The document discusses the composition and structure of the periodontal ligament. It notes that the periodontal ligament consists of cells such as fibroblasts and an extracellular matrix containing collagen fibers, ground substance, and other proteins. It connects tooth cementum to alveolar bone and functions to support teeth and withstand forces. The fibroblasts are responsible for maintaining and remodeling the ligament in response to forces through synthesis and degradation of the extracellular matrix.

1. Dr Harshavardhan Patwal

2.  Introduction
 How unique is periodontal ligament?
 Periodontal ligament cells & extracellular matrix constituents.
 Ground substances.
 Periodontal ligament fibers.
 Collagen
 Periodontal ligament homeostasis and adaptation
to functional demand.
 Degradation And Remodelling Of Connective Tissue Matrix.

3.  Mechanisms of degradation and remodelling of connective tissue
matrix.
 Fibroblast To Matrix Adhesions And Tractions

4.  The periodontal ligament is composed of a complex vascular & highly cellular
connective tissue that surrounds the tooth root & connects it to the inner wall of
the alveolar bone (McCulloch CA et al).
 It is continuous with the connective tissue of the gingiva & communicates with
the marrow spaces through vascular channels in the bone.
 Average width of periodontal ligament space is about 0.2mm
 Space is diminished around teeth
 Not in function
 Unerupted teeth.

5.  Increased in teeth subjected to hyperfunction.
 The periodontal ligament is unique among the various ligament and
tendon systems of the body in that it is the only ligament to span two
distinct hard tissues – namely, tooth cementum and bone.
 Function:
 Supporting the teeth in their sockets.
 Withstand considerable forces of mastication.
 It has the capacity to act as a sensory receptor necessary for the
proper positioning of the jaws during mastication.
 It is a cell reservoir for tissue homeostasis and repair ⁄ regeneration.

6.  When periodontal ligament cells are removed from the
cementum by mechanical means or displaced or altered under
the influence of certain compounds-ankylosis may occur.
 Bone tissue invade the periodontal ligament space & establish
a direct connection between the tooth & the wall of the
alveolar socket.

7.  When periodontal ligament fibroblasts or their progenitors are
allowed to gain access to the root & repopulate the area.
 These cells may come from the adjacent periodontal ligament.
 Invasion of periodontal ligament fibroblasts in an ankylosed site must
be preceded by cells that have the capacity to resorb bone /
cementum.
 A new periodontal ligament space may be created at the cost of the
cementum & also of the alveolar process.

8.  Shortly, this space is colonized by periodontal ligament cells that form
new periodonal ligament fibers,new cementum & a new alveolar wall
in which the fibers insert (Wasserlink et al 1994).
 Moreover,masticatory function can accelerate the resolution of
ankylotic areas & restoration of normal periodontal ligament width
(Andersson et al 1985).
 Boyko et al 1981- seeded fibroblasts from either periodonal ligament or
gingiva onto the surface of tooth roots, which were subsequently
inserted in artificial sockets.

9.  Apart from ankylotic areas, - areas with newely formed periodonal
ligament fibers inserted in newely formed cementum.
 Observed only when periodontal ligament fibroblasts were used as
seeding cells.
 Gingival cells could not induce regeneration of the periodontal
ligament (Lang et al).
 Indicating that the periodontal ligament cells have a specialized
properties.

10.  Cellular elements.
 Periodontal fiber groups.
 Ground substances.

11.  The periodontal ligament consists of cells and an extracellular
compartment comprising collagenous and noncollagenous matrix
constituents.
 The cells include osteoblasts and osteoclasts, fibroblasts, epithelial cell rests
of Malassez, monocytes and macrophages, undifferentiated mesenchymal
cells, and cementoblasts and odontoclasts.
 The extracellular compartment consists mainly of well-defined collagen
fiber bundles embedded in an amorphous background material, known as
ground substance.

12.  The predominant cell type is the fibroblast, which occupies about
30% of the volume of the periodontal ligament space in rodents
(Beertsen W,).

13.  FIBROBLASTS

14.  The fibroblasts of the periodontal ligament originate in part from the
ectomesenchyme of the investing layer of the dental papilla and from the
dental follicle (Ten Cate AR et al) and are different from cells in other
connective tissues in a number of respects.
 Freeman & Ten Cate demonstrated that periodontal ligament
fibroblasts near the cementum are derived from the ectomesenchymal
cells of the investing layer of the dental papilla, while fibroblasts near
the alveolar bone are derived from perivascular mesenchyme.
 Although periodontal ligament cells are frequently considered as a
homogeneous population, there are some data indicating that the
periodontal ligament contains a variety of fibroblast populations with
different functional characteristics (McCulloch CAG et al).

15.  Whether these subsets are derived from a single type of progenitor cell
is unknown.
 The fibroblasts on the bone side of the periodontal ligament exhibit
more abundant alkaline phosphatase activity than those on the tooth
side (Groeneveld MC et al).
 This enzyme plays a key role in phosphate metabolism probably in the
mineralization process (Beertsen et al) & perhaps also in acellular
(afibrillar) cementum formation (Groeneveld et al)

16.  Alkaline phosphatase activity in rat molar PDL shows a high corelation
with acellular cementum thickness- patient suffering from
hyphophosphatasia.
 Cementum formation is greatly impaired.
 The enzyme is not restricted to cementoblasts & osteoblasts but is
found in all periodontal ligament fibroblasts, especially along their
outer plasma-membrane.
 The rapid degradation of collagen by fibroblast phagocytosis is the
basis for the very fast turnover of collagen in the periodontal ligament
(Everts V,et al).

17.  The generation of highly specialized cell populations that can remodel
and heal damaged tissues in a temporally and spatially appropriate
manner is thought to be essential for the repopulation and
differentiation responses in healing periodontium.
 The signals that regulate these processes include cell matrix and direct
cell-cell interactions which are known to control cell proliferation,
differentiation and cell function (Gumbiner BM et al).
 For example, periodontal ligament cells produce cell adhesion proteins
like vitronectin, tenascin and undulin as well as several integrin
subunits (Steffensen B et al).

18.  The fibroblasts of the periodontal ligament are connected by
specialized junctional complexes which include as gap junctions
(Shore RC et al).
 Notably, mechanical stimulation of the periodontal ligament
stimulates the expression of connexin, an important protein in the
formation of gap junctions in periodontal ligament cells (Lewis JE et
al).

19.  The PDL fibroblast contains a prominent nucleus, which has a single
distinct nucleolus & clearly defined nuclear pores.
 It has been suggested that periodontal fibroblasts have a role in
generating the force of eruption by contraction, in a way similar to
myofibroblasts that are responsible for wound contraction.
 Sodek 1977 – as fibroblasts produce the extracellular matrix of the PDL
which demostrates a very high rate of turn over, the cells contain
significant amounts of the organelles involved in protein synthesis &
degradation.

20.  Epithelial cells:
 The epithelial cells in the periodontal ligament are remnants of HERS
and known as the epithelial cell rests of Malassez.
 They occur close to the cementum as a cluster of cells that form an
epithelial network, and seem to be more evident or abundant in furcation
areas.
 Undifferentiated mesenchymal cells:
 An important cellular constituent of the periodontal ligament is the
undifferentiated mesenchymal cell, or progenitor cell.
 The fact that new cells are being produced for the periodontal ligament
whereas cells of the ligament are in a steady state means that selective
deletion of cells by apoptosis must balance the production of new cells.

21.  In periodontal wound healing, the periodontal ligament contributes
cells not only for its own repair but also to restore lost bone and
cementum ( Anusaksathien O,et al).
 Recently, cells with stem cell characteristics have been isolated from
the human periodontal ligament (Hollenbach E,et al).

22.  Filling The Space Between The Fibers & Cells.
 Glycosaminoglycans
 Hyaluronic Acid
 Proteoglycans
 Glycoproteins
Fibronectin
Laminin

23.  The cell surface proteoglycans participate in several biological
functions, including:
 Cell adhesion,
 Cell-cell & cell matrix interactions,
 Binding to various growth factors as co-receptors
 Cell repair (Worapamorn W et al)

24. Ground substance:
 The periodontal ligament ground substance has been estimated to be
70% water and is thought to have a significant effect on the tooth’s
ability to withstand stress loads.
 There is an increase in tissue fluids within the amorphous matrix of the
ground substance in areas of injury and inflammation.

26.  Periodontal ligament Fibers:
 The predominant collagens of the periodontal ligament are type I, III, and
XII, with individual fibrils having a relatively smaller average diameter than
tendon collagen fibrils.
 The vast majority of collagen fibrils in the periodontal ligament are arranged
in definite and distinct fibre bundles, and these are termed principal fibers.
 Elastic fibers: There are three types of elastic fibers:
 Elastin, Oxytalan, and Elaunin.
 Only oxytalan fibers are present within the periodontal ligament; however,
elaunin fibers may also be found in association with fiber bundles in the
gingival ligament.

27.  Oxytalan fibers:
 These are bundles of microfibrils that run more or less vertically from the
cementum surface, forming a three-dimensional branching meshwork that
surrounds the root and terminates in the apical complex of arteries, veins,
and lymphatics.
 They are also associated with neural elements.
 They are thought to regulate vascular flow in relation to tooth function.
 Because they are elastic, they can expand in response to tensional
variations, with such variations then registered on the walls of the vascular
structures.

28.  Principal fibers- which are collagenous & arranged in bundles & follow a
wavy course.
 The terminal portions of the principle fibers that are inserted into
cementum & bone – sharpeys fibers.
 They are associated with abundant noncollagenous proteins typically
found in bone & cementum (Mc Kee MD et al)
 Osteopontin
 Bone sialoprotein
 These proteins are thought to contribute to the regulation of
mineralization & to tissue cohesion at sites of increased biomechanical
strain.

29.  The principal fibers of the periodontal ligament are arranged in six
groups:(Sloan & Carter)
 Transseptal
 Alveolar crest
 Horizontal
 Oblique
 Apical
 Interradicular

30.  Components of the extracellular matrix:
 Collagen:
 Collagens are a large family of triple helical proteins that are widespread
throughout the body and are important for a broad range of functions, including
tissue scaffolding, cell adhesion, cell migration, cancer, angiogenesis, tissue
morphogenesis and tissue repair.
 It is a protein composed of different amino acids, the most important of which are
glycine, proline, hydroxylysine & hydroxyproline (Carneiro J et al).

32.  Each polypeptide chain has a repeating Gly-XY triplet in which glycyl
residues occupy every third position and the X and Y positions are
frequently occupied by proline and 4-hydroxyproline, respectively.
 The three α chains are held together by interchain hydrogen bonds.
Highly ordered hydration networks surround the triple helices.
 The amount of collagen in a tissue can be determined by its
hydroxyproline content.
 Collagen is responsible for maintenance of the frame work & the tone
of the tissues & it exhibits a wide range of diversity.

33.  Vertebrate collagens are classified by function and domain
homology:
 Fibril-forming collagens: (I,II,III,V,XI)
 Fibril-associated collagens with interrupted triple helices (FACITs): (IX,XII,XIV,XVI)
 Network-forming collagens :( IV,VIII,X)
 Transmembrane collagens:(XIII,XXV,XVII)
 Endostatin-producing collagens:(XV,XVIII)
 Anchoring fibrils:(VII)
 Beaded-filament-forming collagen:(VI)

34.  Collagen is synthesized by fibroblasts, chondroblasts, osteoblasts,
odontoblasts.
 The principle fibers are composed mainly of collagen type I (Bosshardt
DD et al)
 Reticular fibers are composed of type III.
 Collagen type IV is found in basal lamina (Romanos GE et al)
 Type VI collagen has been immunolocalized in periodontal ligament &
gingiva (Everts V et al)

35.  Recent in vitro studies have shown that type VI collagen simulates
fibroblasts proliferation in a non-integrin-mediated pathway(Atkinson JC et
al)-
 Type V collagen – associated with the cell surface & to coat larger type III &
type I fibrils.
 Immunocytochemical studies have also localized collagen types XII & XIV in
the PDL (Karimbux NY et al).
 Type XII, a member of the fibril associated collagens, forms small fibrils that
have a role in the organization of the network of larger collagen fibrils.

36.  N.Y. Karimbux and I. Nishimura- Temporal and Spatial Expressions of
Type XII Collagen in the Remodeling Periodontal Ligament during
Experimental Tooth Movement.
 The type XII collagen expression may be closely associated with the
functional regeneration of the PDL.

37.  Collagen biosynthesis occurs inside the
fibroblasts to form tropocollagen
molecules.
 In collagen type I & III the fibrils associate
to form fibers, & in collagen type I the
fibers associate to form bundles.

38.  In comparision to collagens,the non-collagenous proteins occur in
small amounts in the PDL.
 The adhesion molecules, fibronectin, tenascin, & vironectin, are
among the glycoproteins found in the PDL (Pitaru et al).
 Fibronectin is widely distributed in the PDL.
 Vitronectin is an attachment factor associated with elastic fibers in
loose connective tissue.

39.  It has been localized throughout the PDL, including in cells lining
cementum & bone surfaces.
 Vitronectin participates in the regulation of blood coagulation,
plasminogen activation, & fibrinolysis.
 The fibroblasts interact with the extracellular matrix through
receptor-ligand interactions.
 Many of the matrix ligands are noncollagenous proteins, such as
extracellular adhesion factors (Hynes et al).
 Eg,binding of a fibronectin fragment to the a5b1 fibronectin receptor
of rabbit synovial fibroblasts leads to secretion of collagenase.

40.  In contrast,when a5b1 receptors are occupied by inact fibronectin
molecules,no collagenolytic response is observed.
 PDL fibroblasts react in a similar way to fibronectin fragments by
increasing the expression of collagenase & stromelysin.
 It is increasingly apparent that receptor-matrix informational
exchange is involved in regulating fibroblast connective tissue
remodelling.

41.  Such a unique and dynamic connective tissue system involving multiple
tissues requires exquisite regulation at the cellular level.
 Maintenance and remodeling of periodontal ligament collagen fibers
(Deporter DA, Ten Cate AR), together with the embedding and
calcification of their extremities to form Sharpey’s fibers (Johnson RB et
al), requires the concerted action of numerous cell types (Garant PR et al)
 Central to these integrated activities is the periodontal ligament
fibroblast, whose responsibilities include the formation and remodeling
of the periodontal ligament fibers, and presumably a signaling system to
maintain periodontal ligament width and thickness across the soft tissue
boundary

42.  In the periodontal ligament, cellular signals are, in part, mediated by the
forces transmitted to the fibroblasts via collagen fibrils with which they
are in direct contact( Garant PR et al).
 At the tissue level, periodontal ligament fibroblasts are rather regularly
dispersed throughout the ligament and are generally oriented with their
long axis parallel to the direction of the collagen fibrils.
 During development and the initial formation of the periodontal
ligament, the cytoplasm-to-nucleus ratio is high, and fibroblasts appear
very active in terms of having an extensive network of rough
endoplasmic reticulum, a well-developed Golgi apparatus and abundant
secretory granules containing predominantly type I collagen

43.  MATRIX REMODELING & ADAPTABILITY.
 Several studies have indicated that the extracellular matrix
collagens of the periodontal ligament have an extremely high
turnover and remodeling rate, much higher than in gingiva, skin and
bone (Sodek J.).
 Turnover and remodelling in the periodontal ligament imply
synthesis and breakdown of matrix components, particularly the
collagenous fiber meshwork that extends between cementum and
bone.

44.  Turnover describes a process in which the structural organization of
the tissue remains unchanged.
 During remodelling the three-dimensional organization of the fiber
meshwork is adapted to accommodate for positional changes of the
tooth in its socket or changes in functional state (such as
hypofunction) (Beertsen et al).
 Both processes can occur simultaneously & may therefore be
indistinguishable.
 Sodek et al rapid remodeling is a unique characteristic of the
periodontal ligament that relates to the adaptability of the periodontal
tissues.

45.  (Sicher H.et al) suggested that remodeling of the ligament is confined
largely to the mid-region of the periodontal ligament where fibers
from the bone and fibers from the tooth interdigitate in an
‘‘intermediate plexus’’.

46.  Turnover and remodeling activity in teeth of limited eruption, like the
molars of rodents, are found throughout the width of the periodontal
ligament from cementum to bone (Rippin JW.).
 To adapt to changes of tooth position, the fiber systems in the
periodontal ligament must be degraded and new fibers synthesized.
 Since the periodontal ligament is not made up of single strands of
straight collagen fibers but consists instead of a complex meshwork,
remodeling does not necessarily occur at all sites synchronously.

47.  There is apparently some flexibility in the system to permit adaptational
changes by breaking down short stretches of collagen fiber bundles or
single fibrils while leaving others intact.
 This highly localized remodeling process Is facilitated by the
phagocytosis of collagen.
 Unlike the bulk removal of collagen that is effected by extracellular
matrix metalloproteinases, collagen phagocytosis enables periodontal
ligament fibroblasts to very precisely remove collagen fibrils at specific
sites (Everts V).

48.  Svoboda et al- relationship between turnover rate of collagen in the
various tissues constituting the periodontium & the amount of
collagen ingested by fibroblasts, the highest amount being found in
the tissues with the highest turnover, most notably the periodontal
ligament.

49.  A remarkable capacity of the periodontal ligament is that it maintains
its width more or less over time, despite the fact that it is squeezed in
between two hard tissues.
 Compelling evidence exists indicating that populations of cells within
the periodontal ligament, both during development and during
regeneration, secrete molecules that can regulate the extent of
mineralization and prevent the fusion of tooth root with surrounding
bone, e.g. ankylosis.

50.  Balance between the activities of bone sialoprotein and osteopontin
may contribute to establishing and maintaining an unmineralized
periodontal ligament region.
 Matrix Gla protein is also present in periodontal tissues; based on its
role as an inhibitor of mineralization, it may also act to preserve the
periodontal ligament width.
 At the cell level, Msx2 prevents the osteogenic differentiation of
periodontal ligament fibroblasts by repressing Runx2 ⁄ Osf2
transcriptional activity (Kanda-Nakamura C et al).

51.  The periodontal ligament has also the capacity to adapt to functional
changes.
 When the functional demand increases, the width of the periodontal
ligament can increase by as much as 50%, and the fiber bundles also
increase markedly in thickness.
 Conversely, a reduction in function leads to narrowing of the ligament
and a decrease in number and thickness of the fiber bundles.
 These functional modifications of the periodontal ligament also
implicate corresponding adaptive changes in the bordering cementum
and alveolar bone.

52.  Breakdown of the collagenous matrix is a normal event in tissues
undergoing morphogenesis, morphostasis and growth.
 However, it is vital that this process is subject to rigid control, as failure
to maintain an appropriate balance between degradation and synthesis
can lead to net destruction or net gain, resulting,

53.  The earliest manifestations of vitamin C deficiency (scurvy) were seen
as intraoral lesions and tooth loosening (Hunt and Paynter, 1959),
which indicated that the periodontal tissues were subject to rapid
turnover of collagen.
 The earliest attempts to determine collagen turnover in the PDL used
autoradiographic techniques (Stallard, 1963 et al).
 These studies (mostly using 3H-labelled proline) suggested that
turnover of collagen in the periodontal tissues was rapid.
 However, the technique fails to discriminate between specific
radiolabelling of collagen and incorporation of the tracer into other
proteins, many of which (particularly intracellular proteins) turn over
at a much higher rate than extracellular proteins.

54.  Orlowski (1976, 1978), overcame these difficulties in part bY
measuring the specific activity of hydroxyproline 24 hours after
injection with 3H-proline.
 Turnover in the PDL was found to be higher than that of the gingival,
with a half life of around 9.5 days and a turnover time of 13.5 days for
the rat incisor PDL.
 Sodek (1976, 1977) utilized a novel short-term approach to measure
incorporation of 3H-labelled proline into newly synthesized (and
therefore salt-extractable) and mature collagen in the rat molar PDL,
gingival, alveolar bone and skin.

55.  Hydroxyproline-specific activities for theses tissues revealed that the
rate of collagen synthesis in the PDL was twice that in gingival, four
time that in skin, and six times that in alveolar bone.
 This study also showed that the conversion of newly synthesized
collagen into mature insoluble collagen was highly efficient in the PDL
compared with the other tissues (which showed up to 50 per cent
degradation of newly synthesized collagen).
 This supported the Suggestion of Guis and Slootweg (1973) based
upon collagen extractability, that newly synthesized tropocollagen
matured rapidly in bovine PDL.

56.  The half life of mature collagen in rat molar PDL was calculated by
Sodek to be 1 day, compared with 5 days in gingival, 6 days in alveolar
bone and 15 days in skin.
 (Sodek, 1978) comparing molar and incisor PDLs calculated a longer
half life of 3 days for mature collagen in the incisor ligament.
 Imberman et al. (1986) applied the so-called ‘pool expansion’
technique in order to determine the half life of collagen in the rat
incisor and molar PDLs; they then compared these with those of the
skin, gingival, and palatal mucosa.

57.  These authors calculated the half lives of collagen to be: 7.8 days for
incisor PDL, 8.8 days for molar PDL, 150 days for incisor gingival
tissue, 8.8 days for molar gingival, 50 days for skin and 21 days for
palatal mucosa.
 Finally, using Poole’s approach (1971), Sodek and Ferrier (1988)
attempted to validate the apparently rapid rate of collagen turnover in
the periodontal tissues of the rat.
 This study yielded half life values for mature, insoluble collagen as 3
days for molar PDL, 6 days for incisor PDL, and 10 days for gingiva.

58.  The discrepancies between values reported by different workers using
a variety of techniques clearly indicate the difficulties involved in
obtaining definitive data for collagen turnover rates in the PDL and
other connective tissues in general.
 However, the general consensus is that the turnover of collagen in PDL
is unusually high.
 This may have important implications in the aetiology of chronic
inflammatory periodontal disease , such that an imbalance in the
synthesis and degradation process may result in net collagen loss
(Page and Ammons, 1974).

59.  Sodek (1989) suggested that the differences in turnover rate with tooth type
might be due to direction of the functional loading on the tissue, i.e
tensional as opposed to compressional.
 Finally, perhaps related to the constraints turnover of collagen in the PDL
also appears to be in some way linked with rate of eruption, (Berkovitz,et
al).
 Rippin (1976, 1978) reported an increase in collagen turnover with
reactivated eruption following extraction of opposing teeth, but no such
increase was observed when eruption was accelerated in the rat incisor (Van
den Bos and Tonino, 1984).
 The precise relationship between rate of eruption and PDL collagen
turnover remains obscure.

60.  Degradation of the extracellular matrix can occur through a number of
different pathways, including activation of matrix metalloproteinases
(MMPs), release of reactive oxygen species and phagocytosis of matrix
components,release of a wide range of cytokines & other
inflammatory mediators that affect the enzyme release & fibroblasts
function.

61. Matrix metalloproteinases:
 Matrix metalloproteinase gene family encodes a total of 24
homologous proteinases, classified into collagenases, gelatinases,
stromelysins, membrane type matrix metalloproteinases and other
matrix metalloproteinases depending on their substrate specificity
and molecular structure (Uitto VJ et al).
 MMPs are secreted by connective tissue cells (predominantly
fibroblasts) but are also produced by some leucocytes
(polymorphonuclear neutrophil leucocytes and macrophages.)

62.  The secreted MMPs are subsequently activated by proteinases such as
plasmin, which in turn are regulated by tissue protein factors such as
plasminogen activators.
 The plasminogen activators are serine proteinases.

63.  Matrix metalloproteinases play a major role in connective
tissue breakdown (Birkedal-Hansen H,et al).
 To date, MMP-1, -2, -3, -8, -9, and -13 have been identified in
inflamed periodontal tissues.
 The enzymes synthesized by fibroblasts & epithelial cells are
belived to be mosetly involved in normal tissue remodeling
(sodek et al)

64.  LPS & TNF-alpha activate MMP synthesis by keratinocytes.
 IL-I enhances MMP expression by fibroblasts, whereas LPS stimulate
PGE2 production by these cells.
 The activities of PGE2 include suppression of cell proliferation,
inhibition of collagen synthesis, & with IL-6, stimulation of bone
resorption (page et al).
 Bacterial plaque products can also stimulate MMP production by a
variety of cells (Birkedal-Hansen et al).

65.  Tissues also contain another group of matrix metalloproteinase
inhibitors known as tissue inhibitor of metalloproteinases (TIMPs)
(Brew K,et al).
 At least four tissue inhibitor of metalloproteinases (TIMP-1, TIMP-2,
TIMP-3, TIMP-4) are expressed by vertebrates and these act by
preventing the conversion of precursor forms of matrix
metalloproteinase to their active forms.
 TGF-b, steroids, & inteferon –y induce the expression of TIMP-1 &
TIMP-2.

66.  Tetracycline & their analogues –inhibit (ingman et al).
 Tetracycline, which also inhibit the expression of 72-kd gelatinase
(MMP-2), does not affect fibroblasts collagenase, indicating that the
MMP in gingival crevicular fluid are derived from PMN (Ingman et
al).

67.  Phagocytosis
 Phagocytosis is also a significant pathway of collagen degradation in the
physiological turnover and remodeling of periodontal connective tissues
(Ten Cate AR et al).
 During normal tissue homeostasis, collagen degradation can take place
within the lysosomal apparatus of phagocytic cells.
 Phagocytosis of collagen has been proposed to involve recognition of
collagen fibrils through specific membrane-bound receptors (possibly
integrins), followed by partial digestion of the fibrils by proteinases
such as gelatinases (MMP- 2, -9) and final degradation by lysosomal
enzymes cysteine proteinases such as cathepsin B or L (Everts V,et al).

68.  Cells defective in phagocytosis may contribute to gingival overgrowth
and fibrosis as well as compromise normal wound repair and tissue
regeneration (McCulloch CA et al).

69.  Reactive oxygen species:
 Oxygen-derived free radicals such as the superoxide radical and the
hydroxyl radical are integral reaction products of normal cellular
metabolism, but these are elevated in cells undergoing active respiratory
bursts (Baboir BM).
 Tissues and cells may be exposed to oxygen-derived free radicals during
inflammatory reactions, particularly where polymorphonuclear leukocytes
and macrophages are in abundance.
 Oxygen-derived free radicals are highly reactive molecular species which
can disrupt cellular proteins, nucleic acids and membrane lipids as well as
cause depolymerization of matrix components such as collagen,
hyaluronan and proteoglycans (Freeman BA).

70. What advantage could there be in degrading collagen
intracellularly instead of extracellularly?
 Phagocytosis allows a more precise & selective control for the collagen
fibers to be degraded.
 Whereas the release of extracellular collagenolytic enzyme affords a
more rapid, extensive degradation around the cells,observed during
inflammation.
 Together all these studies indicate that the PDL is characterized by a
rapid turnover & a high remodeling capacity.

71.  Under steady-state conditions, collagen degradation in this
tissue is carried out via a process of collagen phagocytosis
without the involvement of collagenase.

72.  When cells come into contact with one another, & sometimes with the extracellular matrix,
specialized junctions may form at specific sites on the contacting cell membranes.
 The specialized junctions may be classified as
 Occluding (tight) junctions (zonula occludens)
 Adhesive junctions
 Cell-cell
 Zonula adherens
 Macula adherens
 Cell-matrix
 Focal adhesions
 Hemidesmosomes
 Communicating (gap) junctions.

73.  Cell-matrix junctions have a structural organization similar to that of
cell-cell adhesive junctions, but they use different molecular
components & attach the cell to the extracellular matrix.
 In focal adhesions the transmembrane component is a member of the
integrin family of adhesion molecules.
 Integrins are heterodimers of different alpha & beta subunits that
occur in different combinations with specificity for various
extracellular matrix molecules.

74.  The cytoplasmic adapter proteins, which include the actin-binding
proteins a-actinin,vinculin, & talin, link the transmembrane integrins
to the actin cytoskeleton.
 Binding of the integrin to collagen, laminin, fibronectin, & other
extracellular matrix proteins results in recruitement & remodeling of
actin cytoskeleton.
 Ligand binding by integrins also leads to the recruitment & activation
of various intrcellular signaling molecules, including guanine
nucleotide-binding proteins & several protein kinases.

75. Fibroblast To Matrix Adhesions And Tractions
• Fibroblast attached to the substratum of the extra cellular matrix via
surface receptors for collagen and fibronectin.
 Attachment to the substratum is essential for cellular migration and for
organization for the extra cellular fibrillar matrix.
 In the formation of these adherent contacts, the cell membrane integrin
a5 b1 attaches to the arginine- glycine-aspartic acid sequences of
fibronectin.
 Assembly is initiated by binding of soluble fibro nectin molecules to the
cell of its integrin receptors A5b1 and avb3

77.  The cytoplasmic domain of the integrin receptor attaches to the
peripheral cytoplasmic protein talin, which in term interacts with the
protein called vinculin.
 With the binding of fibronectin to collagen fibrils , the molecular
linkage extends from the cytoplasmic contractile apparatus to an
extracellular fibrin network-exerting traction on collagen fibers.
 In fibroblast under tension, filamentous actin and smooth muscle
myocin associate to from contractive stress fiber that terminated
plasma membrane in fibro nexus junctions.

78.  Through such cell to matrix contact the extra cellular
matrix can exert and effect on cells shape and behaviour.
 Tension in the ECM is transmitted to fibroblasts integrin
receptors, leading to signaling event that alters the activity
of te cell.

80.  Human PDL fibroblast respond to increased tension by upregulating the
expressions of IL-1b103 and by secreting prostaglandin E2.
 In this “outside –in” type of signalling, tension transmitted to the fibroblast
causes a rise in the activity of several small guanosine triphosphatases, which
regulate the enzyme cascades that lead to changes in cell shape and function.
 Cells can also alter the binding strength of their integrin receptors through
cytoplasmic signaling pathways.
 This represents a form of “inside-out” signal transduction.
 The linkage between the cell surface and the immediate extracellular matrix
serves as a node whereby the cell can receive regulatory information from the
outside and provides a mechanism for exerting an organizing influence on the
adjacent matrix.

81.  When fibroblasts move through a collagen gel matrix, they tend to
align collagen fibrils parallel to the long axis of migration.
 If the substratum on which cells are placed is sufficiently anchored-
cells move over the substratum.
 Not anchored-pulled towards the cells.
 In vitro studies of matrix contraction have shown that fibroblasts are
able to generate the same degree of tension-contracting wound.
 PDGF & IGF-1 & TGF-b promote fibroblasts contraction of type I
collagens gels.

82.  PDGF promotes gel contraction by stimulating actin cytosketetal polymerization &
increase the expression of integrins.
 Both gingival & PDL fibroblasts exhibit high gel-contracting abilities.
 Each fibroblasts may have a minimum of 105 fibronectin receptors.
 On highly mobile fibroblasts-receptors are diffusely distributed.
 Stationary cells-arranged in linear arrays.
 Immunocytochemical localization of fibronectin in the periodontal ligament has
shown that it forms aggregates about 90nm thick.(Cho Mi et al)

83.  The fibronexus is a terminal for anchorage of stress fibers to the cell
surface and the ECM .
 The stress fibers comprise well defined bundles of actin and non
sarcomeric myosin oriented parallel to the long axis of the fibroblast .
 Stress fibers are found in fibroblasts involved in transferring tension to an
extracellular fibre network that is firmly attached to stable structures in
their immediate vicinity and PDL fibroblasts can form robust stress fibers
similar to those observed in myofibroblasts of healing wounds.
 Well developed stress fibers and fibronexus contact have been observed
in fibroblasts of the transseptal fiber group between molar teeth of
monkeys.(Garant PR)

84.  V.-J. Uitto- Expression of Fibronectin and Integrins in Cultured
Periodontal Ligament Epithelial Cells.
 During the differentstages of cell growth and spreading, the integrins
appeared to become reorganized, which facilitated cell-cell contact
and allowed for migration of the cell colonies.

85.  PDL has a high potential for regeneration & repair.
 Regeneration of a functional ligament requires corelated
development of new cementum & bone for the attachment of
sharpeys fibers.
 Repair of the PDL involves the replacement of small areas of
damaged ligament.
 In repair, new fibroblasts are derived from perivascular progenitor
cells in the adjacent normal PDL.

86.  Migration of fibroblasts into the area to be repired is facilitated by the
presence of fibrin & fibronectin networks.
 New collagen fibers are laid down raoidly & often without functional
orientation or attachment to the adjacent hard tissues.
 Studies of potential progenitor-cell pools have shown that the marrow
spaces of the alveolar bone,particularly along lateral communications
between the PDL & the marrow, are sites of cell proliferation.

87.  After extensive damage to the PDL connective tissue, the PDL
compartment is populated by an increased number of bone-forming
cells, & ankylosis of the tooth to the alveolar bone results.
 Surgical attempts to regenerate new PDL attachment have revealed
that success depends on:
 After removal of inflamed tissue, the root surface must be debrided of
contaminants,such as bacterial endotoxins.
 Gingival epithelial cells must be prevented from gaining access to the
root surface.

88.  The geometric nature of the lesion to be repired is a factor in the
prognosis for success.
 Defects with intact lateral bone surfaces & with an ample amount of
normal PDL adjacent to the area to be repaired are more likely to
undergo satisfactory regeneration than are lesions that have
horizontal loss of attachment.
 Application of growth factors:
 Single application of PDGF & IGF to the root surface produced new
cementum with functionally oriented PDL fibers & new crestal
alveolar bone 4wks postsurgery in monkeys.

89.  Regeneration of PDL is accelerated by the application of PDGF-BB to
acid – demineralized root surfaces in membrane-guided repair of
furcation defects in beagle dogs.
 BMP-7-significant new bone,cementum,& PDL within 8weeks
following itts application to furcation defects in beagle dogs.
 Recent studies have shown that amelogenin acts as a cell adhesion
factor.

90.  Clinical periodontology-carranza.
 Oral cells & tissues-garant.
 Biology of the periodontal connective tissues
 Oral histology-tencate.
 Perio 2000-2000
 Perio2000-1997.
 Perio2000-2006.
 The periodontal ligament-berkovitz.