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.
Periodontal ligament development
Cells from the dental follicle give rise to the periodontal ligaments (PDL).
Formation of the periodontal ligaments begins with ligament fibroblasts from the dental follicle. These fibroblasts secrete collagen, which interacts with fibers on the surfaces of adjacent bone and cementum. This interaction leads to an attachment that develops as the tooth erupts into the mouth. The occlusion, which is the arrangement of teeth and how teeth in opposite arches come in contact with one another, continually affects the formation of periodontal ligaments. This perpetual creation of periodontal ligaments leads to the formation of groups of fibers in different orientations, such as horizontal and oblique fibers.
Classification of Cementum
- Embryologically
Primary and secondary
2. According to cellular component
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
Active
Cells are round, plump with basophilic cytoplasm (rough endoplasmic reticulum)
Inactive
Cells have little cytoplasm
Cementocytes
- Cementocyte lacuna
- cementocyte canaliculus
Cells have fewer organelles compared to cementoblasts. They are found in lacunae and have numerous processes toward the periodontal ligament. Eventually they die due to avascularity
Cementicles
a) free
b) attached
c) embedded
The Transition from the Deciduous to the Permanent Dentition.
1. The transition begins with the eruption of the four first permanent molars, and replacement of the lower deciduous central incisors by the permanent lower central incisors.
2. Complete resorption of the deciduous tooth roots permits exfoliation of that tooth and replacement by the permanent (successional) teeth
3. The mixed dentition exists from approximately age 6 years to approximately age 12 years. In contrast, the intact deciduous dentition is functional from age 2 - 2 /2 years of age to 6 years of age.
4. The enamel organ of each permanent anterior tooth is connected to the oral epithelium via a fibrous cord, the gubernaculum. The foramina through which it passes can be seen in youthful skulls
The deciduous second molars are particularly important. It is imperative that the deciduous second molars be preserved until their normal time of exfoliation. This prevent mesial migration of the first permanent molars.
Use a space maintainer in the event that a second deciduous molar is lost prematurely
The mixed dentition
I. Transition dentition between 6 and 12 years of age with primary tooth exfoliation and permanent tooth eruption
2. Its characteristic features have led this to be called the ugly duckling stage because of
a. Edentulated areas
b. Disproportionately sized teeth
c. Various clinical crown heights
d. Crowding
e. Enlarged and edematous gingiva
f. Different tooth colors
The very first histological evidence of tooth development appear during the second month of intrauterine life. Calcification of deciduous incisors begins at 3-4 months in utero.
Crown stage
Hard tissues, including enamel and dentin, develop during the next stage of tooth development. This stage is called the crown, or maturation, stage by some researchers. Important cellular changes occur at this time. In prior stages, all of the inner enamel epithelium cells were dividing to increase the overall size of the tooth bud, but rapid dividing, called mitosis, stops during the crown stage at the location where the cusps of the teeth form. The first mineralized hard tissues form at this location. At the same time, the inner enamel epithelial cells change in shape from cuboidal to columnar. The nuclei of these cells move closer to the stratum intermedium and away from the dental papilla.
The adjacent layer of cells in the dental papilla suddenly increases in size and differentiates into odontoblasts, which are the cells that form dentin. Researchers believe that the odontoblasts would not form if it were not for the changes occurring in the inner enamel epithelium. As the changes to the inner enamel epithelium and the formation of odontoblasts continue from the tips of the cusps, the odontoblasts secrete a substance, an organic matrix, into their immediate surrounding. The organic matrix contains the material needed for dentin formation. As odontoblasts deposit organic matrix, they migrate toward the center of the dental papilla. Thus, unlike enamel, dentin starts forming in the surface closest to the outside of the tooth and proceeds inward. Cytoplasmic extensions are left behind as the odontoblasts move inward. The unique, tubular microscopic appearance of dentin is a result of the formation of dentin around these extensions.
After dentin formation begins, the cells of the inner enamel epithelium secrete an organic matrix against the dentin. This matrix immediately mineralizes and becomes the tooth's enamel. Outside the dentin are ameloblasts, which are cells that continue the process of enamel formation; therefore, enamel formation moves outwards, adding new material to the outer surface of the developing tooth.