no slide title · 2020-03-16 · physical characteristics cementum human cementum is avascular and...
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CEMENTUM
Objectives: By the end of the lecture, you should be able to
1-Identify various types of cementum.
2-Recongnize the similarities and differences between
cementum and bone.
3- understand the histological structure, functions and age
changes of cementum.
Cementum
Definition: Cementum is a mineralized dental
tissue covering the anatomical part of the root of
the human teeth, it begins at the amelo-cemental
junction and continues to the apex.
It serves as a medium for the attachment of collagen
fibers of the periodontal ligament that bind the
tooth to the surrounding structures.
Origin: from the most inner layer of the dental sac
cells that differentiat into cementoblasts
Physical characteristics
Cementum human cementum is avascular and with no nerve supply.
Thickness: the thickness of cementum at the cervical area is about 20-50 microns and
it gradually increases in thickness till it reaches its maximum about (150-200u) at the apex
and at the bifurcation of the root. Some times cementum extends to the inner wall of the
root dentin for short distance and so a lining of the root canal.
Colour: is light yellow in Colour it is somewhat lighter in colour than dentin. It can be
distinguished from enamel by its lack of lustre and darker hue.
Permeability: Cementum has been shown to be permeable to a variety of materials, it is
permeable from the dentin side as well as from the periodontal ligament side, the
permeability decrease with age from the ligament side.
Chemical Composition
In fully formed permanent teeth, cementum contains
Organic materials
50-55% Inorganic materials
45-50%
Calcium and Phosphate
in the form of
hydroxyappatite
crystals
Protein
polysaccharides (proteoglycanes),
glycoprotein, and
Phosphoproteins.
Collagen (type I mainly) 1- Intrinsic produced by
cementoblasts.
2- Extrinsic (Sharpye’s Fibers)
of the Periodontal ligament
fibroblasts.
N.B. Numerous trace elements are found in varying amounts. It is of interest that
cementum contains the greatest amount of fluoride in all mineralized tissues.
Classification of Cementum and Histological structure
Cementum of human teeth is classified into:
1-ACellular cementum
2-Cellular cementum
3- Intermediate cementum
4- Afibrillar cementum
Acellular cementum
It is clear and structureless, it directly over the granular layer of Tomes. It
covers root dentin starting from the amelocemental junction to the apex,
but it is often missing at the apical third of the root, where the cementum
may be entirely of the cellular type. In general acellular cementum usually
predominates at the coronal half of the root, whereas cellular cementum is
more frequent on the apical half. Alternating layers of cellular and acellular
cementum could be present in decalcified specimens.
Collagen fibrils form the main bulk of the organic portion, in between the
collagen fibrils there are protein polysaccharide materials which form the
ground substance. Collagen fibrils in both acellular and cellular cementum
are arranged in a very complex fashion.
When cementum is thin, Sharpey's fibers cross the entire thickness of the
cementum, with increase deposition of the cementum a larger part of the
fibers is incorporated in the cementum. It has to be noted that the increase
in thickness of cementum does not enhance the functional efficiency by
increasing the strength of attachment of the individual fibers, because the
deeper portion of the fibers is calcified and the attachment proper is
confined to the superficial cementum only.
Cellular cementum
It has the same structure as the acellular cementum but contains
cells (cementocytes), the cementocytes are cells incorporated
into cellular cementum.
These cells are similar to the osteocytes. cementocytes has
granular cytoplasm and faintly stained nucleus, the cell body lies
in space called lacunae and their processes present in
canaliculi. these processes may branch and anastomose with
those of the neighboring cells connected by gap junction.
Most of these processes are directed toward the periodontal
surface of the cementum, to provide nutrition for the
cementum.
Cementocytes present in the deepest layer of cementum show
signs of degeneration, the cytoplasm contains degenerated
organelles, the endoplasmic reticulum is dilated and
mitochondria are spaced.
Signs of degeneration such as cytoplasmic clumping are seen
among cementocytes present in the most deep layers.
Cementocytes are best demonstrated in dry ground section
where the cells are degenerated.
Cementocyte And Osteocyte
Dentin side
PDL side
Lacuna
Canaliculi
Osteocyte
Differences & similarities between bone and cementum
Differences
Processes of cementocytes are numerous and longer toward the periodontal ligament site while of osteocytes are similar
Cementocytes arranged haphazard while osteocytes arranged in Haverian system.
No periosteum.
No trabeculae & No bone marrow.
No blood supply.
No nerve supply.
Almost no remodelling.
Similarities
The forming cells are cementoblasts in Cementum & osteoblasts in bone. Entraped cells are cementocytes and osteocytes.
Lacunae and canaliculi for cells & their processes.
Similar matrix = Mineral+collagen fibers.
Apposional growth from surface.
Incremental growth line in matrix.
Resorption by multinucleated giant cells, cementoclast in cementum & osteoclast in bone.
Cellullar cementum Acellular cementum
It is more frequent on the apical
third of the root,
It predominates in the coronal two
thirds of the root.
It has only about 60% of its
collagen content derived from
Sharpey’s fibers and the remainder is
intrinsic fibers.
More permeable.
It consists of mainly Sharpey’s
fibers and rest few fibrils are of
intrinsic group.
Less permeable.
It appears by the time of tooth
eruption.
Faster rate of formation.
It is formed before tooth
eruption.
Slower rate of formation.
It contains cementocytes It does not contain cementocytes.
Incremental lines of Salter
Both cellular and acellular cementum are separated by incremental
lines into layers which indicate periodic formation.
Histochemical studies indicate that incremental lines are highly
mineralized areas with less collagen and more ground substance
than other portions in cementum. the incremental lines are roughly
parallel to the long axis of the root.
Cellular cementum is formed at a faster rate than the acellular
cementum and the Incremental lines in the cellular cementum
located further apart than in acellular cementum.
Question?
Intermediate cementum
Dentin is separated from cementum by a zone known as the intermediate
cementum layer.
This intermediate cementum does not resemble either dentin or cementum.
Usually it is present in the apical two thirds of the roots of molars and
premolars, rarely seen in incisors or deciduous teeth.
Origin: It is believed that this layer represents areas where cells of Hertwig's
epithelial root sheath become trapped in a rapidly deposited dentin or
cementum matrix. Some times the intermediate cementum is found as a
continuous layer and sometimes it is found in isolated areas.
.
Function
* It seals the surface of the sensitive root dentin.
* It cements the dentin with cementum.
Cemento-dentinal junction
The cemento-dentinal junction is relatively smooth in permanent teeth but in
deciduous teeth it is scalloped.
The attachment of cementum to dentin in either case is quite firm, although
nature of this attachment is not fully understood, the collagen fibrils of
cementum are arranged in relatively distinct bundles, while those of dentin are
arranged somewhat hazardly.
The collagen fibrils of cementum and dentin interwine at their interface in a
very complex fashion, so it is not possible to precisely determine which fibrils
are of dentinal origin and which are of cemental origin.
CEMENTUM: Boundaries
Cemento-enamel junction CEJ
Dentino-cemental junction DCJ
Ligamento-cemental junction
APICAL FORAMEN
Cemento-Enamel Junction
cemento-enamel junction
The relation between cementum and enamel at the cervical
region of teeth is variable.
In 30% of all teeth, cementum meets the cervical end of enamel
in a relatively sharp line.
When the epithelial root sheath of Hertwig's is delayed in its
separation from the dentin, a zone the root is devoid of
cementum. In such a case the cemento enamel junction here
consists only a layer of dentin without enamel and cementum.
This occurs in about 10% of the teeth.
In about 60% of the teeth cementum overlaps the cervical end
of enamel for a short distance. it occurs when the reduced
enamel epithelium degenerates at the cervical termination
The undifferentiated mesenchymal cells of the dental sac to come in direct contact with the enamel surface.
They differentiate and produce cells that produce a laminated electron
dense reticular material called afibrillar cementum.
It does not contain collagen fibers. If it continues in contact with
connective tissue cells. Fibrillar cementum with collagen fibers will form
over it.
1- The main function of cementum is to form a medium for attachment
of collagen fibers that bind the tooth to the alveolar bone . The
cementum is a continuous deposits and is not liable for resorption by
existence of cementoid tissue , this keep the attachment apparatus
intact. .
2- Cementum serves as the major reparative tissue for root surface so
damage or resorption of the root can be repaired by the deposition of
cementum. .
3- Deposition of cementum at the apical area of the root can
compensated for the loss of tooth substance from occlusal wear.
Function of cementum:
Cementogenesis This comprises matrix formation and mineralization. Matrix formation The development of the root begins at the
crown is formed. The outer and inner dental epithelium form the epithelial root sheath Hertwig's. The inner dental epithelium will induce the neighboring cells of the dental papilla to differentiate into odontoblasts. The predentin and dentin formation occur in a similar manner to that of the crown.
Once the dentin formation is started, the epithelial root sheath of Hertwig's will loose its continuity, undifferentiated mesenchymal cells from adjacent connective tissue of the tooth follicle will differentiate into cementoblasts.
The cementoblasts have the ultrastructural characteristics typical of cells actively producing protein-polysaccharide complexes i.e. an abundant endoplasmic reticulum, a well developed Golgi apparatus and several mitochondria. The ultrastructural features are not unique to cementoblasts and can be observed in other cells actively producing proteins and polysaccharides. The main product of cementoblast is collagen which constitutes the major portion of organic matrix. Also cementoblasts produce the ground substances.
The remnants of the epithelial root sheath of Hertwig's migrate toward the
dental sac and become the epithelial rests of Malassez found in the
periodontal ligament of the fully developed tooth. Epithelial cells which
fail to migrate from the dentin surface may be incorporated in the
cementum .
Growth of cementum is a rhythmic process and as a new layer of
cementoid is formed, the old one calcifies. A thin layer of cementoid can
usually be observed on cementum surface which is covered by
cementoblasts .
Mineralization
Mineralization occurs after some matrix production has taken place.
The mineral originate from tissue fluids, where calcium and phosphate ions
are present.
The mineral crystals are deposited within, on the surface of and between the
collagen fibers.
The long axes of the crystals are arranged parallel to the long axes of the
collagen fibers. The crystals are composed of unit of hydroxy apatite.
In order to form one single crystal in cementum, several thousands of unit
cells are required. The crystals are plate-like in shape and they measure
about 530 Å long.
Age changes of cementum :
1) Permeability: by aging the permeability of cementum
decreases gradually, the permeability from the periodontal
ligament side is lost except in the most recent formed layers of
cementum, while permeability from the dentin side remains
only in the apical region
2) Hypercementosis: is an abnormal thickening of cementum it
may affect all teeth of dentition. The increase in cementum
thickness is around the apex of all teeth and in the furcation of
multirooted teeth and on other areas of the root.
Hypercementosis is viewed in two forms ;
Cementum hypertrophy: when the increase of the cement
thickness improve the functional qualities of the cementum. This type
of cementum is typical form of cellular cementum. It occurs in
functioning teeth with numerous Sharpey’s fibers.
Cementum hyperplasia: it occurs in non functioning or embedded
teeth. this type of hypercementosis is characterized by calcification of
Sharpey's fibers and is accompanied by numerous cementicles .
Cementicles: they are ovoid or round calcified structure that exhibits
by dystrophic calcification of the degenerated periodontal tissue or the
epithelial rests of Malassez they have nidus favoring the deposition of
concentric layers of calcospherites around the degenerated or
hemorrhagic areas.
Hypercementosis
Attached and embedded
cementicles
Attached cementicle Embedded cementicle
According to formation;
1- Primary cementum (acellular cementum): formed before tooth
eruption.
2- Secondary cementum (cellular cementum):formed by the time of
tooth eruption.
According to cellularity:
1- Cellular cementum: Cementum containing cementocytes in lacunae
within the cementum matrix.
2- Acellular cementum: Cementum without any cells in its matrix.
• According to the presence of collagen fibrils in the matrix:
1- Fibrillar cementum: Cementum with a matrix that contains well-defined
fibrils of type I collagen.
2- Afibrillar cementum: Cementum that has a matrix devoid of detectable
type I collagen fibrils. Instead, the matrix tends to have a fine, granular
consistency.
Types of cementum
(Cont.)Types of cementum
According to the origin of the matrix fibers (applies only to fibrillar forms of collagen):
1- Extrinsic fiber cementum: Cementum that contains primarily extrinsic fibers, i.e. Sharpey's fibers that are continuous with the principal fibers of the periodontal ligament. Since the fibers were originally produced by periodontal ligament fibroblasts, they are considered "extrinsic" to the cementum. These fibers are orientated more or less perpendicular to the cementum surface and play a major role in tooth anchorage.
2- Intrinsic fiber cementum: Cementum that contains primarily intrinsic fibers, i.e. fibers produced by cementoblasts and that are orientated more or less parallel to the cementum surface. This form of cementum is located predominantly at sites undergoing repair, following surface resorption. It plays no role in tooth anchorage.
3- Mixed fiber cementum: Cementum that contains a mixture of extrinsic and intrinsic fiber cementum.
•Afibrillar type cementum (cement overlap enamel) •Intermediate cementum type
The main differences between
extrinsic and intrinsic fibers
Intrinsic fibers Extrinsic fibers
Fine collagen fibers Course collagen fibers
They are secreted from
cementoblasts
They are secreted from fibroblasts of PDL
They run parallel to the cementum
surface
They runs nearly perpendicular to intrinsic
fibers
They are seen by decalcified section
with silver stain only
They are seen by ground and decalcified
section
----------------------- Serve in the attachment of the tooth with the
alveolar bone
The descriptors for the classes of cementum described above can be
used in various combinations to more precisely describe a specific type
of cementum. Examples follow:
1. Acellular, afibrillar cementum
-This cementum is mostly composed of mineralized matrix, without detectable
collagen fibrils or cementocytes.
-It is produced exclusively by cementoblasts. It is typically found as coronal
cementum on human teeth.
2. Acellular, extrinsic fiber cementum
-This type of cementum has a matrix of well-defined, type I collagen
fibrils. The fibrils are part of the, densely packed Sharpey's fibers, that are
continuous with the principal fibers of the periodontal ligament. Because of
their dense packing, the individual Sharpey's fibers that form the bulk of the
matrix may no longer be identifiable as individual fibers within the cementum
layer.
-This cementum, which is acellular, is located in the cervical two-thirds of the
root of human teeth. It plays a major role in tooth anchorage.
. Cellular, intrinsic fiber cementum3
-This cementum contains cementocytes in a matrix composed almost exclusively of intrinsic fiber cementum.
.cementum repairat sites of It is located almost exclusively -
However, it may be covered over in tooth anchorage.no part It plays -by extrinsic or mixed fiber cementum, both of which are able to provide new anchorage.
4. Cellular, mixed fiber cementum - It is found on the apical third of the root and in furcation (i.e. between
roots). In these locations, the rate of cementum formation is usually more rapid
than in the cervical region.
- The mineralized, extrinsic collagen fibers (Sharpey's fibers) run a more
irregular course than in acellular, extrinsic fiber cementum.
- Intrinsic fibers are found interspersed among the extrinsic fibers of the
cementum matrix, so that individual Sharpey’s fibers are more readily
identifiable than in extrinsic fiber cementum.
- Cementoblasts are trapped in hollow chambers (or lacunae) where they
become cementocytes
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