NEET MDS Lessons
Dental Anatomy
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.
Types of dentitions:
1. Diphyodont. Teeth develop and erupt into their jaws in two generations of teeth. The term literally means two generations of teeth.
2. Monophyodont. a single generation of teeth.
3. Polyphyodont. Teeth develop a lifetime of generations of successional teeth
4. Homodont. all of the teeth in the jaw are alike. They differ from each other only in size.
5. Heterodont. There is distinctive classes of teeth that are regionally specialized.
FORMATION OF THE ROOT AND ITS ROLE IN ERUPTION
- As dentin and enamel is deposited the shape of the future crown appears.
- The cells just superficial to the horizontal diaphragm start to proliferate and grow pushing the horizontal diaphragm down into the mesenchym.
- This forms a tube.
- This tube is the epithelial root sheath of Hertwig's.
- The mesenchym cells lying inside the tube nearest to the epithelial root sheath are induced to differentiate into odontoblasts, which then start to deposit dentin.
- After the first dentin of the root has been laid down the inner epithelial cells of the sheath start to deposit an enameloid substance called intermediate cementum.
- The root sheath cells then separate from the intermediate cementum and breaks up in a network of epithelial strands.
- The mesenchym on the outside comes into contact with the intermediate cementum and differentiate into
cementoblasts, which will deposit the cementum.
- This cementum traps the collagenic fibres, of the periodontal ligament, which are also formed.
- Epithelium of the root sheath persists as epithelial rests of Malassez. Because the epithelium of the root sheath forms from enamel epithelium it can develop into ameloblasts which will deposit enamel pearls.
- There is little space for the root to develop.
- To create space the crown is pushed out.
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
Dental Formula, Dental Notation, Universal Numbering System
A. Dental Formula. The dental formula expresses the type and number of teeth per side
The Universal Numbering System. The rules are as follows:
1. Permanent teeth are designated by number, beginning with the last tooth on the upper right side, going on to the last tooth on the left side, then lower left to lower right
2. Deciduous teeth are designated by letter, beginning with the last tooth on the upper right side and proceeding in clockwise fashion
ARTICULAR SURFACES COVERED BY FIBROUS TISSUE
TMJ is an exception form other synovial joints. Two other joints, the acromio- and sternoclavicular joints are similar to the TMJ. Mandible & clavicle derive from intramembranous ossificiation.
Histologic
- Fibrous layer: collagen type I, avascular (self-contained and replicating)
- Proliferating zone that formes condylar cartilage
- Condylar cartilage is fibrocartilage that does not play role in articulation nor has formal function
- Capsule: dense collagenous tissue (includes the articular eminence)
- Synovial membrane: lines capsule (does not cover disk except posterior region); contains folds (increase in pathologic conditions) and villi
Two layers: a cellular intima (synovial cells in fiber-free matrix) and a vascular subintima
Synovial cells: A (macrophage-like) syntesize hyaluronate
B (fibroblast-like) add protein in the fluid
Synovial fluid: plasma with mucin and proteins, cells
Liquid environment: lubrication, ?nutrition - Disk: separates the cavity into two comprartments, type I collagen
anterior and posterior portions
anetiorly it divides into two lamellae one towards the capsule, the other towards the condyle
vascular in the preiphery, avascular in the center - Ligaments: nonelastic collagenous structures. One ligament worth mentioning is the lateral or temporomandibular ligament. Also there are the spheno- and stylomandibular with debatable functional role.
Innervations
|
Ruffini |
Posture |
Dynamic and static balance |
|
Pacini |
Dynamic mechanoreception |
Movement accelerator |
|
Golgi |
Static mechanoreception |
Protection (ligament) |
|
Free |
Pain |
Protection joint |
Dentinogenesis
Dentin formation, known as dentinogenesis, is the first identifiable feature in the crown stage of tooth development. The formation of dentin must always occur before the formation of enamel. The different stages of dentin formation result in different types of dentin: mantle dentin, primary dentin, secondary dentin, and tertiary dentin.
Odontoblasts, the dentin-forming cells, differentiate from cells of the dental papilla. They begin secreting an organic matrix around the area directly adjacent to the inner enamel epithelium, closest to the area of the future cusp of a tooth. The organic matrix contains collagen fibers with large diameters (0.1-0.2 μm in diameter). The odontoblasts begin to move toward the center of the tooth, forming an extension called the odontoblast process. Thus, dentin formation proceeds toward the inside of the tooth. The odontoblast process causes the secretion of hydroxyapatite crystals and mineralization of the matrix. This area of mineralization is known as mantle dentin and is a layer usually about 150 μm thick.
Whereas mantle dentin forms from the preexisting ground substance of the dental papilla, primary dentin forms through a different process. Odontoblasts increase in size, eliminating the availability of any extracellular resources to contribute to an organic matrix for mineralization. Additionally, the larger odontoblasts cause collagen to be secreted in smaller amounts, which results in more tightly arranged, heterogenous nucleation that is used for mineralization. Other materials (such as lipids, phosphoproteins, and phospholipids) are also secreted.
Secondary dentin is formed after root formation is finished and occurs at a much slower rate. It is not formed at a uniform rate along the tooth, but instead forms faster along sections closer to the crown of a tooth. This development continues throughout life and accounts for the smaller areas of pulp found in older individuals. Tertiary dentin, also known as reparative dentin, forms in reaction to stimuli, such as attrition or dental caries.
The dentin in the root of a tooth forms only after the presence of Hertwig's epithelial root sheath (HERS), near the cervical loop of the enamel organ. Root dentin is considered different than dentin found in the crown of the tooth (known as coronal dentin) because of the different orientation of collagen fibers, the decrease of phosphoryn levels, and the less amount of mineralization.