HISTOLOGIC CHANGES OF THE PULP

Regressive changes

Pulp decreases in size by the deposition of dentin.
This can be caused by age, attrition, abrasion, operative procedures, etc.
Cellular organelles decrease in number.

Fibrous changes

They are more obvious in injury rather than aging. Occasionally, scarring may also be apparent.

Pulpal stones or denticles

They can be: a)free, b)attached and/or c)embedded. Also they are devided in two groups: true or false. The true stones (denticles) contain dentinal tubules. The false predominate over the the true and are characterized by concentric layers of calcified material.

Diffuse calcifications

Calcified deposits along the collagen fiber bundles or blood vessels may be observed. They are more often in the root canal portion than the coronal area.

Histology of the Cementum

Cementum is a hard connective tissue that derives from ectomesenchyme.

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.

TEMPOROMANDIBULAR JOINT

There are three kind of joints:
 

·  Fibrous
Two bones connected with fibrous tissue
Examples
suture (little or no movement)
gomphosis (tooth - PDL - bone)
syndesmosis (fibula & tibia, radius and ulna; interosseous ligament)

·  Cartilagenous
Two subtypes:
2a) primary: bone<--->cartilage (costochondral joint)
2b) secondary: bone<-->cartilage<-->FT<-->cartilage<--> bone (pubic symphysis)

·  Synovial
Two bones; each articular surface covered with hyaline cartilage in most cases
The bones are united with a capsule (joint cavity)
In the capsule there is presence of synovial fluid
The capsule is lined by a synovial membrane
In many synovial joints there maybe an articular disk
Synovial joints are characterized by the presence of ligaments
Synovial joints are classified according to the number of axes of bone movement: uniaxial, biaxial, multiaxial

the shapes of articulating surfaces: planar, ginglymoid (=hinged), pivot, condyloid

The movement of the joints is controlled by muscles

The temporomandibular joint is a synovial, sliding-ginglymoid joint (humans)

Embryology of the TMJ
Primary TMJ: Meckel's cartilage --> malleus & incal cartilage. It lasts for 4 months.
Secondary TMJ: Starts developing around the third month of gestation
Two blastemas (temporal and condylar); condylar grows toward the temporal (temporal appears and ossifies first)
Formation of two cavities: inferior and upper
Appearance of disk
Bones: glenoid fossa (temporal bone) and condyle (mandible)
 

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.

Bell stage

The bell stage is known for the histodifferentiation and morphodifferentiation that takes place. The dental organ is bell-shaped during this stage, and the majority of its cells are called stellate reticulum because of their star-shaped appearance. Cells on the periphery of the enamel organ separate into three important layers. Cuboidal cells on the periphery of the dental organ are known as outer enamel epithelium.The cells of the enamel organ adjacent to the dental papilla are known as inner enamel epithelium. The cells between the inner enamel epithelium and the stellate reticulum form a layer known as the stratum intermedium. The rim of the dental organ where the outer and inner enamel epithelium join is called the cervical loop

Other events occur during the bell stage. The dental lamina disintegrates, leaving the developing teeth completely separated from the epithelium of the oral cavity; the two will not join again until the final eruption of the tooth into the mouth

The crown of the tooth, which is influenced by the shape of the internal enamel epithelium, also takes shape during this stage. Throughout the mouth, all teeth undergo this same process; it is still uncertain why teeth form various crown shapes—for instance, incisors versus canines. There are two dominant hypotheses. The "field model" proposes there are components for each type of tooth shape found in the ectomesenchyme during tooth development. The components for particular types of teeth, such as incisors, are localized in one area and dissipate rapidly in different parts of the mouth. Thus, for example, the "incisor field" has factors that develop teeth into incisor shape, and this field is concentrated in the central incisor area, but decreases rapidly in the canine area. The other dominant hypothesis, the "clone model", proposes that the epithelium programs a group of ectomesenchymal cells to generate teeth of particular shapes. This group of cells, called a clone, coaxes the dental lamina into tooth development, causing a tooth bud to form. Growth of the dental lamina continues in an area called the "progress zone". Once the progress zone travels a certain distance from the first tooth bud, a second tooth bud will start to develop. These two models are not necessarily mutually exclusive, nor does widely accepted dental science consider them to be so: it is postulated that both models influence tooth development at different times.Other structures that may appear in a developing tooth in this stage are enamel knots, enamel cords, and enamel niche.

Enamel

Structural characteristics and microscopic features

a.  Enamel rods or prisms

(1) Basic structural unit of enamel.

(2) Consists of tightly packed hydroxyapatite crystals. Hydroxyapatite crystals in enamel are four times larger and more tightly packed than hydroxyapatite found in other calcified

tissues (i.e., it is harder than bone).

(3) Each rod extends the entire thickness of enamel and is perpendicular to the dentinoenamel junction (DEJ).
 

b. Aprismatic enamel

(1) The thin outer layer of enamel found on the surface of newly erupted teeth.

(2) Consists of enamel crystals that are aligned perpendicular to the surface.

(3) It is aprismatic (i.e., prismless) and is more mineralized than the enamel beneath it.

(4) It results from the absence of Tomes processes on the ameloblasts during the final stages of enamel deposition.

c. Lines of Retzius (enamel striae)

(1) Microscopic features

 (a) In longitudinal sections, they are observed as brown lines that extend from the DEJ to the

tooth surface.

 (b) In transverse sections, they appear as dark, concentric rings similar to growth rings in a tree.
 

(2) The lines appear weekly during the formation of enamel.
 

(3) Although the cause of striae formation is unknown, the lines may represent appositional or incremental growth of enamel. They may also result from metabolic disturbances of ameloblasts.

(4) Neonatal line

(a) An accentuated, dark line of Retzius that results from the effect of physiological changes

on ameloblasts at birth.

(b) Found in all primary teeth and some cusps of permanent first molars.

d. Perikymata

(1) Lines of Retzius terminate on the tooth surface in shallow grooves known a perikymata.

(2) These grooves are usually lost through wear but may be observed on the surfaces of developing teeth or nonmasticatory surfaces of formed teeth.
 

e. Hunter-Schreger bands

(1) Enamel rods run in different directions. In longitudinal sections, these changes in direction result in a banding pattern known as HunterSchreger bands.

(2) These bands represent an optical phenomenon of enamel and consist of a series of  alternating dark and light lines when the section is viewed with reflected or polarized

light.

f. Enamel tufts

(1) Consist of hypomineralized groups of enamel rods.

(2) They are observed as short, dark projections found near or at the DEJ.

(3) They have no known clinical significance.

g. Enamel lamellae
 

(1) Small, sheet-like cracks found on the surface of enamel that extend its entire thickness.

(2) Consist of hypocalcified enamel.

(3) The open crack may be filled with organic material from leftover enamel organ components, connective tissues of the developing tooth, or debris from the oral cavity.

(4) Both enamel tufts and lamellae may be likened to geological faults in mature enamel.
 

h. Enamel spindle
 

(1) Remnants of odontoblastic processes that become trapped after crossing the DEJ during the differentiation of ameloblasts.
 

(2) Spindles are more pronounced beneath the cusps or incisal edges of teeth (i.e., areas where occlusal stresses are the greatest).
 

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.