The connective tissue of the PDL consists of:
(A) Cells
(B) Extracellular substance formed of ground substance and fibers.
There are also rich supplies of blood vessels, nerves and
lymphatics.
 
[A] cells
The cells of the PDL may be divided into the following categories; 1-Synthetic
cells; fibroblasts; cementoblasts; and osteoblasts. 2-Resorpative cells;
fibroblasts; osteoclasts and cementoclasts. 3-Progenitor cells; undifferentiated
mesenchymal cells. 4-Epithelial rests of Malassez.
5-Defense cells; macrophages; mast, plasma cells; and leukocytes.
[ 1 ] Synthetic cells
There are certain general cytological criteria that distinguish all cells that
are synthesizing proteins and these can be applied to the fibroblasts,
cementoblasts and osteoblasts.
 
The criteria of protein synthesizing cells include:
□ The presence of a large open-faced or vesicular nucleus containing prominent
nucleoli. This reflects the increase transcription of RNA and production of
ribosomes in the nucleolus.
□ Presence of large quantities of rough endoplasmic reticulum (RER) and
prominent Golgi saccules which are readily recognized by EM and which reflect
translation and transport of protein.
□ Presence of large number of mitochondria which reflects the increased
requirements for energy.
□ Large amount of cytoplasm to accommodate these organelles.
 
The cell that is actively secreting protein will be seen under the light
microscope to exhibit a large open-faced or vesicular nucleus with prominent
nucleoli and to have abundant cytoplasm. The cytoplasm tend to be
haematoxyphilic which reflects the presence of large quantities of RER. A clear
area representing the Golgi saccules may be recognized in the cytoplasm if the
plane of section is favorable.
Low or nonactive secreting cells will be seen under the light microscope to
exhibit little of cytoplasm, very few organelles and a close-faced nucleus.
Cells of different stages of activity are present in the PDL.
□ Fibroblasts
They are found in the PDL surrounded by fibers and ground substance and oriented
mostly parallel to the collagen fibers. They are the most important PDL cells
because of the high density of collagen composing the tissue.
PDL fibroblasts are motile – contractile cells and they are capable of
generating a force responsible for tooth eruption.
Beside the above described criteria of the protein synthetic cells, the PDL
fibroblasts show numerous microtubules and microfilaments which run along the
length of the cell (assume the property of motile ceils).
There is evidence that a single cell can function information {fibroblastic
function) and in resorption or destruction (jibroclastic function) of the
ligament collagen. One end of the cell is active in phagocytizing collagen and
contains lysosomal system as well as intracellular collagen profiles and
vacuoles, while the other end of the same cell is active in assembling the
pro-collagen molecules.
 
□ Cementoblasts
They are found distributed on the cementoid tissue of the root surface, as
described in the cementum section.
 
□ Osteoblasts
They are found at the periodontal surface of the alveolar bone in various stages
of differentiation. It is an oval cell with eccentric nucleus and show few
cytoplasmic processes. Under the EM, it shows the above criteria of the protein
synthetic cells. It is also rich in alkaline phosphatase and pyrophosphatase
enzymes which are concerned with calcification.
They synthesize organic components of bone matrix to form osteoid tissue as well
as its maturation.
[21 Resorptive cells
□ The fibroblasts : having a fibroclastic action.
□ Osteoclasts:
The presence of osteoclasts on the periodontal surface of the alveolar bone
indicates that resorption is active. Osteoclasts are seen regularly in normal
functioning periodontal ligament in case of bone remodeling. They are found in
Howships's lacunae present on surface of bone or surrounding the end of bone
spicule.
The cells are formed as a result of fusion of multiple blood monocytes.
They are large multinucleated (6-12 nuclei) cells presenting striated or ruffled
border and eosinophilic cytoplasm rich in lysosomes.
They show high activity of acid phosphatase, collagenase, and other proteolytic
enzymes that are important in bone resorption.
They also liberate carbon dioxide (C02) which is important in decalcification of
bone matrix.
The osteoclasts appear io accomplish both demineralization of the inorganic
substances and disaggregation of the organic bone matrix, as well as elimination
of debris during bone resorption.
Under the EM, the osteoclasts show numerous mitochondria, lysosomeSj abundant
Golgi saccules and free ribosomes with little RER.
 
□ Cementoclasts
Because cementum does not remodel regularly, cementoclasts or more properly
odontoclasts ( since they destroy dentin and enamel ) are rarely found in the
PDL.
 
They may be seen during:
a) Resorption of deciduous teeth (exfoliation).
b) When excessive force is applied during orthodontic tooth movements.
c) Due to traumatic occlusion.
d) In certain pathological conditions.
 
The cementoclasts are similar in appearance to the osteoclasts. They are mono or
multinucleated giant cells often located in Howship's lacunae on the surface of
cementum.
Their origin is unknown, but they may arise in the same manner as osteoclasts.
They are rich in acid phosphatase and other proteolytic enzymes.
 
[3] Progenitor cells
They are undifferentiated mesenchymal cells that have the capacity to undergo
mitotic division and can differentiate to different specialized cells.
Progenitor cells tend to have small, closed-faced nucleus and very little
cytoplasm and found in highest concentrations close to blood vessels.
It is not known whether a single population of progenitor cells gives rise to
all of the specialized synthetic cells in the PDL, or if there are a number of
populations, each of which gives rise to different specialized cells.
 
[41 Epithelial cells
Remnants of the epithelial root sheath of Hertwig are found close to the
cementum as isolated columns, network, or islands of epithelial cells
(epithelial rests of Malassez ).
EM shows that the epithelial rest cells exhibit tonofilaments and are attached
to each other by desmosomes.
The functional capability of these cells is unknown, however, in certain
pathological conditions, cells of the epithelial rests can undergo rapid
proliferation and can produce a variety of cysts and tumors.
[5] Defensive cells
Defensive cells including macrophages, mast cells, plasma cells and leucocytes
may be found in the PDL whenever an inflammatory reaction takes place. They are
discussed in the pulp section.
 
[B1 extracellular substances :
It consists of a ground substance with embedded fibers and rich supply of blood
vessels, lymphatics and nerves
 
The ground substance
It is formed of 70 % water and the rest is organic material formed of:
□ Glycosaminoglycans with much hyaluronate.
□ Glycoproteins (including fibronectin and tenascin).
□ Proteoglycans such as proteoglycans dermatan sulphate and chondroitin sulphate.
□ Glycolipids.
 
The ground substance has many important functions, Lit controls collagen
fibrillogenesis.
2. If assists fiber orientation.
3. It is especially important in ion and water binding and exchange.
 
Fibers
The fibers of the PDL consists of:
1. Mainly collagen fibers.
2. Oxytalan fibers. 3.Eluanin fibers.
4. Elastic fibers.
 
1- Collagen fibers
90 % of the PDL fibrils are collagen.
 
They are mostly type I collagen, while only 20 % are type III collagen.
The collagen fibrils may be either:
a) Gathered together forming distinct fiber bundles that constitutes the highly
oriented Principal fibers. These fiber bundles can be easily recognized by the
light microscope. They run a wavy course and straighten out under tension. This
gives the fibers the appearance of being elastic.
b) Collagen fibrils may show random fashion distribution ramifying between the
principal fibers. This minor fibril group can only be observed by the EM.
 
The principal bundles of the collagen fibers are arranged in particular groups.
They are named according to their location with respect to teeth into:
1. Gingival ligament.
2. Transseptal (interdental) ligament.
3. Alveolodental ligament which consists of:
 
□ Alveolar crest group.
□ Horizontal group.
□ Oblique group.
□ Apical group.
□ Interradicular group.
 

a. The gingival ligament
Its fibers spread radially from the cervical cementum to the lamina propria of
the gingiva. Its function is to hold the gum tightly against the neck of the
tooth.