The periodontal ligament is a soft, vascular connective tissue that connects tooth roots to the alveolar bone socket. It develops from the dental follicle during root formation and tooth eruption. The periodontal ligament contains principal collagen fiber bundles oriented in different directions, as well as fibroblasts, cementoblasts, osteoblasts, and progenitor cells. It maintains homeostasis through a balance of synthetic and resorptive cells and extracellular substances. The unique structure and cellular composition of the periodontal ligament allow it to function in tooth attachment and as a sensory organ.

1. P E R I O D O N TA L L I G A M E N T
D R . T. D I N E S H K U M A R , M D S
D E P T. O F O R A L PAT H O LO G Y
S R M D E N TA L C O L L E G E , R A M A P U R A M
S R M I N S T I T U T E O F S C I E N C E & T E C H N O LO G Y

2. CONTENTS
INTRODUCTION
DEFINITION
SYNONYMS
EXTENT AND SHAPE
DEVELOPMENT
PERIODONTAL LIGAMENT HOMEOSTASIS
CELL BIOLOGY OF THE NORMAL PERIODONTIUM
CELLULAR EVENTS
EXTRACELLULAR SUBSTANCES
STRUCTURES OF THE CONNECTIVE TISSUE
FUNCTIONS OF THE PDL
AGE CHANGES
UNIQUE FEATURES
CLINICAL CONSIDERATIONS

3. INTRODUCTION
Unique and complex
dynamic structure.
Soft, richly vascular and
cellular connective tissue.
Surrounds the root.
Joins the root cementum
with the socket wall.

4. DEFINITIONS
• Periodontal ligament is composed of soft complex vascular and highly
cellular connective tissue that surrounds the tooth roots and connects to
the inner wall of the alveolar bone. (Mc Culloch CA, Lekic P, Mc Kee MD
Periodontal 2000 24:56,2000)
• It is that soft, specialized CT situated between the cementum covering the
root of the tooth and bone forming the socket wall. (A.R.Tencate 1971)
• The periodontal ligament occupies the periodontal space, which is located
between the cementum and the periodontal surface of alveolar bone and
extends coronally to the most apical part of the lamina propria of the
gingiva. (Orban)

5. SYNONYMS
Desmodont
Gomphosis
Pericementum
Dental Periosteum
Alveodental Ligament
Periodontal Membrane
REFERENCE: ORBAN’S ORAL HISTOLOGY
AND EMBRYOLOGY

6. EXTENT AND SHAPE
Shape : An hour glass and is
narrowest at the mid root level
Width : 0.15-0.38mm.
Radiographic appearance:
radiolucent bounded by
radiopaque cementum and lamina
dura of the alveolar bone proper
The average width of the
periodontal space is 0.1–0.25 mm
REFERENCE: ORBAN’S ORAL HISTOLOGY
AND EMBRYOLOGY

7. DEVELOPMENT
 All periodontal tissues originate from the dental
follicle
 Begins with root formation & prior to tooth
eruption. (Tencate et al.,1972).
 At Late bell stage - when amelogenesis and
dentinogenesis are well advanced, the inner and
the outer enamel epithelium at the cervical loop
of enamel organs form a doubled layered
epithelial root sheath which proliferates apically
to map out the shape of future root called the
HERTWIG’S EPITHELIAL ROOT SHEATH REFERENCE: ORBAN’S ORAL
HISTOLOGY AND EMBRYOLOGY; TEN
CATE’S ORAL HISTOLOGY

8. DEVELOPMENT
Establish continuity across the ligament space
Assembles as collagen bundles on the bone and cementum surface
Type I collagen is secreted
Actively synthesize & deposit collagen fibrils and glycoproteins in
developing PDL (Grant’s 1989; Ten Cate’s 1971)
As the root formation continues, cells in the peri follicular
gain their polarity, cellular volume & become widely separated

9. DEVELOPMENT OF
PRINCIPLE FIBRES
Active fibroblasts adjacent to cementum of the
coronal third of the root, appear to become
aligned in an oblique direction to the long axis of
the tooth.
The first collagen fiber bundles of the
ligament become discernible.
These are the precursors of the alveolar
crest fiber bundle group.
By the time of first occlusal contact of the
tooth with its antagonist, the horizontal group
are almost completely developed.
The oblique fibers in the middle third of
the root are still being formed.
REFERENCE: TEN CATE’S ORAL HISTOLOGY

10. Bone related region
– rich in cells
Middle zone – few
cells , thin collagen
fibres
Cementum related
region – dense,
organised collagen
fibres

11. PERIODONTAL LIGAMENT HOMEOSTASIS
• Various molecules have been proposed, which play a role in maintaining an
unmineralized periodontal ligament.
Msx-2
Osteopontin
and Bone
Sialoprotein
Matrix ‘Gla’
protein
RGD-
cementum
attachment
protein
TGF-𝜷
Prostaglandins
REFERENCE: ORBAN’S ORAL
HISTOLOGY AND EMBRYOLOGY
Msx – 2
Prevents osteogenic
differentiation of
PDL fibroblasts by
repressing Runx2
transcriptional
activity
Matrix ‘Gla’ protein
Inhibitor of
mineralisation –
preserves the width
of the ligament
Prostaglandins
Inhibits mineralised
bone nodule
formation
Bone sialoprotein vs
osteopontin
Helps to maintain
the unmineralized
ligament region

12. HOMEOSTASIS
↑Functional demand
Width increases by 50%
Fibre bundles increased
in thickness
↓Functional demand
Narrowing of the
ligament
↓Number & thickness of
fibre bundles

13. TOOTH
AT TACHMENT
MECHANISM:
ENTHESIS

14. S T R U C T U R E O F T H E
P E R I O D O N TA L L I G A M E N T

15. CELLS OF PERIODONTAL LIGAMENT
Synthetic Resorptive Progenitor
Epithelial
rests of
Malassez
Défense
cells

16. SYNTHETIC
CELLS OF
PDL
FIBROBLAST OSTEOBLAST
CEMENTOBLAST

17. Most
predominant
cell type of
periodontal
ligament.
Found between
the fibers of
the periodontal
ligament,
where they are
surrounded by
fibers and
ground
substance.
STRUCTURE:
Ovoid or
elongated cells
oriented along
the principal
fibres.
Large cells with
an extensive
cytoplasm
Associated with
protein
synthesis &
secretion (e.g.
RER, several
Golgi complex,
& many
secretary
vesicles)
FIBROBLASTS

18. DIFFERENCES
PERIODONTAL LIGAMENT FIBROBLASTS GINGIVAL FIBROBLASTS
Ectomesenchymal origin Mesodermal origin
Expression of alkaline phosphatase and cyclic
AMP
Expression of alkaline phosphatase and
cyclic AMP
More proliferative Less proliferative
Motile and contractile. Generate tooth
movement.
Non motile
Also cause collagen degradation Not capable of collagen degradation

19. Bone forming cells derived from
osteoprogenitor cells.
Found on the surface of the alveolar
bone .
Seen on surfaces of bone giving an
epithelium like appearance. Shape
varies (seen as oral, triangular, cuboidal)
etc.
Responsible for laying down the organic
matrix of bone including the collagen
fibers. Alkaline phosphatase present in
osteoblasts is responsible for its
mineralization.
OSTEOBLASTS-STRUCTURE

20. Maintain the integrity of the
lacunae and canaliculi.
Open the
channels for
diffusion of
nutrition through
the bone.
Play a role in
removal or
deposition of
matrix and of
calcium when
required.
Osteoblasts first migrate away
from the bone surface into the
body of periodontal ligament
before eventually taking up their
functional position.
Osteoblasts form
a cell layer over
forming bone
surface and act
as barrier that
control ion flux
into out bone.
OSTEOBLASTS-
FUNCTIONS

21. These are the cells
responsible for secreting
the organic matrix (mainly
collagen) of cementum.
As distinct layer of cells on
the root surface, similar to
osteoblastic layer but not
regular in arrangement.
CEMENTOBL ASTS

22. RESORPTIVE CELLS OF PDL
Osteoclast
Cementoclast Fibroblast

23. FIBROBLASTS
Capable of both
synthesis and
resorption.
Collagen
degradation
Extracellular
degradation
of Collagen
Intracellular
degradation
of Collagen

24. OSTEOCLASTS-STRUCTURE
Present on the
surface of bone
where it is removed.
At such locations
cells occupy pits
called Resorption
bays or lacunae of
Howship's.
Osteoclasts are
formed by fusion of
mononuclear cells
arising from bone
marrow.
They do not cover
the whole of
resorbing surface at
any one time, rather
they service a much
larger area by
demonstrating
considerable
Inhibitor: Osteo-
protegrin

25. CEMENTOCLASTS
Resemble Osteoclasts.
Located in
Howship's
lacunae.
Origin unknown but believed
be same as that of Osteoclasts.
Since constant
deposition of
cementum
occurs, these
cells are not
seen during
normal
functioning of
cementum.

26. PROGENITOR CELLS OF PDL

27. EPITHELIAL RESTS OF MALASSEZ
Represent the remnants
of developmental
Hertwig’s epithelial root
sheath which is involved
in differentiation of root
odontoblasts and also
secrete enamel like
proteins on the root
surface.
Form a latticework in the
periodontal ligament
appear as either isolated
clusters of cells or
interlacing strands.
Arranged in close
packing of cuboidal cells,
completely surrounded
by connective tissue

28. DEFENSE
CELLS

29. E X T R A C E L L U L A R
S U B S TA N C E S
Fibers
Collagen
Principal
Elastic
Secondary
Reticular
Indifferent fibre plexus
Sharpey's fibres
Ground substance
Glycosaminoglycans
Proteoglycans
Glycoproteins

30. C O L L A G E N
The collagen is gathered to form bundles approximately
5 µm in diameter. These bundles are termed PRINCIPAL
FIBERS.
Composed mainly of amino acids.
The main types of: Type I and Type III.
More than 70% of periodontal ligament collagen is type

31. SYNTHESIS OF COLLAGEN
Fibroblast
Tropocollagen
Microfibril
Fibril
Fiber
Bundle

32. PRINCIPLE
FIBERS
Alveolar
Crest
Horizontal Oblique Interradicular Apical

33. INTERMEDIATE PLEXUS
Earlier it was believed that
principal fibers follow a wavy
course from cementum to bone
and are joined in the mid region
of the periodontal space giving
rise to a zone of distinct
appearance i.e the Intermediate
plexus .
Research over past years suggests
that cemental fibers meet and
fuse with osseous fibers, no such
plexus remains. Secondly the
entire PDL is metabolically active ,
not just the middle or
intermediate zone (Thomas M.
The recent concept is that, fibers
cross the entire width of
periodontal space but branch en
route and join neighboring fibers
to from a complex three
dimensional network .

34. EL A STIC FI BRES
ELASTIC
FIBRES
IMMATURE
ELAUNIN OXYTALIN
MATURE
ELASTIN

35. O T H E R F I B R E S

36. RETICULAR FIBERS
• These are immature collagen fibers with argyrophilic staining properties and
are related to basement membrane of blood vessels and epithelial cells
which lie within the periodontal ligament.
SECONDARY FIBERS
 Represent the newly formed collagenous elements, not yet incorporated
into principal fiber bundle.
 Located between and among the principal fibers.
 These are relatively non-directional and randomly oriented.
 Appear to transverse the periodontal ligament space corono-apically and
are often associated with path of vasculature and nervous elements.

37. SHARPEY’S FIBERS
Collagen fibers are embedded
into cementum on one side of
the periodontal space and into
alveolar bone on the other.