Clinical importance of cementum

1) Deposition of cementum continues throughout life.
The effects of the continuous deposition of cementum are the maintenance of total length of the tooth (good) and constriction of the apical foramen (bad).
2) With age, the smooth surface of cementum becomes more irregular due to calcification of some ligament fiber bundles. This is referred to as spikes.

Behavior of cementum in pathologic conditions

The periodontium, which is the supporting structure of a tooth, consists of the cementum, periodontal ligaments, gingiva, and alveolar bone. Cementum is the only one of these that is a part of a tooth. Alveolar bone surrounds the roots of teeth to provide support and creates what is commonly called a "socket". Periodontal ligaments connect the alveolar bone to the cementum, and the gingiva is the surrounding tissue visible in the mouth.

Periodontal ligaments

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.

Abnormalities

There are a number of tooth abnormalities relating to development.

Anodontia is a complete lack of tooth development, and hypodontia is a lack of some tooth development. Anodontia is rare, most often occurring in a condition called hipohidrotic ectodermal dysplasia, while hypodontia is one of the most common developmental abnormalities, affecting 3.5–8.0% of the population (not including third molars). The absence of third molars is very common, occurring in 20–23% of the population, followed in prevalence by the second premolar and lateral incisor. Hypodontia is often associated with the absence of a dental lamina, which is vulnerable to environmental forces, such as infection and chemotherapy medications, and is also associated with many syndromes, such as Down syndrome and Crouzon syndrome.

Hyperdontia is the development of extraneous teeth. It occurs in 1–3% of Caucasians and is more frequent in Asians. About 86% of these cases involve a single extra tooth in the mouth, most commonly found in the maxilla, where the incisors are located. Hyperdontia is believed to be associated with an excess of dental lamina.

Dilaceration is an abnormal bend found on a tooth, and is nearly always associated with trauma that moves the developing tooth bud. As a tooth is forming, a force can move the tooth from its original position, leaving the rest of the tooth to form at an abnormal angle. Cysts or tumors adjacent to a tooth bud are forces known to cause dilaceration, as are primary (baby) teeth pushed upward by trauma into the gingiva where it moves the tooth bud of the permanent tooth.

Regional odontodysplasia is rare, but is most likely to occur in the maxilla and anterior teeth. The cause is unknown; a number of causes have been postulated, including a disturbance in the neural crest cells, infection, radiation therapy, and a decrease in vascular supply (the most widely held hypothesis).Teeth affected by regional odontodysplasia never erupt into the mouth, have small crowns, are yellow-brown, and have irregular shapes. The appearance of these teeth in radiographs is translucent and "wispy," resulting in the nickname "ghost teeth"

Types of dentitions:

1. Diphyodont. Teeth develop and erupt into their jaws in two generations of teeth. The term literally means two generations of teeth.

2. Monophyodont. a single generation of teeth.

3. Polyphyodont. Teeth develop a lifetime of generations of successional teeth

4. Homodont. all of the teeth in the jaw are alike. They differ from each other only in size.

5. Heterodont. There is distinctive classes of teeth that are regionally specialized.

Dentin

Composition: 70% inorganic, 20% organic, 10% water by weight and 45%, 33%, and 22% in volume respectively
Hydroxyapatite crystals and collagen type I
Physical characteristics: Harder than bone and softer than enamel
Yellow in color in normal teeth
Radiographic appearance: More radiolucent than enamel

Primary (circumpulpal) dentin: forms most of the tooth
Mantle dentin: first dentin to form; forms the outline of dentin in the adult tooth
Predentin: lines the innermost portion of dentin (faces the pulp)
Secondary dentin: after root formation dentin continues to form, continuous to primary dentin but with structural irregularities
Tertiary dentin: reactive or reparative dentin; may or may not have characteristics of primary dentin; produced in the area of an external stimulus; osteodentin

Dentin is formed by cells called odontoblasts.
These cells derive from the ectomesenchyme and produce the organic matrix that will calcify and become the dentin.
Formation of dentin initiates formation of enamel.
The formation of dentin starts during late bell-stage in the area of the future cusp.

First coronal dentin and then root dentin.

Completion of dentin does not occur until about 18 months after eruption of primary and 2-3 years after eruption of permanent teeth.

The rate of dentin development varies.

The role of the internal (inner) dental (enamel) epithelium
Cuboidal - Columnar (reverse polarization)
Ectomesenchymal cells of the dental papilla become preodontoblasts - odontoblasts
Acellular zone disappears

Histologic features of dentin
Odontoblasts
Dentinal tubules
Extend through the entire thickness of dentin
S-shaped (primary curvatures) path in the crown, less S-shaped in the root, almost straight in the cervical aspect
Secondary curvatures
Tubular microbranches
Presence of fluid
 

Intratubular dentin
Dentin in the tubule that is hypermineralized

The term peritubular dentin should not be used
 

Sclerotic dentin
Dentinal tubules that are occluded with calcified material
Most likely a physiologic response
Reduction of permeability of dentin
 

Intertubular dentin
Dentin between the tubules
 

Interglobular dentin
Areas of unmineralized or hypomineralized dentin
The defect affects mineralization and not the architecture of dentin
 

Incremental lines
Lines of von Ebner: lines associated with 5-day rythmic pattern of dentin deposition
Contour lines of Owen: Originally described by Owen they result from a coincidence of the secondary curvatures between neighboring dentinal tubules.
 

Granular Layer of Tomes
Seen only in ground sections in the root area covered by cementum
Originally, they were thought to be areas of hypomineralization
They are true spaces obtained by sections going through the looped terminal portions dentinal tubules

DE junction :Scalloped area

Enamel tissue with incremental lines of Retzius and dentin tissue with parallel, curved dentinal tubules are in contact at the irregular dentino-enamel junction. The junction often has a scalloped-shaped morphology

DC junction Dentin Cemental Junction

Tooth development is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth.. For human teeth to have a healthy oral environment, enamel, dentin, cementum, and the periodontium must all develop during appropriate stages of fetal development. Primary teeth start to form between the sixth and eighth weeks in utero, and permanent teeth begin to form in the twentieth week in utero.

 Overview

The tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth.These cells are derived from the ectoderm of the first branchial arch and the ectomesenchyme of the neural crest.The tooth bud is organized into three parts: the enamel organ, the dental papilla and the dental follicle.

The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium.These cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. The location where the outer enamel epithelium and inner enamel epithelium join is called the cervical loop. The growth of cervical loop cells into the deeper tissues forms Hertwig's Epithelial Root Sheath, which determines the root shape of the tooth.

The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells. Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth. Mesenchymal cells within the dental papilla are responsible for formation of tooth pulp.

The dental follicle gives rise to three important entities: cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the alveolar bone around the roots of teeth. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum.