Histology of the Cementum

HO Sedano & RS Edmunds: Laboratory Manual and Study Guide
Section III: (c) Cementum - Periodontal Ligament (frames B19 through B24)

Cementum

Cementum is a hard connective tissue that derives from ectomesenchyme.
Review cementogenesis from your book and class notes
What is the role of Hertwig's root sheath?
What is the hyaline layer of Hopewell Smith?

Embryologically, there are two types of cementum:
Primary cementum: It is acellular and develops slowly as the tooth erupts. It covers the coronal 2/3 of the root and consists of intrinsic and extrinsic fibers (PDL).
Secondary cementum: It is formed after the tooth is in occlusion and consists of extrinsic and intrinsic (they derive from cementoblasts) fibers. It covers mainly the root surface.

Functions of Cementum

It protects the dentin (occludes the dentinal tubules)
It provides attachment of the periodontal fibers
It reverses tooth resorption

Cementum is composed of 90% collagen I and III and ground substance.
50% of cementum is mineralized with hydroxyapatite. Thin at the CE junction, thicker apically.

CEMENTUM vs. BONE

Cementum simulates bone
1) Organic fibrous framework, ground substance, crystal type, development
2) Lacunae
3) Canaliculi
4) Cellular components
5) Incremental lines (also known as "resting" lines; they are produced by continuous but phasic, deposition of cementum)

Differences between cementum and bone
1) Cementum is not vascularized
2) Cementum has minor ability to remodel
3) Cementum is more resistant to resorption compared to bone
4) Cementum lacks neural component
5) Cementum contains a unique proteoglycan interfibrillar substance
6) 70% of bone is made by inorganic salts (cementum only 46%)

Relation of Cementum to Enamel at the Cementoenamel Junction (CEJ)

"OMG rule"

Refer to Picture 1

In 60% of the teeth cementum Overlaps enamel
In 30% of the teeth cementum just Meets enamel
In 10% of the teeth there is a small Gap between cementum and enamel

Classification of Cementum

Please review your book and notes on this subject

1. Embryologically
Primary and secondary
2. According to cellular component

Refer to Picture 2

Acellular
Thin
Amorphous
First layer to seal the dentin tubules
Cellular
Thick
Better structure
Apical surface

Layers of cellular and acellular cementum alternate (randomly)

3. Based on the origin of the collagenous matrix
Extrinsic
Intrinsic
Mixed

4. Combined classification
a. Primary acellular intinsic fiber cementum
b. Primary acellualar extrinsic fiber cementum
c. Secondary cellular intrinsic fiber cementum
d. Secondary cellular mixed fiber cementum
e. Acellular afibrillar cementum

5. Depending on the location and patterning
Intermediate and mixed stratified cementum

Participating Cells

Cementoblasts

Refer to Picture 3

Active
 Cells are round, plump with basophilic cytoplasm (rough endoplasmic reticulum)
Inactive
Cells have little cytoplasm
Cementocytes

Refer to Picture 4


1 Cementocyte lacuna
2 Cementocyte canaliculus

Cells have fewer organelles compared to cementoblasts. They are found in lacunae and have numerous processes toward the periodontal ligament. (why?)
Eventually they die due to avascularity.

Cementicles

Please refer to Picture 5


1 Dentin
5 Acellular cementum
6 Periodontal ligament
7 Alveolar bone
8 Attached cementicle
9 Free cementicle

a) free
b) attached
c) embedded

Clinical importance of cementum

1) Deposition of cementum continues throughout life.
The effects of the continuous deposition of cementum are the maintenance of total length of the tooth (good) and constriction of the apical foramen (bad).
2) With age, the smooth surface of cementum becomes more irregular due to calcification of some ligament fiber bundles. This is referred to as spikes.

Behavior of cementum in pathologic conditions

Refer to Picture 6 and 7 and review your notes