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.

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

1. Fibrous layer: collagen type I, avascular (self-contained and replicating)
2. Proliferating zone that formes condylar cartilage
3. Condylar cartilage is fibrocartilage that does not play role in articulation nor has formal function
4. Capsule: dense collagenous tissue (includes the articular eminence)
5. 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
6. 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
7. 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
 

Periodontal ligament

Composition

a. Consists mostly of collagenous (alveolodental) fibers.
Note: the portions of the fibers embedded in cementum and the alveolar bone proper are known as Sharpey’s fibers.

b. Oxytalan fibers (a type of elastic fiber) are also present. Although their function is unknown, they may play a role in the regulation of vascular flow.

c. Contains mostly type I collagen, although smaller amounts of type III and XII collagen are also present.

d. Has a rich vascular and nerve supply.

Both sensory and autonomic nerves are present.

(1) The sensory nerves in the PDL differ from pulpal nerves in that PDL nerve endings can detect both proprioception (via mechanoreceptors) and pain (via nociceptors).

(2) The autonomic nerve fibers are associated with the regulation of periodontal vascular flow.

(3) Nerve fibers may be myelinated (sensory) or unmyelinated (sensory or autonomic).

Cells

a. Cells present in the PDL include fibroblasts; epithelial cells; cementoblasts and cementoclasts; osteoblasts and osteoclasts; and immune cells such as macrophages, mast cells, or eosinophils.

b. These cells play a role in forming or destroying cementum, alveolar bone, or PDL.

c. Epithelial cells often appear in clusters, known as rests of Malassez.

Types of alveolodental fibers

a. Alveolar crest fibers—radiate downward from cementum, just below the cementoenamel junction (CEJ), to the crest of alveolar bone.

b. Horizontal fibers—radiate perpendicular to the tooth surface from cementum to alveolar bone, just below the alveolar crest.

c. Oblique fibers

(1) Radiate downward from the alveolar bone to cementum.

(2) The most numerous type of PDL fiber.

(3) Resist occlusal forces that occur along the long axis of the tooth.

d. Apical fibers

(1) Radiate from the cementum at the apex of the tooth into the alveolar bone.

(2) Resist forces that pull the tooth in an occlusal direction (i.e., forces that try to pull the tooth from its socket).

e. Interradicular fibers

(1) Only found in the furcal area of multi-rooted teeth.

(2) Resist forces that pull the tooth in an occlusal direction.

Gingival fibers

a. The fibers of the gingival ligament are not strictly part of the PDL, but they play a role in the maintainence of the periodontium.

b. Gingival fibers are packed in groups and are found in the lamina propria of gingiva

c. Gingival fiber groups:

(1) Transseptal (interdental) fibers

(a) Extend from the cementum of one tooth (just apical to the junctional epithelium), over the alveolar crest, to the corresponding area of the cementum of the adjacent tooth.

(b) Collectively, these fibers form the interdental ligament , which functions to resist rotational forces and retain adjacent teeth in interproximal contact.

(c) These fibers have been implicated as a major cause of postretention relapse of teeth that have undergone orthodontic treatment.

(2) Circular (circumferential) fibers

(a) Extend around tooth near the CEJ.

(b) Function in binding free gingiva to the tooth and resisting rotational forces.

(3) Alveologingival fibers—extend from the alveolar crest to lamina propria of free and attached gingiva.

(4) Dentogingival fibers—extend from cervical cementum to the lamina propria of free and attached gingiva.

(5) Dentoperiosteal fibers—extend from cervical cementum, over the alveolar crest, to the periosteum of the alveolar bone.

Alveolar bone (process)

1. The bone in the jaws that contains the teeth alveoli (sockets).

2. Three types of bone :

a. Cribriform plate (alveolar bone proper)

(1) Directly lines and forms the tooth socket. It is compact bone that contains many holes, allowing for the passage of blood vessels. It has no periosteum.

(2) Serves as the attachment site for PDL (Sharpey’s) fibers.

(3) The tooth socket is constantly being remodeled in response to occlusal forces. The bone laid down on the cribriform plate, which also provides attachment for PDL fibers, is known as bundle bone.

(4) It is radiographically known as the lamina dura.

b. Cortical (compact) bone

(1) Lines the buccal and lingual surfaces of the mandible and maxilla.

(2) Is typical compact bone with a periosteum and contains Haversian systems.

(3) Is generally thinner in the maxilla and thicker in the mandible, especially around the buccal area of  the mandibular premolar and molar.

c. Trabecular (cancellous, spongy) bone

(1) Is typical cancellous bone containing Haversian systems.

(2) Is absent in the maxillary anterior teeth region.

3. Alveolar crest (septa)

a. The height of the alveolar crest is usually 1.5 to 2 mm below the CEJ junction.

b. The width is determined by the shape of adjacent teeth.

(1) Narrow crests—found between teeth with relatively flat surfaces.

(2) Widened crests—found between teeth with convex surfaces or teeth spaced apart.

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.

Genetics and Environment: Introduction

The size of the teeth and the timing of the developing dentition and its eruption are genetically determined. Teeth are highly independent in their development. Also, teeth tend to develop along a genetically predetermined course.: tooth development and general physical development are rather independent of one another. Serious illness, nutritional deprivation, and trauma can significantly impact development of the teeth. This genetic independence (and their durability) gives teeth special importance in the study of evolution.

Teeth erupt full size and are ideal for study throughout life. Most important, age and sex can be recorded.

When teeth erupt into the oral cavity, a new set of factors influence tooth position. As the teeth come into function, genetic and environment determine tooth position.

In real life, however, girls shed deciduous teeth and receive their permanent teeth slightly earlier than boys, possibly reflecting the earlier physical maturation achieved by girls. Teeth are slightly larger in boys that in girls