Digestive Tract I: Oral Cavity

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Peristalsis and Secretions

• Digestive, gastrointestinal or alimentary tract consists of muscular tube which is affected by peristalsis associated with glands (diverticula of the gut for exocrine glands) that elaborate secretions to the tube lumen.

• Pancreas has both endocrine & exocrine components.

• Enteroendocrine cells elaborate their secretions to the interstitial compartment where they effect local paracrine control.

Basic Segments of GI Tract

Lips are the entrance to the oral cavity. Lips are composed of:

• Outer surface of keratinized stratified squamous epithelium with hair follicles, sweat glands and sebaceous glands.

• Hypodermis

• Orbicularis oris

• Minor mucous glands

• Muco-cutaneous junction

• Transition zone or vermilion border, the red/pink part with no sweat or sebaceous glands

• Inner surface with stratified squamous mucosal & mucous glands.

Cheeks

• Cheeks are generally composed of the same elements as the lip.

• Stratified squamous mucosal inside.

• Hair on outside.

• Buccinator muscle keeps food between the teeth.

• Mucous membrane is tethered to the muscle by elastic fibers in the submucosa.

• Prevents biting the cheek when chewing.

Tongue

• The tongue is fundamentally a bag of skeletal muscle with fibers going in several directions.

• Intrinsic muscles are restricted to the tongue proper, insert into CT & alter the shape of the tongue.

• Ex. roll it up laterally.

• Extrinsic muscles extend from the tongue to the mandible, styloid process & soft palate & alters the position of the tongue (Physician to Patient, Say ah & stick out your tongue).

Functions of the Tongue

1. Assist in speech

2. Involved in initiation of swallowing.

3. Position food bolus in the mouth.

4. Chewing.

5. Catch food (frogs flip out their tongue to catch flies).

6. Pick up food (dogs & cats).

Anatomy of the Tongue

• Tongue is separated into right & left parts by a CT septum.

• Anchored by a lingual frenulum (short frenulum leads to "tongue tied").

• Epithelium under tongue is thin and replete with many vessels (site for administration of nitroglycerin).

• Dorsal surface: V-shaped sulcus terminalis divides into anterior 1/3 & posterior 2/3.

• Foramen caecum at tip of sulcus terminalis.

• Site of origin of the thyroid gland.

Lingual Papillae

4 types of papillae: filiform, fungiform, vallate or circumvallate, and foliate.

Filiform

• Most numerous.

• Highly keratinized.

• On anterior 1/3.

• Very well developed in other animals.

• Allows us to lick ice cream effectively.

• Brush your tongue to avoid Bad Breath.

• As exfoliated cells build up, tongue can have a gray or white color.

Fungiform

• Scattered between filiform.

• Have blood vessels close to the surface which makes them appear as red spots.

• Nonkeratinized.

Vallate or Circumvallate

• Distributed along sulcus terminalis.

• 8-12 in number.

• Papilla surrounded by a trough filled with serous fluid.

• Secretions of serous glands of von Ebner Sides of papillae have taste buds.

Foliate

Not very numerous in man & are ridge shaped folds.

Taste Buds

• Taste buds along sides of vallate papillae, some on filliform & randomly on fungiform.

• Assists in monitoring taste, however most is olfaction.

• Chemorecptors & supportive (sustentacular) cells that communicate with the oral cavity via a taste pore.

• 4 basic tastes.

Hard Palate

• Core of bone.

• Stratified squamous, lightly keratinized CT ties epithelium closely to bone.

• In anterior region, CT passes through adipose before connecting to bone.

• Small adipose cushions allow for some give in the palate.

• This is important when the tongue pushes food up & forward during eating.

• Posteriorly, the adipose is replaced by mucous glandular tissue that adds secretions to the oral cavity.

Soft Palate

• Core of muscle.

• Inferior surface - stratified squamous, lightly keratinized.

• Superior surface - pseudostratified columnar with cilia.

• This is a moveable flap that is drawn forward during swallowing closing the nasopharynx & preventing food from being pushed into the nasal cavities.

• Inferior surface contains many small salivary glands.

• Free posterior border is the uvula containing the voluntary muscle, the musculus uvulae.

Waldeyer's Ring

• Lingual tonsils.

• On posterior 1/3 of tongue.

• Stratified squamous.

• Mucosal surface overlies lymphatic tissue & forms crypts.

• Mucous glands occur at base of crypts & wash them out.

• Palatine tonsils do not have glands & therefore tend to get infected.

• Have same type of epithelium.

• Pharyngeal tonsils (adenoids).

• Single with pseudostratified columnar with cilia.

• Pharynx: Stratified squamous lightly keratinized.

• Core - pharyngeal muscles covered with bucco-pharyngeal fascia.

Salivary Glands

• Three tyeps: parotid, submandibular, and sublingual.

• Glands are predominantly serous, mucous or mixed.

Acini

• Secretory acini can be all serous, all mucous or be arranged as a crescentic serous demilune at the terminal of a mucous acinus.

• Ducts that drain the acini & are within the glandular lobules are intralobular & include cuboidal intercalated ducts & striated ducts.

• Interlobular ducts are simple columnar & excretory ducts are simple to stratified columnar.

Parotid Gland

• Largest salivary gland.

• Lies between mastoid process & ramus of mandible & extends below the zygomatic arch.

• Stenson's duct opens at 2nd molar of upper jaw.

• Predominantly serous.

• Adipose common.

• Prominent intralobular ducts.

• Myoepithelial cells around acini & ducts.

• Tubuloalveolar acini.

Submandibular

• Predominantly serous (mixed).

• Tubuloalveolar.

• Serous demilunes.

• Lies against inner aspect of body of mandible.

• Ducts open into floor of oral cavity posterior to incisor teeth of lower jaw, adjacent to lingual frenulum.

• Mucous, not adipose.

Sublingual

• Predominantly mucous with serous demilunes.

• Lies in midline of floor of mouth & ducts open posterior to submandibular.

Saliva

• pH 6.0-7.4

• 1,000 - 1,500 ml/day in adult.

Functions of Saliva

• Ptyalin digests starch.

• Mucous secretions lubricate.

• Cleanse the mouth of food, exfoliated cells, bacteria, etc. any disease process that inhibits salivary secretion results in halitosis due to decomposition of food by bacteria.

• Solvent action for substances to stimulate taste buds, they must be in solution.

• Excretion of organic & inorganic materials, ex. potassium, iodine, mercury, lead, rabies virus.

Teeth Histology

Tooth is made of 3 calcified CT substances: enamel, dentin, & cementum.

Enamel

• Anatomical crown - part covered by enamel.

• Clinical crown - part above gingiva.

• Hardest substance in the body.

• Similar to bone.

• 96% hydroxyapatite.

• Ameloblasts produce matrix that then precipitates hydroxyapatite.

Dentin

• (enamel stripped away).

• Composes majority of tooth.

• Lies between enamel & cementum.

• Not as dense as enamel.

• 70% hydroxyapatite.

• Produced by odontoblasts

Cementum

• Similar to bone but no Haversian canals.

• Lies outside the root of the tooth.

• Produced by cementoblasts.

• Functions to anchor collagen of periodontal ligament to tooth.

• Laid down as cementoid that is subsequently calcified In upper root cementum is acellular.

• In lower part, cells occur.

• Cementoblasts live in lacunae & are nourished by way of canaliculi.

Pulp Cavity

• Highly innervated & vascularized.

• Continuous with underlying CT via apical foramen.

• Upper part is pulp chamber.

• Lower part is root canal.

Periodontal Ligament

• Collagen suspends tooth in bone socket.

• Attaches tooth via cementum to bone.

• Basis of orthodontia.

• Permits limited motion, hydraulic shock absorber.

• Ligament is richly innervated with sensory fibers & can detect hard substances in soft food (ex. grit or sesame seed).

Gingiva

• Where epithelium of oral cavity meets tooth.

• During eruption, gingiva attaches to enamel crown.

• Point of attachment moves down during life.

• Functions to repel bacterial invasion.

• Because of fluoride in drinking water, most tooth loss is due to periodontal disease.

Primary, Deciduous or Milk Teeth

• Serves during early life.

• Usually replaced by 18.

• Divide jaw into quadrants, each quadrant has 5 teeth, medial to lateral.

• 2 incisors 1 canine 2 molars 20 total Dental formula 2-1-2.

Secondary or Permanent Dentition

• Normally 8 per quadrant.

• 2 incisors 1 canine 2 premolars 3 molars.

• 3rd set of molars are wisdom teeth & some never develop or erupt.

• Dental formula 2-1-2-3.

Origin of Enamel

• Enamel is derived from oral ectoderm.

• Dentin, cementum & pulp comes from neural crest mesenchyme.

• Dental lamina is a thickening of oral ectoderm.

• Grows down into mesenchyme.

• Ectoderm forms tooth bud.

Form and Function of Teeth

• Tooth bud assumes a bell shape & becomes the enamel organ & captures mesenchyme as the dental papilla which will form odontoblasts.

• Mesenchyme also condenses around the enamel organ as the dental sac & differentiates into cementoblasts & the periodontal ligament.

• Outer enamel epithelium is continuous with the dental lamina.

• Inner enamel epithelium becomes ameloblasts & produces enamel.

• Stellate reticulum is between.

• Differentiation of ameloblasts is induced by dental papilla and dental papilla is induced by ameloblasts to differentiate into odontoblasts, therefore a reciprocal induction.

• Odontoblasts form predentin that is mineralized as dentin.

• Odontoblasts cell bodies retreat but cell processes remain in tunnels in dentin as dentinal tubules or Tombe's fibers.

• They span the thickness of dentin.

• The basis for sensitivity to cold & heat.

• Stimulate nerve endings in pulp.

Formation of Enamel

• Ameloblasts form enamel & do not leave behind any cell processes.

• The enamel is in the form of rods or prisims.

• The matrix secreting portion of the cell is called Tombe's processes.

• The enamel matrix is then calcified.

Formation of Root

• The root of the tooth forms after the crown.

• Tooth eruption is caused by root growth.

Tartar

Yellow-brown deposit at or below the gingival margin (floss it off).

Calculus

A build-up of calcified tartar (scrape it off).

Plaque

A mass of bacteria attached to tooth surface (brush it off).

Dental Caries

• Progressive destruction of enamel by dissolution of the matrix.

• To prevent this acidic action, neutralize the environment.

• Mandible contains bone with attached teeth.

• If teeth are lost, bone is resorbed.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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