated with mineralization

exact role not fully understood

acid phosphatase

a lysosomal enzyme

involved in digestion of resorbed predentin matrix

active odontoblast

numerous organelles

actively producing primary dentin

resting (inactive) odontoblast

fewer organelles

• seen after:

  • completion of root development

  • tooth eruption

• dentin formation shifts from:

  • primary dentin → secondary dentin

odontoblast process

living extensions of pulpal cells

process extends through the entire thickness of dentin

each dentinal tubule forms around a major odontoblastic process

• the process:

  • occupies most of the tubule space

  • coordinates formation of peritubular dentin

• represents a cytoplasmic extension of the odontoblast cell body

• explains why:

  • dentin is a vital tissue

  • destruction of dentin affects the pulp

• during cavity preparation:

  • odontoblast processes may be disrupted

  • leading to pulpal irritation or injury

according to transmission electron microscopy (TEM)

odontoblastic process is limited to inner third of dentin

possibly due to shrinkage artifacts

according to scanning electron microscopy (SEM)

odontoblastic process appears to extend to DEJ

but structures observed may be lamina limitans, not the process itself

confocal microscopy indings

• in rat molars:

  • processes do not reach outer dentin or DEJ

  • except during early tooth development

conclusion:

• proteins derived from odontoblasts may remain in tubule walls even after the process retracts

• dentin matrix does not remodel, so these antigens persist

cytoskeletal / ultrastructural components of odontoblast

microtubules

microfilaments

functions of microtubules & microfilaments

cytoplasmic extension

structural framework

intracellular transport of materials

microtubules

give rigidity and structural support

extend from the odontoblast cell body

run parallel to the long axis of the process

microfilaments

are thin for movement

present in the main process and lateral branches

occasional mitochondria

found where the process passes through predentin

suggest metabolic activity within the process

lamina limitans

• lined the dentinal tubule wall via:

  • electron-dense limiting membrane

• a narrow space separates:

  • lamina limitans

  • plasma membrane of the odontoblast process

• except where the process is constricted

• microtubules contain tubulin

collagen synthesis pathway

1. RER

   • rapid incorporation of isotope

   • synthesis of procollagen

2. golgi apparatus

   • procollagen packaged and concentrated into secretory vesicles

   • proteoglycan modification

3. vesicular transport

   • vesicles migrate to the base of the odontoblast process

4. exocytosis

   • vesicles fuse with plasma membrane

   • release tropocollagen into predentin

3H-proline

processed in the Golgi

measures new collagen formation

tracing shows collagen is synthesized in the RER

demonstrate protein synthesis and secretion pathways in odontoblasts

transported via secretory vesicles to the odontoblast process base for exocytosis into predentin.

• after intraperitoneal injection:

  • label appears first in odontoblasts

  • then in predentin matrix

fibrillogenesis

occurs on the outer surface of the odontoblast plasma membrane

forms predentin/dentin matrix, provides structural strength, guides mineralization

process by which collagen molecules assemble into fibrils, which then form fibers

• fibrils:

  • ~15 nm diameter near odontoblast process

  • increase to ~50 nm near calcification front

• released into predentin and thicken toward mineralized dentin

tropocollagen

precipitates extracellularly

aggregates into collagen fibrils at the cell surface

fibrils thicken from 15 nm (base) to 50 nm (calcification front)

the basic structural unit of collagen—a triple helix of three polypeptide chains

proteoglycans

ex: chondroitin sulfate

inhibit mineralization

accumulate near the calcification front

may inhibit calcification by binding calcium

synthesized in the RER, modified in the golgi, and secreted into predentin

lysosomal enzymes

used to destroy ingested materials

likely remove proteoglycans before mineralization begins

mineralization begins only after proteoglycans are removed from the predentin matrix

pulp fibroblast

found throughout the pulp

produces and destroys collagen

most abundant cells in the pulp (cell-rich zone)

tissue-specific cells capable of:

• maintaining extracellular matrix (ECM)

• differentiating into odontoblast-like cells when properly stimulated

function of pulp fibroblast

maintain extracellular matrix

phagocytose and digest collagen → collagen turnover

participate in pulpal repair and inflammation

• synthesize:

  • type I & III collagen

  • proteoglycans

  • glycosaminoglycans (GAGs)

immature fibroblasts

polygonal, widely spaced in ground substance

inactive cells which maintain structure and communicate with other cell

• organelles:

  • inconspicuous golgi

  • sparse RER

  • many free ribosomes

• multiple processes → form gap junctions

  • enable electrical and chemical signaling

mature fibroblasts

fully functional actively producing proteins and helping build and repair the pulp

characteristics:

• appear as active protein-secreting cells

• collagen fibrils accumulate along cell surface

• proliferated RER, prominent golgi complex

• secretory vesicles present, stellate (star-shaped) in shape

fibroblast

stimulated by neuropeptides

• with aging:

  • blood vessels, nerves, and collagen fibers increase

  • relative number of fibroblasts decreases

• many pulpal fibroblasts remain relatively undifferentiated

  • functionally similar to stem cells

• pulp contains many argyrophilic fibers

  • previously thought to be reticulin

  • now believed to be collagen fibers with a GAG sheath

• non-argyrophilic collagen fibers increase with age

functions of fibroblast

multiply near the injury site

can turn into odontoblast-like cells to help form a dentinal bridge that seals the damage

they produce collagen and other proteins needed to rebuild and support the pulp’s structure

• secretes:

  • Nerve Growth Factor (NGF) → links pulp inflammation and nerve response

  • Pro-inflammatory cytokines → to guide healing and manage inflammation

• NGF:

  • regulates neuronal development

  • influences odontoblast response to injury

macrophage

commonly located near blood vessels

involved in signaling pathways within the pulp

identified by antigenic markers in immunohistochemical studies

derived from blood monocytes, digestion occurs via lysosomal enzymes

migrate into pulp tissue and differentiate into various subpopulations

• highly active in:

  • endocytosis

  • phagocytosis

functions of macrophage

phagocytosis & scavenging

• remove:

  • foreign bodies

  • dead or damaged cells

  • extravasated red blood cells

antigen presentation:

• process antigens and present them to T lymphocytes

• processed antigen binds to:

  • MHC class II molecules

• essential for T cell–dependent immunity

signaling & inflammation:

• participate actively in pulpal signaling pathways

released by macrophage:

other cytokines

growth factors (GF)

interleukin 1 (IL1)

tumor necrosis factor (TNF)

dendritic cell

aka: antigen-presenting cells (APCs)

accessory immune cells

equivalent to langerhans cells in epithelium (in the epidermis & mucous membranes)

• primarily found in lymphoid tissues but also present in:

  • connective tissues

  • dental pulp

• in normal pulp:

  • located mainly in peripheral coronal pulp near predentin

• after antigenic challenge:

  • migrate centrally within the pulp

dendritic cell is characterized by:

possess dendritic cytoplasmic processes

function as professional antigen-presenting cells

express major histocompatibility complex (MHC) class II molecules

function of dendritic cell

play a central role in T cell–dependent immunity

• process protein antigens and present:

  • peptide fragments + MHC class II

• recognized by:

  • T-cell receptors

• leads to T-cell activation

lymphocyte

core soldiers of the immune system

works w/ macrophages and dendritic cells

B lymphocytes → rarely found in normal pulp

T lymphocytes → are present in normal human pulp

• observed in:

  • normal pulps

  • impacted teeth

  • inflamed pulps

lymphocyte contains:

T lymphocytes

macrophages, dendritic cells

T8 suppressor → the predominant subset

presence of macrophages, dendritic cells, T lymphocytes

indicates that dental pulp is:

• immunocompetent tissue

• capable of initiating immune responses

mast cell

rare in normal pulp

found near blood vessels

common in chronically inflamed pulp

widely distributed in connective tissues

mast cells contain granules loaded with:

heparin → anticoagulant

histamine → major inflammatory mediator

additional chemical factors involved in inflammation

functions of mast cells

chronic pulpitis

vascular changes

inflammatory reactions