Dentin

1. Composition

a. Inorganic (70%)—calcium hydroxyapatite crystals.

b. Organic (30%)—water and type I collagen.

2. Types of dentin

a. Primary dentin

(1) Dentin formed during tooth development, before completion of root formation.

It constitutes the majority of dentin found in a tooth.

(2) It consists of a normal organization of dentinal tubules.

(3) Circumpulpal dentin

(a) The layer of primary dentin that surrounds the pulp chamber.  It is formed after the mantle dentin.

(b) Its collagen fibers are parallel to the DEJ.

b. Secondary dentin

(1) Dentin formed after root formation is complete.

(2) Is deposited unevenly around the pulp chamber, forming along the layer of dentin closest to the pulp.

It therefore contributes to the decrease in the size of the pulp chamber as one ages.

(3) It consists of a normal, or slightly less regular, organization of dentinal tubules. However,

as compared to primary dentin, it is deposited at a slower rate.

(4) Although the dentinal tubules in secondary dentin can be continuous with those in primary

dentin, there is usually a tubular angle change between the two layers.

c. Tertiary (reparative, reactive) dentin

(1) Dentin that is formed in localized areas in response to trauma or other stimuli such as caries, tooth wear, or dental work.

(2) Its consistency and organization vary. It has no defined dentinal tubule pattern

d. Mantle dentin
 

(1) The outermost layer of dentin
(2) Is the first layer of dentin laid down by odontoblasts adjacent to the DEJ.

(3) Is slightly less mineralized than primary dentin.

(4) Has collagen fibers that are perpendicular to the DEJ.

(5) Dentinal tubules branch abundantly in this area.

e. Sclerotic (transparent) dentin

(1) Describes dentinal tubules that have become occluded with calcified material .

(2) Occurs when the odontoblastic processes retreat, filling the dentinal tubule with calcium phosphate crystals.

(3) Occurs with aging.

f. Dead tracts

(1) When odontoblasts die, they leave behind empty dentinal tubules, or dead tracts.

(2) Occurs with aging or trauma.

(3) Empty tubules are potential paths for bacterial invasion.

3. Structural characteristics and microscopic features:
 

a. Dentinal tubules

(1) Tubules extend from the DEJ to the pulp chamber.

(2) The tubules taper peripherally (i.e., their diameters are wider as they get closer to the pulp). Since the tubules are distanced farther apart at the periphery, the density of tubules is greater closer to the pulp.

(3) Each tubule contains an odontoblastic process or Tomes’ fiber.

Odontoblastic processes are characterized by the presence of a network of microtubules, with

Occasional mitochondria and vesicles present.

Note: the odontoblast’s cell body remains in the pulp chamber.
 

(4) Coronal tubules follow an S-shaped path, which may result from the crowding of  odontoblasts as they migrate toward the pulp during dentin formation.

b. Peritubular dentin (intratubular dentin)

(1) Is deposited on the walls of the dentinal tubule, which affects (i.e., narrows)the diameter of the tubule .

(2) It differs from intertubular dentin by lacking a collagenous fibrous matrix. It is also more mineralized than intertubular dentin.
 

c. Intertubular dentin

(1) The main part of dentin, which fills the space between dentinal tubules

 (2) Is mineralized and contains a collagenous matrix.

d. Interglobular dentin

(1) Areas of hypomineralized or unmineralized dentin caused by the failure of globules or calcospherites to fuse uniformly with mature dentin.

(2) Dentinal tubules are left undisturbed as they pass through interglobular dentin; however,

No peritubular dentin is present.

(3) Interglobular dentin is found in the:

(a) Crown—just beneath the mantle dentin.

(b) Root—beneath the dentinocemental junction, giving the root the appearance of a granular

layer (of Tomes).

e. Incremental lines

(1) Dentin is deposited at a daily rate of approximately 4 microns.

(2) As dentin is laid down, small differences in collagen fiber orientation result in the formation of incremental lines.

(3) Called imbrication lines of von Ebner.

(a) Every 5 days, or about every 20 µm, the changes in collagen fiber orientation appear more

accentuated. This results in a darker staining line, known as the imbrication line of von

Ebner.

(b) These lines are similar to the lines of Retzius seen in enamel.

f. Contour lines of Owen

(1) An optical phenomenon that occurs when the secondary curvatures of adjacent dentinal tubules coincide, resulting in the appearance of lines known as contour lines of Owen.
 

(2) Contour lines of Owen may also refer to lines that appear similar to those just described; however, these lines result from disturbances in mineralization.

g. Granular layer of Tomes

(1) A granular or spotty-appearing band that can be observed on the root surface adjacent to the dentinocemental junction, just beneath the cementum.

Gingiva

The connection between the gingiva and the tooth is called the dentogingival junction. This junction has three epithelial types: gingival, sulcular, and junctional epithelium. These three types form from a mass of epithelial cells known as the epithelial cuff between the tooth and the mouth.

Much about gingival formation is not fully understood, but it is known that hemidesmosomes form between the gingival epithelium and the tooth and are responsible for the primary epithelial attachment. Hemidesmosomes provide anchorage between cells through small filament-like structures provided by the remnants of ameloblasts. Once this occurs, junctional epithelium forms from reduced enamel epithelium, one of the products of the enamel organ, and divides rapidly. This results in the perpetually increasing size of the junctional epithelial layer and the isolation of the remenants of ameloblasts from any source of nutrition. As the ameloblasts degenerate, a gingival sulcus is created.

Enamel

Composition: 96% mineral, 4% organic material and water
Crystalline calcium phosphate, hydroxyapatite
Physical characteristics: Hardness compared to mild steel; enamel is brittle
Support from dentin is necessary
Enamel has varies in thickness

Structure of enamel

Ground sections of enamel disclose the information that we have about enamel
Enamel is composed of rods
In the past we used the term prism (do not use)
 

Enamel rod
The rod has a cylinder-like shape and is composed of crystals that run parallel to the longitudinal axis of the rod. At the periphery of the rod the crystals flare laterally.
Interrod region: surrounds each rod; contain more enamel protein (fish scale appearance)
Rod sheath: boundary where crystals of rods meet those of the interrod region at sharp angles (We used to describe that as a keyhole configuration)
Each ameloblast forms one rod and together with adjacent ameloblasts the interrod region Very close to dentin there is no rod structure since the Tomes' processes develop after the first enamel is formed.
Striae of Retzius and cross striations
Incremental lines
Enamel structure is altered along these lines
Cross striations are also a form of incremental lines highlighting the daily secretory activity of ameloblasts

Bands of Hunter and Schreger
Optical phenomenon produced by changes in rod direction

Gnarled enamel
Twisting of rods around each other over the cusps of teeth

Enamel tufts and lamellae
They are like geologic faults
Tufts project from the DE junction, appear branched and contain greater concentrations of enamel protein than enamel
Lamellae extend from the enamel surface
Enamel spindles

Perikymata
Shallow furrows on surface of enamel formed by the striae of Retzius

Soft Oral Tissues

Oral Mucosa

The oral mucosa consists mainly of two types of tissues: the oral epithelium, which consists of stratified, squamous epithelium, and the underlying connective tissue layer, known as the lamina propria.  There are three variations of oral mucosa.

A. Oral epithelium

1. Consists of stratified, squamous epithelium.

2. Four layers (Note: Cells mature as they progress from the deepest [basal] layer to the most superficial [cornified] layer) a. Basal layer (stratum germinativum or basale)

(1) A single layer of cuboidal or columnar cells overlying the lamina propria.

(2) Contains progenitor cells and thus provides cells to the epithelial layers above.

(3) Site of cell division (mitosis).

b. Prickle cell layer (stratum spinosum)

(1) Consists of several layers of larger, ovoid-shaped cells.

c. Granular layer (stratum granulosum)

(1) Cells appear larger and flattened.

(2) Granules (known as keratohyaline granules) are present in the cells.

(3) This layer is absent in nonkeratinized epithelium.

d. Cornified layer (stratum corneum, keratin, or horny layer)

(1) In keratinized epithelium:

(a) Orthokeratinized epithelium the squamous cells on the surface appear flat and contain keratin. They have no nuclei present.

(b) Parakeratinized epithelium the squamous cells appear flat and contain keratin; nuclei are present within the cells.

(2) In parakeratinized epithelium, both squamous cells without nuclei and cells with shriveled (pyknotic) nuclei are present.

(3) In nonkeratinized epithelium, the cells appear slightly flattened and contain nuclei.

B. Lamina propria

1. Consists of type I and III collagen, elastic fibers, and ground substance. It also contains many cell types, including fibroblasts, endothelial cells, immune cells, and a rich vascular and nerve supply.

2. Two layers:

a. Superficial, papillary layer

(1) Located around and between the epithelial ridges.

(2) Collagen fibers are thin and loosely arranged.

b. Reticular layer

(1) Located beneath the papillary layer.

(2) Collagen fibers are organized in thick, parallel bundles.

C. Types of oral mucosa

1. Masticatory mucosa

a. Found in areas that have to withstand compressive and shear forces.

b. Clinically, it has a rubbery, firm texture.

c. Regions: gingiva, hard palate.

2. Lining mucosa

a. Found in areas that are exposed to high levels of friction, but must also be mobile and distensible.

b. Clinically, it has a softer, more elastic texture.

c. Regions: alveolar mucosa, buccal mucosa, lips, floor of the mouth, ventral side of the tongue, and soft palate.

3. Specialized mucosa

a. Similar to masticatory mucosa, specialized mucosa is able to tolerate high compressive

and shear forces; however, it is unique in that it forms lingual papillae.

b. Region: dorsum of the tongue.

D. Submucosa

1. The connective tissue found beneath the mucosa . It contains blood vessels and nerves and may also contain fatty tissue and minor salivary glands.

2. Submucosa is not present in all regions of the oral cavity, such as attached gingiva, the tongue, and hard palate. Its presence tends to increase the mobility of the tissue overlying it.

E. Gingiva

1. The portion of oral mucosa that attaches to the teeth and alveolar bone.

2. There are two types of gingiva: attached and free gingiva. The boundary at which they meet is known as the free gingival groove .

a. Attached gingiva

(1) Directly binds to the alveolar bone and tooth.

(2) It extends from the free gingival groove to the mucogingival junction.

b. Free gingiva

(1) Coronal to the attached gingiva, it is not bound to any hard tissue.

(2) It extends from the gingival margin to the free gingival groove.

c. Together, the free and attached gingiva form the interdental papilla.

.F. Alveolar mucosa

1. The tissue just apical to the attached gingiva.

2. The alveolar mucosa and attached gingiva meet at the mucogingival junction .

G. Junctional epithelium

1. Area where the oral mucosa attaches to the tooth, forming the principal seal between the oral cavity and underlying tissues.

2. Is unique in that it consists of two basal lamina, an internal and external . The internal basal lamina, along with hemidesmosomes, comprises the attachment apparatus (the epithelial attachment). This serves to attach the epithelium directly to the tooth.

3. Histologically, it remains as immature, poorly differentiated tissue. This allows it to maintain its ability to develop hemidesmosomal attachments.

4. Has the highest rate of cell turnover of any oral mucosal tissue.

H. Interdental papilla (interdental gingiva)

1. Occupies the interproximal space between two teeth. It is formed by free and attached gingiva.

2. Functions to prevent food from entering the (interproximal) area beneath the contact point of two adjacent teeth. It therefore plays an important role in maintaining the health of the gingiva.

3. Col

a. If the interdental papilla is cross-sectioned in a buccolingual plane, it would show two peaks (buccal and lingual) with a dip between them, known as the col or interdental col. This depression occurs around the contact point of the two adjacent teeth.

b. Histologically, col epithelium is the same as junctional epithelium

Maxillary Second Deciduous Molar.

-The notation is A or J.

-It looks like a first permanent molar

-There are three roots.

-Usually it has four well developed cusps.

-It is somwhat rhomboidal in outline.

-They often have the Carabelli trait.

- the shape the maxillary first permanent molar strongly resembles that of the adjacent deciduous second molar.

HISTOLOGY OF THE ODONTOBLAST

Formation of Dentin

Mantle dentin: First formed dentin
Type I collagen with ground substance
Formation of the odontoblast process

Matrix vesicles
Appearance of hydroxyapatite crystals
 

Predentin
Primary physiologic (circumpulpal) dentin
All organic matrix is formed from odontoblasts
Smaller collagen fibers
Presence of phosphophoryn

Mineralization
Globular calcification
Interglobular dentin: Areas of incomplete calcification
Incremental lines of von Ebner: Daily, 4mm of organic matrix is deposited. Also every 5 days the arrangement of collagen fibers changes. This creates the incremental lines of von Ebner.
Intratubular dentin

Dentin tubules
S-shaped in the coronal aspect, straight in root dentin

Von Korff fibers
They are an artifact