DENTAL PULP AND PERIIRADICULAR TISSUE ,

1. DENTAL PULP AND
PERIRADICULARTISSUE

2.  The pulp and dentin are different
components of a tooth that remain closely
integrated, both functionally and
anatomically, throughout the life of the
tooth.The two tissues are referred to as the
pulp-dentin organ or the pulp-dentin
complex.

3.  The dental pulp has its genesis at about the
sixth week of the intrauterine life, during the
initiation of tooth development .The oral
stratified squamous epithelium covers the
primordia of the future maxillary and
mandibular processes in a horseshoe-shaped
pattern.

5.  Tooth development starts when stratified
squamous epithelium begins to thicken and
forms the dental lamina.
 The cuboidal basal layer of the dental lamina
begins to multiply and thicken in five specific
areas in each quadrant of the jaw to mark the
position of the future primary teeth.

6.  The stratified squamous oral epithelium covers an
embryonic connective tissue that is called the
ectomesenchyme because of its derivation from the
neural crest cells. By a complex interaction with the
epithelium, this ectomesenchyme initiates and
controls the development of the dental structures.
 The ectomesenchyme below the thickened epithelial
areas proliferates and begins to form a capillary
network to support further nutrient activity of the
ectomesenchyme-epithelium complex.This
condensed area of ectomesenchyme forms the future
dental papilla and subsequently the pulp

7.  The thickened epithelial areas continue to
proliferate and to migrate into the
ectomesenchyme and in the process forms a
bud enlargement called the enamel organ.
This point is considered the bud stage of
tooth development

9.  The enamel organ continues to proliferate
into the ectomesenchyme with an uneven
rhythmic cell division producing a convex and
a concave surface characteristic of the cap
stage of tooth development

11.  The ectomesenchyme, which is partially
enclosed by the inner enamel epithelium,
continues to increase its cellular density.The
cells are large and round or polyhedral with a
pale cytoplasm and large nuclei.This
structure is the dental papilla which
differentiates into the dental pulp

12.  Formation of Dental Follicle (or Dental Sac)
When the ectomesenchyme surrounding the
dental papilla and the enamel organ condenses
and becomes more fibrous, it is called the dental
follicle or the dental sac—the precursor of the
cementum, the periodontal ligament (PDL), and
the alveolar bone .The dental lamina continues
to proliferate at the point where it joins the
deciduous enamel organ and thereby produces
the permanent bud lingual to the primary tooth
germ.

13.  The cells of the inner enamel epithelium continue to
divide and thus increase the size of the tooth germ.
During this growth, the inner enamel epithelium
invaginates deeper into the enamel organ, and the
junction of the outer and the inner enamel epithelium
at the rim of the enamel organ becomes a distinct
zone called the cervical loop.
 The deep invagination of the inner enamel epithelium
and the growth of the cervical loop partially enclosing
the dental papilla begins to give the crown its form.
This point is called the bell stage of development

14.  During this stage, the dental lamina that migrated
into the ectomesenchyme degenerates, the primary
and permanent buds are thus separated from the oral
epithelium, and the distal portion of the dental lamina
proliferates to form the buds of the permanent
molars, which have no primary predecessors.
 As the development progresses, several layers of the
squamous cells between the stellate reticulum and
the inner enamel epithelium form the stratum
intermedium.This layer of cells is limited to the area
of the inner enamel epithelium and seems to be
involved with enamel formation.

15.  Š
Stratum intermedium Enamel Š
 Ectomesenchyme Dentin Š
 Dental papilla Pulp Š
 Dental follicle or dental sacCementum, the
periodontal ligament (PDL), and the alveolar
bone

16.  In a complex series of events, the inner
enamel epithelium exerts an inductive
influence on the ectomesenchyme to begin
dentinogenesis, and consequently,
dentinogenesis has an inductive influence on
the inner enamel epithelium to start
amelogenesis.This series of events begins in
the area of the future cusp tips and continues
to the cervical loop, the future
cementoenamel junction.

18.  The periphery of the adjacent dental papilla
consists of the polymorphic mesenchymal cells
that develop into the cuboidal cells and align
themselves parallel to the basement membrane
of the inner enamel epithelium and the acellular
zone.These cuboidal cells stop dividing and
develop into the columnar cells with polarized
nuclei away from the basement membrane of
the inner enamel epithelium. At this stage, these
cells are called preodontoblasts.

19.  The preodontoblasts mature into odontoblasts
by elongating themselves, by contacting
adjacent odontoblasts through an increase in
size, and by sending the cytoplasmic processes
into the acellular zone.These odontoblastic
processes continue to elongate and move the
odontoblast cell body toward the center of the
dental papilla. During this movement, large-
diameter collagen fibers known as von Korff
fibers are deposited at right angles to the
basement membrane in the extracellular matrix
of the acellular zone.

20.  This process creates the organic matrix of the
first-formed dentin or mantle dentin. As more
collagen fibrils are deposited, the inner enamel
epithelium basement membrane starts to
disintegrate.The vesicles carrying apatite
crystals bud off from the odontoblastic
processes and the crystals are deposited in the
organic matrix for the initiation of
mineralization.The dental papilla becomes the
pulp at the moment of the mantle dentin
formation.

21.  After the deposition of mantle dentin, the
odontoblasts continue to move toward the center of
the pulp and to leave the odontoblastic processes
behind.The organic matrix or predentin is deposited
around the odontoblastic processes.
 The predentin later calcifies and thereby forms the
dentinal tubules. Primary dentin differs from the
mantle dentin in which the matrix originates solely in
the odontoblasts.The collagen fibers are smaller, are
more closely packed, and they are at right angles to
the tubules and are interwoven.The mineralization of
primary dentin originates from the previous
mineralized dentin.

22.  As the incremental deposition of dentin
continues toward the center of the pulp, the
diameter of the odontoblastic processes is
reduced peripherally.Along with this, there is
a reduction in size due to the circumferential
deposition of dentin in the walls of the
dentinal tubules.This dentin, which is more
mineralized and is harder than primary
dentin, is called peritubular dentin

23.  Concomitant with dentinogenesis, the cells of the
inner enamel epithelium cease to divide.These cells
are the elongated epithelial cells called
preameloblasts.
 Ameloblasts
 The preameloblasts differentiate into tall columnar
epithelial cells with their nuclei polarized toward the
stratum intermedium and the ameloblasts.
 While the ameloblasts are differentiating, the
basement membrane of the inner enamel epithelium
is being resorbed and dentin is being deposited to
follow the contour established by the basement
membrane.This process forms the future
dentinoenamel junction

24.  The deposition of enamel matrix causes the
ameloblasts to migrate peripherally and form conic
projections calledTomes’ processes on their secretory
surfaces.The migration of ameloblasts peripherally
(as they secrete enamel) outlines the crown of the
tooth, but blocks the source of nutrition from the
dental pulp.To gain a new source of nutrition, the
outer enamel epithelium becomes a flattened layer of
cells that folds because of the loss of the intracellular
material of the stellate reticulum.This change brings
the capillary network of the dental follicle, the new
source of nutrition, closer to the ameloblasts.

25.  The orderly deposition of enamel continues until the form
of the crown is fully developed. At this time, the
ameloblasts lose theirTomes’ processes, and the outer
enamel epithelium, the stellate reticulum, and the stratum
intermedium form a protective layer of stratified
epithelium around the newly formed crown.
 This marks the beginning of enamel maturation or the
higher mineralization of the existing enamel.This
maturation process begins in the dentinoenamel junction
and progresses peripherally to the enamel surface.
 During the final phase of the maturation process, the
ameloblasts join the stratified epithelium to form the
reduced enamel epithelium and to cover and protect the
enamel until eruption of the tooth

26.  On completion of the crown, the cervical loop,
formed by the union of inner and outer enamel
epithelia, proliferates to form Hertwig’s
epithelial root sheath, which determines the size
and shape of the root of the tooth
 In single-rooted teeth, the epithelial diaphragm
has a single opening which guides the formation
of the root, root canal, and apical foramen.
 In multirooted teeth, the epithelial diaphragm
guides the formation of the furca, roots, root
canals, and apical foramina.

27.  As dentin is formed, the basement membrane of the
inner enamel epithelium disintegrates and the
epithelial cells lose their continuity.
 The disintegration of the basement membrane and
the loss of continuity of the epithelial cells allow the
mesenchymal cells from the dental follicle to
penetrate the newly deposited dentin.
 These mesenchymal cells differentiate into
cementoblasts, which are round, plump cells that
have basophilic cytoplasm with an open nucleus in the
active phase of cementogenesis and a closed nucleus
with reduced cytoplasm during the resting phase

28.  The collagen fibers followed by the ground
substance elaborated by the cementoblasts
are deposited between the epithelial cells.
The cluster of cells left behind from the
epithelial root sheath migrates toward the
dental follicle and the future PDL.This cluster
of epithelial cells comprises the cell rests of
Malassez.

29.  When some matrix production has taken
place, mineralization of the cementum starts
by the spread and deposition of the
hydroxyapatite crystals from the dentin into
the collagen fibers and the matrix.
 As dentinogenesis progresses in incremental
phases, the apical foramen or foramina are
formed by an apposition of dentin and
cementum that reduces the size of the
opening of the epithelial diaphragm.

31.  The accessory canals, which are an inefficient
source of collateral circulation for the pulp,
are formed during the development of the
root. A defect in the epithelial root sheath, a
failure in the induction of dentinogenesis, or
the presence of a small blood vessel produces
a gap that results in the formation of an
accessory canal.

32.  Two kinds of cementum are laid down on the root. If
the cementoblasts retract as the cementum is laid, it
will be acellular cementum; on the other hand, if the
cementoblasts do not retract and are surrounded by
the new cementum, the tissue formed will be cellular
cementum and the trapped cementoblasts are called
cementocytes.
 The acellular cementum is found adjacent to the
dentin.
 The cellular cementum is found usually in the apical
third of the root overlying the acellular cementum and
in alternating layers with it.

34.  The periodontal ligament and alveolar bone
develop at the same time as the root of the
tooth.
 The deposition of bone to form the alveolus
and deposition of cementum to cover the
dentin of the root give form to the
attachment apparatus, the periodontium

35.  The blood vessels of the pulp originate from an
oval or circular reticulated plexus.When fully
developed, this plexus encircles the enamel
organ and the dental papilla in the region of the
dental follicle. A series of vessels arise from this
plexus and grow into the dental papilla.
 The sensory nerve fibers traverse the dental
papilla and, on reaching the coronal pulp, they
branch toward the periphery to form a plexus of
nerves called the plexus of Raschkow.

36.  The blood vessels entering the dental papilla
during the development bring with them the
sympathetic nerve fibers which are
unmyelinated.
 These sympathetic nerve fibers play a role in the
vasoconstriction of the blood vessels. As the
apical foramen matures and reduces the size of
its opening, the myelinated nerve fibers form
bundles located in the center of the pulp in
conjunction with the blood vessels.

37.  Pulp is a connective tissue consisting of
nerves, blood vessels, ground substance,
interstitial fluid, odontoblasts, fibroblasts,
and other cellular components.
 The dental pulp consists of vascular
connective tissue contained within rigid
dentinal walls. Although similar to other
connective tissues in the human body, it is
specialized, owing to its functions and
environment.

38.  Formative: Elaboration of dentin to form the
tooth
 Protective: Protection against and repairing
of the effects of noxious stimuli
 Nutritive: Preserving the vitality of all the
cellular elements
 Sensory: Perception of stimuli

39. Starting at the periphery, the pulp is divided
into four zones:
 1. Odontoblastic zone, which surrounds the
periphery of the pulp
 2. Cell-free zone
 3. Cell-rich zone
 4. Central zone

41.  The odontoblasts are specialized cells that
generally last the entire life of the tooth.The
odontoblasts consist of cell bodies and their
cytoplasmic processes.
 The odontoblastic cell bodies form the
odontoblastic zone, whereas the odontoblastic
processes are located within the predentin
matrix and the dentinal tubules, extending into
the dentin. In this odontoblastic zone, capillaries
and unmyelinated sensory nerves are found
around the odontoblastic cell bodies

42.  The odontoblasts lining the predentin
represent the link between the dentin and the
pulp.The odontoblasts are the matrix-
producing cells and show characteristic
features associated with protein synthesis.

43.  The cell-free zone, or zone ofWeil, is a
relatively acellular zone of the pulp, located
centrally to the odontoblast zone.
 This zone, although called cell-free, contains
some fibroblasts, mesenchymal cells, and
macrophages.
 The main constituents of this zone are a
plexus of capillaries, the nerve plexus of
Raschkow, and the ground substance

44.  The cell-rich zone is located central to the
cell-free zone. Its main components are
ground substance, fibroblasts with their
product, i.e., the collagen fibers,
undifferentiated mesenchymal cells, and
macrophages.

46.  Ground substance, the main constituent of the
pulp, is the part of the matrix that surrounds and
supports the cellular and vascular elements of
the pulp. It is a gelatinous substance composed
of proteoglycans, glycoproteins, and water.
Ground substance serves as a transport medium
for metabolites and waste products of cells and
as a barrier against the spread of bacteria. Age
and disease may change the composition and
function of the ground substance.

48.  The fibroblasts are the predominant cells of the
pulp .They may originate from undifferentiated
mesenchymal cells of the pulp or from the
division of existing fibroblasts.The fibroblasts
are stellate in shape, with ovoid nuclei and
cytoplasmic processes.
 As they age, they become rounder, with round
nuclei and short cytoplasmic processes.
Although fibroblasts are present in the cell-free
and central zones of the pulp, they are
concentrated in the cell-rich zone, especially in
the coronal portion

49.  The function of the fibroblasts is elaboration
of ground substance and collagen fibers,
which constitute the matrix of the pulp.
 The fibroblasts are also involved in the
degradation of collagen and the deposition of
calcified tissue.
 They can elaborate denticles and can
differentiate to replace dead odontoblasts,
with the potential for reparative dentin
formation.

50.  The undifferentiated mesenchymal cells are
derived from the mesenchymal cells of the
dental papilla. Because of their function in repair
and regeneration, they retain pluripotential
characteristics and can differentiate into
fibroblasts, odontoblasts, macrophages, or
osteoclasts.
 They resemble fibroblasts as they are stellate in
shape, with a large nucleus and little cytoplasm.
These cells, if present, are usually located
around blood vessels in the cell-rich zone and
are difficult to recognize.

51.  Macrophages are found in the cell-rich zone,
especially near the blood vessels.These cells
are blood monocytes that have migrated into
the pulp tissue.Their function is to
phagocytize necrotic debris and foreign
materials. Lymphocytes and plasma cells, if
present in the normal pulp, are found in the
coronal subodontoblastic region.
 The function of these cells in the normal pulp
may be immune surveillance.

52.  The central zone or pulp proper contains
blood vessels and nerves that are embedded
in the pulp matrix together with fibroblasts.
From their central location, the blood vessels
and the nerves send branches to the
periphery of the pulp.

53.  Afferent circulation of the pulp consists of the
arterioles entering the apical foramen. As these
vessels traverse the center of the pulp, they branch
into terminal arterioles, metarterioles, precapillaries,
and finally capillaries.The capillaries end in the cell-
poor zone and form a rich subodontoblastic plexus.
 Efferent circulation consists of postcapillary venules
and collecting venules, which empty into two or three
venules that exit through the apical foramina and
empty into the vessels in the PDL. Lymphatic vessels
follow this same pattern

54.  Lymphatic vessels are present in the pulp.
The function of these vessels is the removal
of interstitial fluid and metabolic waste
products to maintain the intrapulpal tissue
pressure at a normal level.
 These lymphatic vessels follow the course of
the venules toward the apical foramen.

55.  Interstitial fluid bathes all the pulpal tissues
and fills the dentinal tubules in their distal
extension and around the odontoblastic
processes.
 The interstitial fluid that fills the dentinal
tubules is called the dentinal fluid.As
previously discussed, the encasement of the
pulp in dentin produces a limited
environment permitting only a small amount
of interstitial fluid.

56.  The hydrostatic pressure in the interstitial
fluid surrounding the pulpal cells is called the
pulpal tissue fluid pressure.

57.  The sensory mechanism of the pulp is
composed of sensory afferent and autonomic
efferent systems.
 Approximately, 80% of the nerves of the pulp
are C fibers and the rest areAδ fibers.

58. C NERVE FIBRES
 C fibers are unmyelinated and fine sensory afferents.
 C fibers have a diameter of 0.3–1.2 µm and a conduction velocity
of 0.4–2 m/s.
 The conduction of these fibers, which are of smaller diameter than
Aδ fibers, is slow.
 These fibers are probably distributed throughout the pulp tissue.
With their receptive fields located in the pulp, C fibers transmit
impulses that are experienced as a dull, poorly localized, and
lingering pain; they conduct throbbing and aching pain associated
with pulp tissue damage.
 In addition to the nociceptive alarm signaling, the intradental
sensory axons play a regulatory role in the maintenance and repair
of the pulpodentinal complex.
 Mechanism of stimulation: Inflammation that accompanies tissue
injury leads to increase in tissue pressure and release of chemical
mediators.This in turn stimulates the C fibers that result in pain.

59.  Most apical myelinated axons are fast-conducting Aδ fibers
with their receptive fields located at the pulpal periphery and
inner dentin.
 The Aδ fibers have a diameter of 2–5 µm and a conduction
velocity of 6–30 m/s.
 The Aδ fibers, with a larger diameter than that of the C fibers,
conduct impulses at a higher velocity.
 They conduct impulses that are interpreted as a short, well-
localized, sharp, and pricking pain.
 The Aδ fibers are distributed in the odontoblastic and
subodontoblastic zones and are associated with dentinal pain.

60. Mechanism of stimulation:
 Three theories have been proposed to explain the sensitivity of dentin. –
 Direct Stimulation Theory: The first is the direct stimulation of the nerve
endings of the pulp; the lack of nerve endings at the periphery of the dentin
negates this theory.
 OdontoblasticTheory:The second theory proposes that the
odontoblasts function as nerve endings.This theory cannot be accepted,
however, because no one knows for certain how far the odontoblastic
processes extend in the dentinal tubules, and no evidence indicates that
the odontoblasts are able to function as nerve endings.
 HydrodynamicTheory:The third theory, the hydrodynamic theory, states
that any fluid movement in the dentinal tubules and around the
odontoblasts as the result of a stimulus excites the nerve endings and
produces an impulse.This theory is the most acceptable of the three.

61. Stimulated impulse travels from C or Ad fiber nerve endings
The plexus of Raschkow
Nerve trunk in the central zone of the pulp that exits the tooth
through the apical foramen
Maxillary or the mandibular division of the trigeminal nerve
Pons
Thalamus
Cortex
Interpreted as pain

62.  The other histologic structures found in the
dental pulp are mineralizations. Although their
presence has been related to age and disease,
they are also found in young normal dental
pulps.They are present as:
 Nodules called denticles or pulp stones:The
denticles are either true or false denticles,
according to their histologic structure.
 True denticles are uncommon, are usually found
near the apex, and are composed of dentin or
dentinal-type calcifications with tubules,
surrounded by odontoblast-like cells.

63.  False denticles are of two types histologically: -
Round or ovoid with concentric calcified layers
and smooth surfaces - Amorphous without
lamination and rough surfaces
 Diffuse calcifications: They usually follow the
trajectory of the blood vessels, the nerves, and
the collagen fiber bundles.They are most often
found in the walls of blood vessels. Diffuse
calcifications seem to be related to aging
because their incidence increases with age.

64. Age causes important changes in the pulp.
 The continuous deposition of secondary dentin
throughout the life of the pulp and the
deposition of reparative dentin in response to
stimuli reduces the size of the pulp chambers
and root canals and thereby decreases the pulp
volume.This diminution of the pulp is called
atrophy.
 A concomitant decrease in the diameter of the
dentinal tubules by the continuous deposition of
peritubular dentin also occurs.

65.  Some of these tubules close completely and
form sclerotic dentin.The decrease in pulp
volume reduces cellular, vascular, and neural
content of the pulp.The odontoblasts
undergo atrophy and may disappear
completely under areas of sclerotic dentin.
 A reduction in the fluid content of the
dentinal tubules is also seen.These changes
make the dentin less permeable and more
resistant to external stimuli.

66.  The fibroblasts are reduced in size and numbers, but the
collagen fibers are increased in number and in size,
probably because of the decrease in the collagen solubility
and turnover with advancing age. This change is referred to
as fibrosis. Fibrosis is more evident in the radicular portion
of the pulp than elsewhere.
 The blood vessels decrease in number, and arteries undergo
arteriosclerotic changes. Calcific material is deposited in
the tunica adventitia and tunica media. These changes
reduce the blood supply to the pulp.
 The number of nerves is also reduced.
 The ground substance undergoes metabolic changes that
predispose it to mineralization. Changes in the blood
vessels, nerves, and ground substance predispose the pulp
to dystrophic calcifications.

67. The periradicular tissues consist of the
following:
 Cementum, which covers the roots of the
teeth
 Periodontal ligament, whose collagen fibers,
embedded in the cementum of the roots and
in the alveolar processes, attach the roots to
the surrounding tissues
 Alveolar process, which forms the bony
troughs containing the roots of the teeth

68.  Cementum is a bone-like calcified tissue that
covers the roots of the teeth.
 it is derived from the mesenchymal cells of
the dental follicle that differentiate into
cementoblasts.The cementoblasts deposit a
matrix, called cementoid, that is
incrementally calcified and produces two
types of cementum: acellular and cellular

69.  Chronologically, the acellular cementum is
deposited first against the dentin forming the
cementodentinal junction, and as a rule, it
covers the cervical and the middle thirds of the
root.The cellular cementum is usually deposited
on the acellular cementum in the apical third of
the root and alternates with layers of the
acellular cementum.The cellular cementum is
deposited at a greater rate than the acellular
cementum and thereby entraps the
cementoblasts in the matrix.These entrapped
cells are called cementocytes

70.  The cementocytes lie in the crypts of cementum
known as lacunae. From the lacunae, canals, called
canaliculi, which contain protoplasmic extensions of
the cementocytes and serve as pathways for nutrients
to the cementocytes interlace with other canaliculi of
other lacunae to form a system comparable to the
Haversian system of bone. Because cementum is
avascular, its nutrition comes from the PDL. As
incremental layers of cementum are deposited, the
PDL may be further displaced, and some
cementocytes may die as a result and may leave
empty lacunae.

71.  The thickness of cementum reflects one of its
functions.The greater thickness of cementum at
the apex is due to its continuous deposition
during the eruptive life of the tooth to preserve
its height in the occlusal plane.The continuous
deposition of cementum also gives form to the
mature apical foramen.The foramen, as it
matures, becomes conical, with the apex of the
cone, called the minor diameter (constriction),
facing the pulp and the base, called the major
diameter, facing the PDL.

72.  The fibers of the PDL occur between the
osteoblasts and cementoblasts and are
embedded into the bone and cementum,
respectively.These embedded fibers, called
Sharpey’s fibers, attach the PDL to bone and
cementum.

73.  Repair is another function of the cementum. Root
fractures and resorptions are usually repaired by
cementum.The closing of immature roots by
apexification procedures is accomplished by
deposition of cementum or cementum-like tissue.
 Cementum also has a protective function. It is more
resistant to resorption than bone, probably because of
its avascularity. As a result, orthodontic movement of
roots can usually be performed with a minimum of
resorptive damage.
 Other functions are the maintenance of the
periodontal width by the continuous deposition of
cementum and the sealing of accessory and apical
foramina after root canal therapy

74.  The periodontal ligament is a dense, fibrous
connective tissue that occupies the space
between the cementum and the alveolar
bone. It surrounds the necks and the roots of
the teeth and is continuous with the pulp and
gingiva.The PDL is composed of ground
substance, interstitial tissue, blood and
lymph vessels, nerves, cells, and fiber
bundles.

75.  Variations in width occur from tooth to tooth
and in different areas of the ligament in the
same root.Teeth with heavy occlusal loads
have wider PDLs than teeth with minimal
occlusal loads, in which PDLs are thinner.
With advancing age, the width of the PDL is
reduced.
 The width of the PDL varies from 0.15 to 0.38
mm

76.  The interstitial tissue is the loose connective
tissue that surrounds the blood vessels and the
lymphatic vessels, nerves, and fiber bundles.
 This tissue contains collagen fibers independent
of the fiber bundles of the PDL. Changes in its
configuration are due to continuing changes in
the fiber bundles.
 The spaces in the PDL, filled with interstitial
tissue, blood vessels, lymph vessels, and nerves,
are called interstitial spaces.

77.  The active cells of the PDL are the fibroblasts,
osteoblasts, and cementoblasts

78.  Fibroblasts synthesize collagen and matrix and are involved in the
degradation of collagen for its remodeling.The result is a constant
remodeling of the principal fibers and maintenance of a healthy
PDL. Because of these important functions, the fibroblasts are the
most important cells of the PDL.
 Osteoblasts, or bone-forming cells, are found in the periphery of
the PDL lining the bony socket.They are usually seen in various
stages of differentiation.The function of osteoblasts is the
deposition of collagen and matrix, which is deposited on the
surface of the bone and to which Sharpey’s fibers are attached.
Calcification of the osteoid anchors Sharpey’s fibers.The constant
remodeling of bone provides for the continued renewal of the
attachment of the PDL to bone.

79.  Osteoclasts, or bone-resorbing cells, are found in the bone periphery
during periods of bone remodeling.They are multinucleated cells with a
ruffle or striated border toward the area of bone resorption.As the
osteoclasts demineralize and disintegrate the matrix, scooped-out areas
in the bone, called Howship’s lacunae, are formed. Osteoclasts are
usually found in these lacunae.This pattern of resorption gives the
border of the bone an irregular shape.
 Cementoblasts, as previously discussed, are aligned in the periphery of
the PDL opposite the cementum.Their function is the deposition of a
matrix consisting of collagen fibrils and ground substance called
cementoid. Cementoid is found between calcified cementum and the
layer of cementoblasts that thickens in periods of activity.The fibers of
the PDL are found between cementoblasts and are entrapped in the
cementoid. As the cementoid calcifies, the fibers of the PDL become
anchored in the newly formed cementum and are called Sharpey’s fibers,
the same as PDL fibers anchored in bone.Cementoid may protect the
cementum against erosion

80.  Cementoclasts, or cementum-resorbing cells, are not found
in the normal PDL because cementum does not normally
remodel.They are found only in patients with certain
pathological conditions.
 Other cells present in the normal PDL are the epithelial cell
rests of Malassez, undifferentiated mesenchymal cells, mast
cells, and macrophages.The epithelial cell rests of Malassez
are remnants of Hertwig’s epithelial root sheath.These cells
are located in the cementum side of the PDL.Their function
is unknown, but they can proliferate to form cysts in the
presence of noxious stimuli.
 Undifferentiated mesenchymal cells are usually stellate cells
with large nuclei located near the blood vessels.These cells
may differentiate into fibroblasts, odontoblasts, or
cementoblasts.

81.  The periodontal fibers are the principal structural components of
the PDL.Two types are known: collagen and oxytalan fibers.
Collagen fibrils are organized into fibers, which, in turn, are
organized into bundles.
 The fibers that constitute the bundles are not continuous from
bone to cementum, but consist of strands that can be continually
and individually remodeled by fibroblasts without causing loss of
the continuity of the bundles.The terminal fibers of the bundles
insert into cementum on one side and bone on the other side.
 These terminal fibers are called Sharpey’s fibers regardless of
cementum or bone insertion.The fibers are arranged in bundles
with a definite functional arrangement.These bundles follow an
undulating course that allows some movement of the tooth in its
alveolar socket.

82.  The fiber bundles are arranged into principal fiber groups: trans-septal,
alveolar crest, horizontal, oblique, apical, and inter-radicular.
 The trans-septal group is embedded into the cementum of adjacent
teeth traversing the alveolar crest interproximally.
 The alveolar crest group is embedded into the cementum below the
cementoenamel junction, is situated obliquely, and ends in the alveolar
crest.
 The horizontal group is embedded into the cementum apical to the
alveolar crest group and moves horizontally into the alveolar bone.
 The oblique group is embedded into the cementum apically to the
horizontal group and travels obliquely in a coronal direction to be
embedded into the alveolar bone.
 The apical group is embedded into the apical cementum and the fundus
of the alveolar socket.
 The inter-radicular group is embedded in cementum and alveolar bone of
the furca of multirooted teeth.

83.  The functions of the fibers of the PDL are to
attach the tooth to its alveolar socket, to
suspend it in its socket, to protect the tooth
and the alveolar socket from masticatory
injuries, and to transform vertical masticatory
stresses into tension on the alveolar bone.

84.  The alveolar nerves which originate in the
trigeminal nerve innervate the PDL.They are
divided into ascending periodontal or dental,
interalveolar, and inter-radicular nerves.The
nerves of the PDL, as in any other connective
tissue, follow the distribution of the arteries.The
alveolar branches innervate the apical region,
the interalveolar branches innervate the lateral
PDL, and branches of the inter-radicular nerve
innervate the furcal PDL of the posterior teeth

85.  The nerve endings of the PDL enable one to perceive pain,
touch, pressure, and proprioception. Proprioception, which
gives information on movement and position in space,
enables one to perceive the application of forces to the
teeth, movement of the teeth, and the location of foreign
bodies on or between the surfaces of the teeth.
 This proprioceptive sense may trigger a protective reflex
mechanism that opens the mandible to prevent injury to the
teeth or PDL when one bites into a hard object.
Proprioception permits the localization of areas of
inflammation in the PDL. Such inflammatory reactions in
the PDL can be identified by percussion and palpation tests.

86.  The alveolar process is divided into the
alveolar bone proper and the supporting
alveolar bone.
Alveolar Bone Proper
 The alveolar bone proper is the bone that
lines the alveolus or the bony sockets that
house the roots of the teeth. It begins its
formation by intramembranous ossification
at the initial stage of root formation

87.  The alveolar bone proper consists of bundle
bone in the periphery of the alveoli and
lamellated bone toward the center of the
alveolar process.
 The radiographic image of the alveolar bone
proper is called the lamina dura.The alveolar
bone proper can also be referred to as the
cribriform plate.
 This term refers to the many foramina that
perforate the bone.These foramina contain
vessels and nerves that supply teeth,
periodontal ligament, and bone.

88.  Adjacent to the alveolar bone proper is cancellous (spongy)
bone covered by two outer tables of compact bone. One of the
outer tables of compact bone is vestibular and the other is
lingual or palatal.
 The cancellous bone consists of lamellated bone arranged in
branches called the trabeculae. Between the trabeculae are the
medullary spaces, filled with the marrow. The marrow can be
fatty or hematopoietic. In adults, the marrow in the mandible
and maxilla is usually fatty, but hematopoietic tissue is found
in certain locations such as the maxillary tuberosity, maxillary
and mandibular molar periradicular areas, and premolar
periradicular areas.

89.  Hematopoietic marrow spaces appear radiolucent
in radiographs.
 The cortical (compact) bone covers the
cancellous bone and is formed by the lamellated
bone.
 Bone serves as the calcium reservoir of the body.
The body, under hormonal control, regulates
and maintains calcium metabolism.Therefore,
constant physiologic remodeling of bone by
osteoclastic and osteoblastic activity occurs