The+Histology+of+the+Oral+Cavity

=**Objectives**=


 * 1. Describe and identify the component parts of the lip (skin, vermilion border, labial mucosa, labial glands & orbicularis oris muscle). Relate their structure to function.**

The lips consist of a core of skeletal muscle, the obricularis oris, which gives lips their mobility. Externally, the lips have a stratified squamous keratinized epithelium with hair follicles and sweat glands of thin skin. There’s a red vermilion border region of stratified squamous parakeratinized epithelium that has tall dermal papillae with capillaries that give the red color. The vermilion border has no sweat glands. The oral mucosa has two types of epithelial lining: lining mucosa and masticatory mucosa. The lining mucosa lines most of the oral cavity with a mucosal stratified squamous epithelium on moderately dense CT. The masticatory mucosa lines regions exposed to considerable pressure and shear forces such as the gums, hard palate and dorsum of the tongue; this epithelium is parakeratinized to keratinized stratified squamous epithelium bound tightly to dense irregular CT. Gums lack submucosa and glands.


 * 2. Identify and describe the component parts of the tongue (lingual mucosa, filiform, fungiform, and vallate papillae, taste buds, skeletal muscles, lingual salivary glands, and von Ebner glands). Relate their structure to their function.**

The tongue is covered with the mucosa of stratified squamous non-keratinizing epithelium and lamina propria, has serous and mucus glands, and has a core of skeletal muscle. The extrinsic muscles of the tongue originate outside and insert on the tongue (genioglossus, hyloglossus, styloglossus, and platatoglossus) and are responsible for moving the tongue. Intrinsic muscles originate and insert from within the tongue and are arranged in longitudinal, transverse, and oblique bundles responsible for changing the shape of the tongue (especially during phonation).

The dorsum is divided in an anterior 2/3 and posterior 1/3 by the sulcus terminalis. The foramen cecum is a depression where the thyroid gland descended. The posterior 1/3 of the tongue contains the root of the tongue and the lingual tonsils. There are numerous lingual papillae located on the dorsum of the tongue that have a vascular core of CT covered by the stratified squamous non-keratinizing mucosal type epithelium.

There are four types of papillae: filiform, fungiform, foliate, and circumvallate papillae. Filiform papillae are the most numerous and have keratinized epithelium and no taste buds. They function in increase friction between tongue and food. The fungiform papillae are mushroom shaped and located on the lateral edges of the tongue. They are not keratinized and contain taste buds. Foliate papillae are leaf-shaped and poorly developed in humans, located on the lateral margin of the tongue. They look like pages of a book and lose their taste buds by age 2-3. Cicumvallate papillae are large, round, walled papillae located just in front of the sulcus terminalis. They are epithelial lined and have taste buds located in the groove and sides as well as serous glands of von Ebner draining to the base of the moat.

Each tooth contains a crown, root, neck, and pulp. The clinical crown is the part that is visible by the oral cavity, and does not include the part covered by the gums. The anatomical crown is any part that is covered by enamel, including the areas covered by the gums. The root is the part of the tooth that is buried in the alveolus of the bone where the gingiva junctional epithelium attaches to enamel. The pulp chamber is the space in the center of the tooth that is filled with vascular areolar connective tissue, proteoglycans and glycoproteins. The root canal is a hole that connects the pulp chamber with the periodontal space and is transversed by blood vessels and nerves supplying the tooth. The pulp chamber is lined by odontoblasts which produce dentin.
 * 3. Describe and integrate the component parts of the tooth (enamel, dentin, cementum, pulp, anatomic root and crown, neck, and clinical crown). Relate structure to function.**

The bony tooth is composed of 3 mineralized components: enamel, dentin, and cementum. The bulk of the tooth is made up of dentin which surrounds the pulp and the root canal. The crown is covered by enamel and the root covered by cementum.


 * 4. Describe the composition and structure of the peridontal ligament and relate it to its function.**

The periodontal ligament holds teeth in their sockets. They consist of dense irregular connective tissue. Sharpey’s fibers are collagen fibers in the periodontal ligament and are imbedded in the alveolar bone and cementum to hold the tooth in place as a fibrous gomphosis joint. Gingiva supports the tooth and seals off the oval cavity from the connective tissue.


 * 5. Identify the distinguishing features of the gingiva and gingiva sulcas associated with the alveolar processes bearing teeth.**

Because the gingiva is exposed to considerable pressure and shear forces, it has a stratified squamous epithelium that is parakeratinized or keratinized. It is tightly bound to dense irregular CT. As the epithelium of the gingiva approaches the tooth, it forms a hairpin turn at the gingival groove and proceeds toward the root of the tooth, crating a space adjacent to the tooth and deep to the gingival surface. The junctional epithelium is where epithelium attaches to the enamel surface via hemidesmosomes, forming a sealing barrier around the neck of the tooth.


 * 6. Describe the composition of primary and secondary dentition.**

Humans have two sets of teeth: 20 deciduous teeth and 32 permanent teeth. Both sets of teeth are embedded in the alveolar processes of the maxillary bone and mandible.

There are three types of teeth. Incisors are for cutting, canines for puncturing and holding, and molars for crushing and grinding.


 * 7. Describe the development of the typical tooth. Identify the formation and origins of dental lamina, enamel organ, dental papillae, dental sac, ameloblasts and enamel, odonoblasts, and dentin, epithelial root sheath and the process of eruption.**

Tooth development begins at 6-8 weeks of development and has three stages: bud, cap, and bell stage. During the bud stage, the oral ectoderm proliferates to form dental lamina and froms a tooth bud for each tooth (10 per jaw). Neural crest derived ectomesenchyme surrounds the down growing bud, separated from the dental lamina by basal lamina.

During early cap stage, proliferation of the tooth bud increases the size and changes the bud to a cap-shaped enamel organ with three layers. The other enamel epithelium resides on the outer surface and an inner enamel epithelium resides on the inner surface. A stellate reticulum group of cells develops between the inner and outer layers. By the late phase, the ectomesenchyme condeses to forma dental papilla and gives rise to the odontoblasts and dental pulp beneath the inner enamel layer. The cervical loop is the rim where two ectodermal layers make a sharp bend. The dental sac is the vascular structure surrounding the tooth germ that is formed from the ectomesenchyme. By this time, the ectodermal connection to the surface is lost and a permanent too bud arises.

The cap continues to enlarge until it form a bell in the bell stage. It is characterized by the development of the stratum intermedium in the enamel organ between the stellate reticulum and inner enamel epithelium. The outer enamel epithelium breaks down and the dental sac collapses on the stratum intermedium. This causes the stratum intermedium to induce the inner enamel layer to differentiate into ameloblasts. Differentiation of ameloblasts induces the inner enamel layer of the dermal papillae to differentiate into odontoblasts which begin to lay down dentin. Once dentin is layed down, the enamel is deposited to form the dentinoenamel junction.

The appositional phase is the deposition of the dentin and enamel. Odontoblasts are pushed away from the junction as the dentin is deposited though the distal end of the cell remains at the junction to form the cytoplasmic odontoblastic process within a dentinal tubule. Unmineralized dentinal matrix is laid down and later mineralized. The ameloblast is laying down the enamel in much the same manner but when the unmineralized matrix is laiddown,the apical process is pinched down to form the Tomes process and do not continue throughtout the enamel.

Root formation occurs after the crown is complete. Hertwig epithelial root sheath (HERS) forms root from the cervical root and extends down. Because stratum intermedium does not form,the inner epithelial layer for the root does not develop ameloblasts. However, odontoblasts still form. HERS disintergrates in the cervical loop region and the ectomesenchyme cells contact the dentin and differentiate into cementoblasts to deposit cementum.


 * 8. Describe how the structural characteristics of odontoblasts, ameleoblasts and cementoblasts relate to their function.**

Odontoblasts produce dentin and are present throughout life (see p. 257). Ameloblasts lay down enamel from the dentinal surface outward (see p. 258). Cementoblasts line outer surface at periodontal ligament space and secrete cementum throughout life.


 * 9. Compare and contrast the composition, formation, structure, and function of enamel, dentin, and cementum.**

Enamel is the hardest substance in the body with over 96% hydroxyapatite crystals with the rest being enamelins and amelogenin glycoproteins. They are arranged in keyhole shaped enamel rods that extend from the surface to dentin. Each rod is coated by an enamel sheath of organic material. Interrod regions have hydroxyapatite crystals in different orientations. Enamel is laid down by ameloblasts from the dental surface outward only during tooth development.

Dentin is the second hardest substance in the body with 70% hydroxyapatite crystals. Odontoblasts produce dentin throughout life. Odontoblastic processes are cytoplasmic processes that extend through the dentin to the enamel-dentin junction through a dentinal tubule. Predentin is unmineralized organic matrix that surrounds the apical processes before mineralization. Dentin is produced throughout life and slowly fills in the pulp cavity.

Cementum lies outside the dentin on the root and is about 50% hydroxyapatite crystal. Cementocytes are trapped within lacunae of the cementum matrix and have cell processes running through canaliculi to reach vessels in the periodontal space. The cellular portion of cementim has cementoblasts lining the outer surface at the periodontal ligament space and secretes cementum throughout life.


 * 10. Compare and contrast Tome's fibers (odontoblastic processes), Tome's processes,dental tubules and enamel rods.**

Tome's fibers (Odontoblastic processes) are cytoplasmic processes that extend through dentin to enamel-dentin junction, the dentinal tubule is space that process fills. Tomes' processes are formed during the synthesis of enamel, as the ameloblast moves away from the dentin, forming a projection surrounded by the developing enamel. Enamel rods are keyhole shaped prisms that extend from surface to dentin. Each rod coated by enamel sheath of organic material and the interrod region hydroxyapatite crystals have different orientation. Enamel is laid down by ameloblasts from the dentinal surface outward


 * 11. Describe the composition and location of dental pulp.**

Pulp chamber is space in center of tooth filled with vascular areolar CT that is rich in proteoglycans & glycoproteins. The root canal connects pulp chamber with periodontal space carrying blood vessels & nerves to supply tooth. The Apical foramen is opening at root tip for entry of BV’s & nerves and is lined by odontoblasts producing dentin.


 * 12. Describe what is a salivary gland: groups of minor glands and major glands.**

Salivary glands are divided into major and minor categories: minor salivary glands are all the small salivary glands that reside within the oral mucosa and tongue and are small short tubular glands. Major salivary glands consist of three paired glands: Parotid, submandibular and sublingual glands. They have ducts that drain secretions directly into the oral cavity and all are compound tubuloalveolar.


 * 13. Describe and identify the parotid, submandibular and sublingual glands. Integrate their characteristic structure to their function.**

The __Parotid Gland__ is located over the angle of the mandible and consists of compound tubular alveloar glands divided into lobules and lobes. The gland secretes primarily serous fluid which contains salivary amylase and IgA. The__Submandibular gland__ is similar in structure to the parotid gland (compound tubular alveloar) and is located under the mandible in the floor of the mouth. It contains connective tissue septa dividing it into lobules, and its main excretory duct opens on the frenulum of the tongue. The secretions are 5:1 serious to mucous and it produces 60% of the saliva in the mouth. The__Sublingual gland__ is also compound tubular alveloar and is located on the inferior to the tongue. It contains primarily mucus acini with serous demilunes (serous cells at the distal end of a mucous, tubuloalveolar secretory unit of certain salivary glands).


 * 14. Describe and integrate serous and mucus acini, serous demilunes, intralobular ducts (intercalated and striated) and interlobular ducts into the structure of all salivary glands.**

__Serous acini__ secrete a watry fluid while __mucus acini__ secrete a thicker mucus. __Serous demilunes__ are serous cells at the distal end of a mucous, tubuloalveolar secretory unit of certain salivary glands. __Intercalated ducts__ are the smallest (same size as acini)and drain acini & tubules (easiest to see in parotid gland). They consist of simple cuboidal cells & some myoepthelial cells. __Striated ducts__ are where several intercalated ducts merge together to form larger striated duct. They consist of simple cuboidal to low columnar cells and are more reddish staining due to numerous mitochondria & membrane.The intercalated ducts and striated ducts are within the lobules of the gland and are considered intralobular ducts. They function to resorb Na + & Cl- ions and secrete bicarbonate ions. __Interlobular ducts__ are larger ducts formed from striated ducts between lobules and surrounded by connective tissue.