NEET MDS Lessons
Dental Anatomy
CONTACT POINT.:-The point on the proximal surface where two adjacent teeth actually touch each other is called a contact point.
INTERPROXIMAL SPACE.:-The interproximal space is the area between the teeth. Part of the interproximal space is occupied by the interdental papilla. The interdental papilla is a triangular fold of gingival tissue. The part of the interproximal space not occupied is called the embrasure.
EMBRASURE. :-The embrasure occupies an area bordered by interdental papilla, the proximal surfaces of the two adjacent teeth, and the contact point (fig 4-18). If there is no contact point between the teeth, then the area between them is called a diastema instead of an embrasure.
OCCLUSAL
The occlusal surface is the broad chewing surface found on posterior teeth (bicuspids and molars).
OCCLUSION.:-Occlusion is the relationship between the occlusal surfaces of maxillary and mandibular teeth when they are in contact. Many patterns of tooth contact are possible. Part of the reason for the variety is the mandibular condyle's substantial range of movement within the temporal mandibular joint.
Malocclusion occurs when any abnormality in occlusal relationships exist in the dentition. Centric occlusion, is the centered contact position of the chewing surfaces of mandibular teeth on the chewing surface (occlusal) of the maxillary teeth.
OCCLUSAL PLANE.:-Maxillary and mandibular teeth come into centric occlusion and meet along anteroposterior and lateral curves. The anteroposterior curve is called the Curve of Spee in which the mandibular arch forms a concave (a bowl-like upward curve). The lateral curve is called the Curve of Wilson . The composite (combination) of these curves form a line called the occlusal plane, and is created by the contact of the upper and lower teeth
VERTICAL AND HORIZONTAL OVERLAP. :-Vertical overlap is the extension of the maxillary teeth over the mandibular counterparts in a vertical direction when the dentition is in centric occlusion Horizontal overlap is the projection of maxillary teeth over antagonists (something that opposes another) in a horizontal direction.
KEY TO OCCLUSION.:-The occlusal surfaces of opposing teeth bear a definite relationship to each other. In normal jaw relations and when teeth are of normal size and in the correct position, the mesiofacial cusp of the maxillary first molar occludes in the facial groove of the mandibular first molar. This normal relationship of these two teeth is called the key to occlusion.
PERMANENT DENTITION
The permanent dentition consists of 32 teeth. Each tooth in the permanent dentition is described in this section. It should be remembered that teeth show considerable variation in size, shape, and other characteristics from one person to another. Certain teeth show a greater tendency than others to deviate from the normal. The descriptions that follow are of normal teeth.
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
Histology of the Periodontal Ligament (PDL)
Embryogenesis of the periodontal ligament
The PDL forms from the dental follicle shortly after root development begins
The periodontal ligament is characterized by connective tissue. The thinnest portion is at the middle third of the root. Its width decreases with age. It is a tissue with a high turnover rate.
FUNCTIONS OF PERIODONTIUM
Tooth support
Shock absorber
Sensory (vibrations appreciated in the middle ear/reflex jaw opening)
The following cells can be identified in the periodontal ligament:
a) Osteoblasts and osteoclasts b) Fibroblasts, c) Epithelial cells
Rests of Malassez
d) Macrophages
e) Undifferentiated cells
f) Cementoblasts and cementoclasts (only in pathologic conditions)
The following types of fibers are found in the PDL
-Collagen fibers: groups of fibers
-Oxytalan fibers: variant of elastic fibers, perpendicular to teeth, adjacent to capillaries
-Eluanin: variant of elastic fibers
Ground substance
PERIODONTAL LIGAMENT FIBERS
Principal fibers
These fibers connect the cementum to the alveolar crest. These are:
a. Alveolar crest group: below CE junction, downward, outward
b. Horizontal group: apical to ACG, right angle
c. Oblique group: numerous, coronally to bone, oblique direction
d. Apical group: around the apex, base of socket
e. Interradicular group: multirooted teeth
Gingival ligament fibers
This group is not strictly related to periodontium. These fibers are:
a. Dentogingival: numerous, cervical cementum to f/a gingiva
b. Alveologingival: bone to f/a gingiva
c. Circular: around neck of teeth, free gingiva
d. Dentoperiosteal: cementum to alv. process or vestibule (muscle)
e. Transseptal: cementum between adjacent teeth, over the alveolar crest
Blood supply of the PDL
The PDL gets its blood supply from perforating arteries (from the cribriform plate of the bundle bone). The small capillaries derive from the superior & inferior alveolar arteries. The blood supply is rich because the PDL has a very high turnover as a tissue. The posterior supply is more prominent than the anterior. The mandibular is more prominent than the maxillary.
Nerve supply
The nerve supply originates from the inferior or the superior alveolar nerves. The fibers enter from the apical region and lateral socket walls. The apical region contains more nerve endings (except Upper Incisors)
Dentogingival junction
This area contains the gingival sulcus. The normal depth of the sulcus is 0.5 to 3.0 mm (mean: 1.8 mm). Depth > 3.0 mm is considered pathologic. The sulcus contains the crevicular fluid
The dentogingival junction is surfaced by:
1) Gingival epithelium: stratified squamous keratinized epithelium 2) Sulcular epithelium: stratified squamous non-keratinized epithelium The lack of keratinization is probably due to inflammation and due to high turnover of this epithelium.
3) Junctional epithelium: flattened epithelial cells with widened intercellular spaces. In the epithelium one identifies neutrophils and monocytes.
Connective tissue
The connective tissue of the dentogingival junction contains inflammatory cells, especially polymorphonuclear neutrophils. These cells migrate to the sulcular and junctional epithelium.
The connective tissue that supports the sulcular epithelium is also structurally and functionally different than the connective tissue that supports the junctional epithelium.
Histology of the Col (=depression)
The col is found in the interdental gingiva. It is surfaced by epithelium that is identical to junctional epithelium. It is an important area because of the accumulation of bacteria, food debris and plaque that can cause periodontal disease.
Blood supply: periosteal vessels
Nerve supply: periodontal nerve fibers, infraorbital, palatine, lingual, mental, buccal
ERUPTION
. Root completion (approximately 50% of the root is formed when eruption begins)
Generally mandibular teeth erupt before maxillary teeth,
Primary teeth
I. Emerge into the oral cavity as follows:
Maxillary Mandibular
Central Incisor 7½ months 6 months
Lateral incisor 9 months 7 months
Canine 18 months 16 months
First Molar 14 months 12 months
Second Molar 24months 20 months
The sequence of primary tooth development is central incisor, lateral incisor, first molar, second molar
3. Hard tissue formation begins between 4 and 6 months in utero
4. Crowns completed between 1½ and 10 months of age
5. Roots are completed between I½ and3 yearsof age 6 to 18 months after eruption
6. By age 3 years all of the primary and permanent teeth (except for the third molars) are in some stage of development
7. Root resorption of primary teeth is triggered by the pressure exerted by the developing permanent tooth; it is followed by primary tooth exfoliation in sequential patterns
8. The primary dentition ends when the first permanent tooth erupts
Structure
There are 3 pairs
The functional unit is the adenomere.
The adenomere consists of secreting units and an intercalated duct, which opens, in a striated duct.
An secreting unit can be:
- mucous secreting
- serous secreting
THE SECRETING UNIT
THE CELLS
Serous cells
(seromucus cells=secrete also polysaccharides), They have all the features of a cell specialized for the synthesis, storage, and secretion of protein
Pyramidal, Nuclei are rounded and more centrally placed, In the basal 1/3 there is an accumulation of Granular EPR, In the apex there are proteinaceous secretory granules, Cells stain well with H & E (red), Between cells are intercellular secretory capillaries
Rough endoplasmic reticulum (ribosomal sites-->cisternae)
Prominent Golgi-->carbohydrate moieties are added
Secretory granules-->exocytosis
The secretory process is continuous but cyclic
There are complex foldings of cytoplasmic membrane
The junctional complex consists of: 1) tight junctions (zonula occludens)-->fusion of outer cell layer, 2) intermediate junction (zonula adherens)-->intercellular communication, 3)desmosomes-->firm adhesion
Mucus cells
Pyramidal, Nuclei are flattened and near the base, Have big clear secretory granules
Cells do not stain well with H & E (white)
Production, storage, and secretion of proteinaceous material; smaller enzymatic component
-more carbohydrates-->mucins=more prominent Golgi
-less prominent (conspicuous) rough endoplasmic reticulum, mitochondria
-less interdigitations
Myoepithelial cells
Star-shaped, Centrally located nucleus, Long cytoplasmic arms - bound to the secretory cells by desmosomes, Have fibrils like smooth muscle, Squeeze the secretory cell
One, two or even three myoepithelial cells in each salivary and piece body, four to eight processes
Desmosomes between myoepithelial cells and secretory cells myofilaments frequently aggregated to form dark bodies along the course of the process. The myoepithelial cells of the intercalated ducts are more spindled-shaped and fewer processes
Ultrastructure very similar to that of smooth muscle cells (myofilaments, desmosomal attachments)
Functions of myoepithelial cells
-Support secretory cells
-Contract and widen the diameter of the intercalated ducts
-Contraction may aid in the rupture of acinar cells of epithelial origin
Ductal system
Three classes of ducts:
Intercalated ducts
They have small diameter; lined by small cuboidal cells; nucleus located in the center. They have a well-developed RER, Golgi apparatus, occasionally secretory granules, few microvilli. Myoepithelial cells are also present. Intercalated ducts are prominent in salivary glands having a watery secretion (parotid).
Striated ducts
They have columnar cells, a centrally located nucleus, eosinophilic cytoplasm. Prominenty striations that refer to indentations of the cytoplasmic membrane with many mitochondria present between the folds. Some RER and some Golgi. The cells have short microvilli.
The cells of the striated ducts modify the secretion (hypotonic solution=low sodium and chloride and high potassium). There is also presence of few basal cells.
Terminal excretory ducts
Near the striated ducts they have the same histology as the striated ducts. As the duct reaches the oral mucosa the lining becomes stratified. In the terminal ducts one can find goblet cells, basal cells, clear cells. The terminal ducts alter the electrolyte concentration and add mucoid substance.
Connective tissue
Presence of fibroblasts, inflammatory cells, mast cells, adipose cells
Extracellular matrix (glycoproteins and proteoglycans)
Collagen and oxytalan fibers
Nerve supply
The innervation of salivary glands is very complicated. There is no direct inhibitory innervation. There are parasympathetic and sympathetic impulses, the parasympathetic are more prevalent.
The parasympathetic impulses may occur in isolation, evoke most of the fluid to be excreted, cause exocytosis, induce contraction of myoepithelial cells (sympathetic too) and cause vasodialtion. There are two types of innervation: epilemmal and hypolemmal. There are beta-adrenergic receptors that induce protein secretion and L-adrenergic and cholinergic receptors that induce water and electrolyte secretion.
Hormones can influence the function of the salivary glands. They modify the salivary content but cannot initiate salivary flow.
Age changes
Fibrosis and fatty degenerative changes
Presence of oncocytes (eosinophilic cells containing many mitochondria)
Clinical considerations
Role of drugs, systemic disorders, bacterial or viral infections, therapeutic radiation, obstruction, formation of plaque and calculus.
- Rich capillary networks surround the adenomeres.
Amelogenesis and Enamel
Enamel is highly mineralized: 85% hydroxyapatite crystals
Enamel formation is a two-step process
The first step produces partially mineralized enamel: 30% (secretory)
The second step: Influx of minerals, removal of water and organic matrix (maturative)
Again, dentin is the prerequisite of enamel formation (reciprocal induction)
Stratum intermedium: high alkaline phosphatase activity
Differentiation of ameloblasts: Increase in glycogen contents
Formation of the enamel matrix
Enamel proteins, enzymes, metalloproteinases, phosphatases, etc.
Enamel proteins: amelogenins (90%), enamelin, tuftelin, and amelin
Amelogenins: bulk of organic matrix
Tuftelin: secreted at the early stages of amelogenesis (area of the DE junction)
Enamelin: binds to mineral
Amelin
Mineralization of enamel
No matrix vesicles
Immediate formation of crystallites
Intermingling of enamel crystallites with dentin
"Soft" enamel is formed
Histologic changes
Differentiation of inner enamel epithelium cells. They become ameloblasts
Tomes' processes: saw-toothed appearance
Collapse of dental organ
Formation of the reduced enamel epithelium
Hard tissue formation (Amelogenesis )
Enamel formation is called amelogenesis and occurs in the crown stage of tooth development. "Reciprocal induction" governs the relationship between the formation of dentin and enamel; dentin formation must always occur before enamel formation. Generally, enamel formation occurs in two stages: the secretory and maturation stages. Proteins and an organic matrix form a partially mineralized enamel in the secretory stage; the maturation stage completes enamel mineralization.
In the secretory stage, ameloblasts release enamel proteins that contribute to the enamel matrix, which is then partially mineralized by the enzyme alkaline phosphatase. The appearance of this mineralized tissue, which occurs usually around the third or fourth month of pregnancy, marks the first appearance of enamel in the body. Ameloblasts deposit enamel at the location of what become cusps of teeth alongside dentin. Enamel formation then continues outward, away from the center of the tooth.
In the maturation stage, the ameloblasts transport some of the substances used in enamel formation out of the enamel. Thus, the function of ameloblasts changes from enamel production, as occurs in the secretory stage, to transportation of substances. Most of the materials transported by ameloblasts in this stage are proteins used to complete mineralization. The important proteins involved are amelogenins, ameloblastins, enamelins, and tuftelins. By the end of this stage, the enamel has completed its mineralization.
Nerve and vascular formation
Frequently, nerves and blood vessels run parallel to each other in the body, and the formation of both usually takes place simultaneously and in a similar fashion. However, this is not the case for nerves and blood vessels around the tooth, because of different rates of development.
Nerve formation
Nerve fibers start to near the tooth during the cap stage of tooth development and grow toward the dental follicle. Once there, the nerves develop around the tooth bud and enter the dental papilla when dentin formation has begun. Nerves never proliferate into the enamel organ
Vascular formation
Blood vessels grow in the dental follicle and enter the dental papilla in the cap stage. Groups of blood vessels form at the entrance of the dental papilla. The number of blood vessels reaches a maximum at the beginning of the crown stage, and the dental papilla eventually forms in the pulp of a tooth. Throughout life, the amount of pulpal tissue in a tooth decreases, which means that the blood supply to the tooth decreases with age. The enamel organ is devoid of blood vessels because of its epithelial origin, and the mineralized tissues of enamel and dentin do not need nutrients from the blood.