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

Free Cementicle

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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