Tooth is hard bonelike structure in the jaws of vertebrates, it is used for biting and chewing or for attack and defense.
Tooth is an organ made of different tissues:
- enamel
- dentin
- cementum
- pulp
Enamel provides shape and a hard, durable surface for teeth and a protective cap for the dentin and pulp. Both color and form contribute to the esthetic appearance of enamel. Although is capable of liflong service, its crystalline mineral make up and rigidity, exposed to an oral environment of occlusal, chemical, and bacterial challenges, make it vulnerable to acid demineralization, attrition and fracture.
Enamel is as hard as steel, with a Knoop Hardness Number of 343 (compared with 68 for dentin).
The mature enamel is:
- 96% inorganic hydroxyapatite mineral weight and more than 86% by volume
- a small volume of organic matrix
- 4% to 12% water (which is contained in the intercrystalline spaces and in a netwark of micropores opening to the external surface)
Structure
Ameloblasts cells secrete the organic matrix gel to define the enamel contours and initiate its mineralization. Calcium ions are trasported both extra- and intracellulary to form "seeds" of hydroxyapatite throughout the developing matrix. The repeating molecular units of hydroxyapatite, make up the building bloks of enamel crystal. The divergents directions of the crystals generated from the central and the peripheral surfaces of Tomes' processes, repeated in a symmetric pattern, from the two basic structural units of enamel:
- cylindrical enamel rods
- surrounding interrod enamel
A narrow space filled with organic material around three fourths of each rod, called rod sheath, separates the two enamel units.
Permeability
Various fluid, ions, and low-molecular weight substances can diffuse through semipermeable enamel. Therefore, the dynamics of acid demineralization, reprecipitation or remineralization, fluoride uptake, and vital bleanchingtherapy are not limitate to the surface but are active in three dimentions. When teeth become dehydrated the empty micropores make the enamel appear chalky and lighter in color
Lifelong exposure of semipermeable enamel to the ingress of elements from the oral environment into the mineral structure of the tooth results in coloration intensity and resistence of demineralization.
Fuoride enhances the remineralization repair of enamel demaged by plaque-acids to increase the ratio or conversion of hydroxyapatite to more stable or less acid-soluble crystals of fluorohydroxyapatite or fluoroapatite.
Dentin provides both color and an elastic foundaion for the enamel. The radicular (root) dentin covered with cementum and the coronal (crown) dentin supporting the enamel form the bulk strucure of the tooth. The strength and durability of the coronal structures are relatede to dentin integrity. Dentin is a protective barrier and chamber for vital pulp tissues. As a tissue without substantive vascular supply or innervation, it is neverthless able to respond to external, chemical or mechanical stimuli.
Structure
Dentin is primary composed of small, thin apatite crystal flakes embedded in a protein matrix of cross-linked collagen fibrils. The odontoblast, with its cell body at the pulp periphery and its extended process within the dentinal tubule (to about 1/3 the dentin thickness), secretes the organic dentin matrix and regulates mineralization. The tubule paths form a duble curve or S shape.
Dentin is typically pale yallow in color and is slightly harder than bone. Two many types of dentin are present:
- intertubular dentin, the structural components of the hydroxyapatite-embedded collegen matrix forming the bulk of dentin structure
- peritubular dentin, limitated to the lining of the tuble walls
The mature dentin (by volume) is:
- 45-50% inorganic apatite crystal
- 30% organic matrix
- 25% water
These component ratios vary according to:
- location (depth) in dentin
- age
- trauma history of the tooth
Permeability
The open tubular channels of dentin compromize its function as a protective barrier. When the external covering of enamel or cementum is removed from dentin the exposed tubules become conduits between the pulp and the external oral environment. When injury or active caries affect dentin, the immediate inflammatory response is pulpal vasodilation, increased blood flow, and increased interstitial fluid pressure, which results in an increased outward flow rate of tubular fluid. Albumin and immunoglobulins pass from plasma to dentinal fluid and here agglutinate to limit the diffusion to the pulp of exogenous stimuli and possibly to provide a direct immune response to bacteria.
Newly erupted teeth with relatively open tubules are particulary vulnerable to pulpal effects from active caries and rapid penetration of bacteria.
Cementum is the thin layer of calcified (tough calcium deposits) tissue covering the dentine of the root and is one of four tissues that support the tooth in the jaw (together with alveolar bone, the periodontal ligament and the gingivae). Cementum is pale yellow with a dull surface and is softer than dentine. Cementum is adjacent with the periodontal ligament on its outer surface and is firmly fixed to dentine on its deep surface. Its primary function is to give attachment to collagen fibres of the periodontal ligament. It therefore is a highly responsive tissue maintaining the integrity of the root helping to maintain the tooth in its functional position in the mouth and being involved in tooth repair and regeneration.
Structure
As in dentin the organic matrix is composed of a framework of fine collagen fibrins held together by a ground substance that becomes mineralized. Cementum may contains whole cells and is formed in layers that result in the appositional growth lines seen by an histologic point of view. These lines are parallel to the long axis of the root.
Cementum is similar in chemical composition and physical properties to bone however cementum is avascular (not associated with or supplied by blood vessels).Cementum is volumetrically composed of:
- 45% of inorganic material
- approximately 33% of organic material
- 22% water.
Permeability
The permeability of cementum varies with age and the type of cementum with the cellular variety being more permeable. In general cementum is more permeable than dentine. The relative softness of cementum combined with its thinness means that it is readily removed by abrasion when the root surface is exposed to the oral environment.
Dental pulp is a viscous connettive tissue of collagen fibers and organic ground substance supporing the vital cellular, vascular and nerve structures of the tooth. The vascularization is channeled through one opening, the apical foramen at the root apex, and is encased within rigid dentinal walls.
Dental pulp has different functions:
- Formative (dentinogenesis)
- Nutritive
- Sensory
- Protective (coordinates responses to injury and noxious stimuli)
Structure
The pulp tissue is divided into four zone:
- Odontoblastic layer
- Cell-free zone
- Cell-rich zone
- Pulp core
The most numerous pulpal cells are fibroblasts that produce, maintain and remodel pulp matrix and collagen. The other components of the cells population are undifferentiated mesenchymal cells, stemm cells, pericytes and smooth muscle cells. Immunocompetent cells includes macrophages, lymphocytes and dendritic cells that function as a host defense system against foreign bodies and antigens.
Vascularization
A stimulus that demages the cell initiates neural and chemical signals that increase blood flow and capillary permeability. Plasma proteins, fluids and leukocytes spill into the confined extracellular space and elevate interstitial fluid pressure. Elevated extravascular tissue pressure could collapse the venule and start a destructive ischemia. Pulpal circulation numerous arteriole "U-turns", or reserve flow loops, and arteriole-venule anastomoses, or shunt to bypass the affected capillary bad. An afflicted tooth may become hypersensitive to cold or other stimuli.
Innervation
The innervation of tha pulp consist in:
- sensory axons (afferent), with cell bodies located a great distance away in trigeminal ganglion
- sympathetic axons (efferent), with nuclei in the cervical sympathetic ganglia
There are two types of nerves:
- A-beta nerves, with a proprioceptive or touch pressure function
- A-delta and C nerves, sensory interdental nerves
The terminal nerve ends contain receptors for released or generated bioactive factors caused by injury to the dentin, pulpal cells or interstitial environment. Some of these factor are: nerve growth factor, calcitonin gene-related peptide (CGRP) and substance P .
