Formation and Eruption of Deciduous Teeth.

-Calcification begins during the fourth month of fetal life. By the end of the sixth month, all of the deciduous teeth have begun calcification.

-By the time the deciduous teeth have fully erupted (two to two and one half years of age), cacification of the crowns of permanent teeth is under way. First permanent molars have begun cacification at the time of birth. -Here are some things to know about eruption patterns:

(1) Teeth tend to erupt in pairs. 

(2) Usually, lower deciduous teeth erupt first. Congenitally missing deciduous teeth is infrequent. Usually, the lower deciduous central incisors are thefirst to erupt thus initiating the deciduous dentition. The appearance of the deciduous second molars completes the deciduous dentition by 2 to 2 1/2 years of age.

- Deciduous teeth shed earlier and permanent teeth erupt earlier in girls.

- The orderly pattern of eruption and their orderly replacement by permanent teeth is important.

- order for eruption of the deciduous teeth is as follows:

(1) Central incisor.........Lower 6 ½ months,         Upper 7 ½ months

(2) Lateral incisor.........Lower 7 months,   Upper 8 months

(3) First deciduous molar...Lower 12-16 months, Upper 12-16 months

(4) Deciduous canine........Lower 16-20 months, Upper 16-20 months

(5) Second deciduous molar..Lower 20-30 months, Upper 20-30 months

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.

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.

Maxillary Second Deciduous Molar.

-The notation is A or J.

-It looks like a first permanent molar

-There are three roots.

-Usually it has four well developed cusps.

-It is somwhat rhomboidal in outline.

-They often have the Carabelli trait.

- the shape the maxillary first permanent molar strongly resembles that of the adjacent deciduous second molar.

Classification of Cementum

1. Embryologically

Primary and secondary

2. According to cellular component

Acellular: Thin, Amorphous, First layer to seal the dentin tubules

Cellular: Thick, Better structure, Apical surface

Layers of cellular and acellular cementum alternate (randomly)

3. Based on the origin of the collagenous matrix
Extrinsic
Intrinsic
Mixed

4. Combined classification
a. Primary acellular intinsic fiber cementum
b. Primary acellualar extrinsic fiber cementum
c. Secondary cellular intrinsic fiber cementum
d. Secondary cellular mixed fiber cementum
e. Acellular afibrillar cementum

5. Depending on the location and patterning
Intermediate and mixed stratified cementum

Participating Cells

Cementoblasts

Active
Cells are round, plump with basophilic cytoplasm (rough endoplasmic reticulum)
Inactive
Cells have little cytoplasm
Cementocytes

1. Cementocyte lacuna
2. cementocyte canaliculus

Cells have fewer organelles compared to cementoblasts. They are found in lacunae and have numerous processes toward the periodontal ligament. Eventually they die due to avascularity

Cementicles

a) free
b) attached
c) embedded

The very first histological evidence of tooth development appear during the second month of intrauterine life. Calcification of deciduous incisors begins at 3-4 months in utero.