The Transition from the Deciduous to the Permanent Dentition.

1. The transition begins with the eruption of the four first permanent molars, and replacement of the lower deciduous central incisors by the permanent lower central incisors.

2. Complete resorption of the deciduous tooth roots permits exfoliation of that tooth and replacement by the permanent (successional) teeth

3. The mixed dentition exists from approximately age 6 years to approximately age 12 years. In contrast, the intact deciduous dentition is functional from age 2 - 2 /2 years of age to 6 years of age.

4. The enamel organ of each permanent anterior tooth is connected to the oral epithelium via a fibrous cord, the gubernaculum. The foramina through which it passes can be seen in youthful skulls

The deciduous second molars are particularly important. It is imperative that the deciduous second molars be preserved until their normal time of exfoliation. This prevent mesial migration of the first permanent molars.

Use a space maintainer in the event that a second deciduous molar is lost prematurely

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

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.

MAXILLARY SECOND MOLAR

The second molars are often called 12-year molars because they erupt when a child is about 12 years

Facial: The crown is shorter occluso-cervically and narrower mesiodistally whe compared to the first molar. The distobuccal cusp is visibly smaller than the mesiobuccal cusp. The two buccal roots are more nearly parallel. The roots are more parallel; the apex of the mesial root is on line with the with the buccal developmental groove. Mesial and distal roots tend to be about the same length.

Lingual: The distolingual cusp is smaller than the mesiolingual cusp. The Carabelli trait is absent.

Proximal: The crown is shorter than the first molar and the palatal root has less diverence. The roots tend to remain within the crown profile.

Occlusal: The distolingual cusp is smaller on the second than on the first molar. When it is much reduced in size, the crown outline is described as 'heart-shaped.' The Carabelli trait is usually absent. The order of cusp size, largest to smallest, is the same as the first but is more exaggerated: mesiolingual, mesiobuccal, distobuccal, and distolingual.

Contact Points; Height of Curvature: Both mesial and distal contacts tend to be centered buccolingually below the marginal ridges. Since themolars become shorter, moving from first to this molar, the contacts tend to appear more toward the center of the proximal surfaces.

Roots: There are three roots, two buccal and one lingual. The roots are less divergent than the first with their apices usually falling within the crown profile. The buccal roots tend to incline to the distal.

Note: The distolingual cusp is the most variable feature of this tooth. When it is large, the occlusal is somewhat rhomboidal; when reduced in size the crown is described as triangual or 'heart-shaped.' At times, the root may be fused.