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"

MANDIBULAR THIRD MOLAR

Facial: The crown is often short and has a rounded outline.

Lingual: Similarly, the crown is short and the crown is bulbous.

Proximal: Mesially and distally, this tooth resembles the first and second molars. The crown of the third molar, however, is shorter than either of the other molars

Occlusal: Four or five cusps may be present. Occlusal surface is a same as of the first or second molar, or poorly developed with many accessory grooves. The occlusal outline is often ovoid and the occlusal surface is constricted. Occasionally, the surface has so many grooves that it is described as crenulated--a condition seen in the great apes

Contact Points; The rounded mesial surface has its contact area more cervical than any other lower molar. There is no tooth distal to the third molar..

Roots:-The roots, two in number, are shorter in length and tend to be fused together. they show a distinct distal curve

Soft Oral Tissues

Oral Mucosa

The oral mucosa consists mainly of two types of tissues: the oral epithelium, which consists of stratified, squamous epithelium, and the underlying connective tissue layer, known as the lamina propria.  There are three variations of oral mucosa.

A. Oral epithelium

1. Consists of stratified, squamous epithelium.

2. Four layers (Note: Cells mature as they progress from the deepest [basal] layer to the most superficial [cornified] layer) a. Basal layer (stratum germinativum or basale)

(1) A single layer of cuboidal or columnar cells overlying the lamina propria.

(2) Contains progenitor cells and thus provides cells to the epithelial layers above.

(3) Site of cell division (mitosis).

b. Prickle cell layer (stratum spinosum)

(1) Consists of several layers of larger, ovoid-shaped cells.

c. Granular layer (stratum granulosum)

(1) Cells appear larger and flattened.

(2) Granules (known as keratohyaline granules) are present in the cells.

(3) This layer is absent in nonkeratinized epithelium.

d. Cornified layer (stratum corneum, keratin, or horny layer)

(1) In keratinized epithelium:

(a) Orthokeratinized epithelium the squamous cells on the surface appear flat and contain keratin. They have no nuclei present.

(b) Parakeratinized epithelium the squamous cells appear flat and contain keratin; nuclei are present within the cells.

(2) In parakeratinized epithelium, both squamous cells without nuclei and cells with shriveled (pyknotic) nuclei are present.

(3) In nonkeratinized epithelium, the cells appear slightly flattened and contain nuclei.

B. Lamina propria

1. Consists of type I and III collagen, elastic fibers, and ground substance. It also contains many cell types, including fibroblasts, endothelial cells, immune cells, and a rich vascular and nerve supply.

2. Two layers:

a. Superficial, papillary layer

(1) Located around and between the epithelial ridges.

(2) Collagen fibers are thin and loosely arranged.

b. Reticular layer

(1) Located beneath the papillary layer.

(2) Collagen fibers are organized in thick, parallel bundles.

C. Types of oral mucosa

1. Masticatory mucosa

a. Found in areas that have to withstand compressive and shear forces.

b. Clinically, it has a rubbery, firm texture.

c. Regions: gingiva, hard palate.

2. Lining mucosa

a. Found in areas that are exposed to high levels of friction, but must also be mobile and distensible.

b. Clinically, it has a softer, more elastic texture.

c. Regions: alveolar mucosa, buccal mucosa, lips, floor of the mouth, ventral side of the tongue, and soft palate.

3. Specialized mucosa

a. Similar to masticatory mucosa, specialized mucosa is able to tolerate high compressive

and shear forces; however, it is unique in that it forms lingual papillae.

b. Region: dorsum of the tongue.

D. Submucosa

1. The connective tissue found beneath the mucosa . It contains blood vessels and nerves and may also contain fatty tissue and minor salivary glands.

2. Submucosa is not present in all regions of the oral cavity, such as attached gingiva, the tongue, and hard palate. Its presence tends to increase the mobility of the tissue overlying it.

E. Gingiva

1. The portion of oral mucosa that attaches to the teeth and alveolar bone.

2. There are two types of gingiva: attached and free gingiva. The boundary at which they meet is known as the free gingival groove .

a. Attached gingiva

(1) Directly binds to the alveolar bone and tooth.

(2) It extends from the free gingival groove to the mucogingival junction.

b. Free gingiva

(1) Coronal to the attached gingiva, it is not bound to any hard tissue.

(2) It extends from the gingival margin to the free gingival groove.

c. Together, the free and attached gingiva form the interdental papilla.

.F. Alveolar mucosa

1. The tissue just apical to the attached gingiva.

2. The alveolar mucosa and attached gingiva meet at the mucogingival junction .

G. Junctional epithelium

1. Area where the oral mucosa attaches to the tooth, forming the principal seal between the oral cavity and underlying tissues.

2. Is unique in that it consists of two basal lamina, an internal and external . The internal basal lamina, along with hemidesmosomes, comprises the attachment apparatus (the epithelial attachment). This serves to attach the epithelium directly to the tooth.

3. Histologically, it remains as immature, poorly differentiated tissue. This allows it to maintain its ability to develop hemidesmosomal attachments.

4. Has the highest rate of cell turnover of any oral mucosal tissue.

H. Interdental papilla (interdental gingiva)

1. Occupies the interproximal space between two teeth. It is formed by free and attached gingiva.

2. Functions to prevent food from entering the (interproximal) area beneath the contact point of two adjacent teeth. It therefore plays an important role in maintaining the health of the gingiva.

3. Col

a. If the interdental papilla is cross-sectioned in a buccolingual plane, it would show two peaks (buccal and lingual) with a dip between them, known as the col or interdental col. This depression occurs around the contact point of the two adjacent teeth.

b. Histologically, col epithelium is the same as junctional epithelium

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.

Cementum

Composition

a. Inorganic (50%)—calcium hydroxyapatite crystals.

b. Organic (50%)—water, proteins, and type I collagen.

c. Note: Compared to the other dental tissues, the composition of cementum is most similar to bone; however, unlike bone, cementum is avascular (i.e., no Haversian systems or other vessels are present).

Main function of cementum is to attach PDL fibers to the root surface.

Cementum is generally thickest at the root apex and in interradicular areas of multirooted 

Types of cementum

a. Acellular (primary) cementum

(1) A thin layer of cementum that surrounds the root, adjacent to the dentin.

(2) May be covered by a layer of cellular cementum, which most often occurs in the middle and apical root.

(3) It does not contain any cells.

b. Cellular (secondary) cementum

(1) A thicker, less-mineralized layer of cementum that is most prevalent along the apical root and in interradicular (furcal) areas of multirooted teeth.

(2) Contains cementocytes.

(3) Lacunae and canaliculi:

(a) Cementocytes (cementoblasts that become trapped in the extracellular matrix during cementogenesis) are observed in their entrapped spaces, known as lacunae.

(b) The processes of cementocytes extend through narrow channels called canaliculi.

(4) Microscopically, the best way to differentiate between acellular and cellular cementum is the presence of lacunae in cellular cementum.

Tooth development is commonly divided into the following stages: the bud stage, the cap, the bell, and finally maturation. The staging of tooth development is an attempt to categorize changes that take place along a continuum; frequently it is difficult to decide what stage should be assigned to a particular developing tooth. This determination is further complicated by the varying appearance of different histological sections of the same developing tooth, which can appear to be different stages.

Bud stage

The bud stage is characterized by the appearance of a tooth bud without a clear arrangement of cells. The stage technically begins once epithelial cells proliferate into the ectomesenchyme of the jaw. The tooth bud itself is the group of cells at the end of the dental lamina.