HISOLOGY- upper digestive

The upper digestive tract →

oral cavity (lips, teeth, palate, tongue, cheeks), pharynx and esophagus

The lower tract→

digestion, absorption, and excretion, and includes stomach, small and large intestines and anal canal.

The Oral cavity

* A chamber in which food is mechanically fragmented by teeth, chemically modified by enzymes and lubricated by saliva
* narrow space between lips and teeth called vestibule
* Is bounded by hard and soft palates above, back of gums and teeth anteriorly and palatoglossal folds of cheeks laterally

Lining mucosa - Oral

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* forms inner lining of lips, cheeks, soft palate, floor of mouth and undersurface of tongue
* Nonkeratinized stratified squamous epithelium

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Masticatory  mucosa – Oral

* found in gingivae (gums) and hard palate 
* Lightly keratinized stratified squamous epithelium

Specialized mucosa – Oral

* found on dorsal surface of tongue
* Nonkeratinized stratified squamous epithelium

Histology of the Tongue

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* It engages in mastication, swallowing, speech and taste
* Epithelium smooth nonkeratinized stratified squamous
* Consists of striated muscles
* V-shaped groove called sulcus terminalis

 The Lingual Papillae

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* __Filiform__ – most numerous, slender and conical projections, have heavily keratinized cells – grey color is due to keratin
* __Fungiform__ – have narrow base, widely scattered among filiform papillae and have connective tissue core
* __Circumvallate__ – one row of 8-12 on the posterior side of tongue, largest in size, epithelium non keratinized or incompletely keratinized, with serous glands of von Ebner

**The Taste Buds**

* lateral surfaces of the epithelium of fungiform and circumvallate papillae
* 5,000 taste buds on tongue, about 2,500 on soft palate, 900 on epiglottis and 600 in larynx and pharynx
* Transduce- sensory receptors
* Gustatory sensations- chemical stimuli into nerve impulses

Dark cells (Type I) – taste buds

long microvilli, apical cytoplasmic granules, found to be supporting cells and lie in periphery

Light cells (Type II) –taste buds

lie centrally, have long microvilli, granules, abundant smooth ER and little rough ER

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Pale cells (Type III) -taste buds

Light staining, long apical processes that are several times the diameter of microvilli – believed to contain __gustatory cells__

The Teeth

adult- 32 permanent teeth, 4 incisors & premolar, 2 canines & 6 molar

* Crown – projects above gum  or gingiva
* Neck – between crown and root(s)
* Root – fits into the underlying bone, one or more roots are  embedded in a bony socket or alveolus  of the jaws.

odontogenesis

Teeth develop by a complex process

* __Enamel__ arises from __oral__ __*ectoderm*__
* __Dentin__, __pulp__, __cementum__ & __periodontal__ membrane arise from __mesenchyme__

Fluoroapatite is

chemically more stable, less soluble, and more resistant to breakdown by acid bacteria in plaque

Dentin 

* An avascular, mineralized tissue, harder than the bone mineral but similar in composition
* 70 %  hydroxyapatite, about 18 %  type I collagen and the rest about 12 % water.
* Dentin is synthesized by __odontoblasts:__
* tall columnar cells that line the pulp cavity

Enamel

* hardest substance in the body
* 96 % → hydroxyapatite, the rest (4 % ) is organic made of unique glycoproteins called amelogenins and enamelins – it lacks collagen
* Enamel is composed of rods or prisms that radiate from dentin
* Enamel is produced by a layer of columnar cells called __ameloblasts-__ 
* degenerate after the complete formation of enamel

Periodontal Membrane or Ligament

* A layer of collagen enveloping roots of tooth
* Is interposed between the cementum & the surrounding alveolar bone
* It firmly anchors tooth→ socket

Gingiva

* The thicker portion of the oral mucous membrane which is firmly bound to the *periosteum*
* separated from enamel forming gingival crevice (space) with squamous epithelium
* The seal between epithelium and enamel prevents entry of bacteri

The Major Salivary Glands

* Parotid, Submandibular and Sublingual
* Located away from oral epithelium & connected by branching system of ducts
* Produce a large volume of secretions on mechanical and chemical stimulation of nerve endings in mucosa, smells can cause 

The Minor Salivary Glands

* Short and numerous mucous cells in mucosa 
* Secrete continuously and contribute to saliva that moistens and lubricates oral cavity

Viscous secretion

Mucous acini are single layers of pyramidal cells, cytoplasm with mucigen – a precursor of mucus; abundant RER_____.

Watery secretion

Serous acini consist of columnar cells, cytoplasm with secretory granules, have intercellular secretory canaliculi, abundant RER – products: amylase, lysozyme, peroxidase, DNAse, RNAse _____.

The salvia

* Viscous, colorless and opalescent fluid
* Composition: 97-99 % water
* __Inorganic ions__ (Na+, K+, Ca++, Cl-, traces of Fe++)
* __Proteins__ (mucoproteins, immunoglobulins (IgA), enzymes –salivary amylase or ptyalin for digestion of carbohydrates
* __Salivary corpuscles__ (degenerating granulocytes and lymphocytes)

The Tonsils

not fully encapsulated and have __crypts__ that trap bacteria

* __Palatine tonsils__ (paired), the largest and most often infected
* __Lingual tonsils__ (paired) at the base of the tongue
* __Pharyngeal tonsils__ (single) – called adenoids if enlarged

The Pharynx 

* Mucosa – __stratified squamous epithelium__ in oral and laryngeal pharynx and __stratified ciliated columnar__ with goblet cells in the nasal pharynx
* Lamina propria – loose CT, no muscularis mucosae
* Submucosa – loose CT
* Muscularis externa – inner longitudinal skeletal and outer oblique or longitudinal with fibroelastic tissue in between.

The Mucosa (GI wall)

* __Epithelium__
* is conductive, secretory or absorptive
* __Lamina Propria__
* Vascular loose CT w/ fibroblasts, reticular and elastic fibers, macrophages and lymphoid tissue
* __Muscularis mucosae__
* smooth muscle fibers – __inner circular__ and __outer longitudinal__

The Submucosa (GI wall)

moderately dense CT with many blood vessels and a plexus of sympathetic nerves – Meissner’s plexus for control of motility

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The Tunica Muscularis or Muscularis Externa (GI wall)

* Moderately thick smooth muscles – inner circular and outer longitudinal
* a plexus of sympathetic nerves in between these two layers– __Auerbach’s plexus for peristalsis__

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The Serosa (GI wall)

* mesothelium→ squamous epithelium lining abdominal cavity and organs 
* Omentum lines stomach
* mesentery lines intestine

The Esophagus 

* Tube- 10 inches or 25 cm
* Conveys food from pharynx to stomach
* Esophagus Wall- Mucosa, submucosa, muscularis externa and tunica adventitia

The Esophageal Glands

* **Mucosal or cardiac esophageal glands** – __simple tubular__
* Limited to lamina propria - Found __only__ in upper esophagus and at esophagus-stomach junction
* **Submucosal glands** – compound tubular - more widespread with mucus droplets

Gastroesophageal reflux disease (GERD) is a 

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* common chronic condition – common in adults over 40 years
* often accompanies hiatal hernia which causes failure of gastroesophageal sphincter to close resulting in reflux of acidic food into the esophagus causing heart burns

Organs of GI tract include

mouth, pharynx, esophagus, stomach, small intestine, and large intestine which terminates in anus

Accessory digestive organs include

teeth, tongue, gallbladder, and glands such as salivary glands, liver and pancreas – glands produce saliva, bile and digestive enzymes

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Mucosa- (Esophagus wall)

* __Epithelium__ stratified squamous 
* At the __junction__ of esophagus and stomach an abrupt transition from stratified squamous to simple columnar epithelium
* __Lamina propria__ – loose CT w/ collagen and elastic fibers

Submucosa- (Esophagus wall)

* Dense CT w/ collagen and elastic fibers, many small blood vessels
* Forms __numerous folds__ that smooth out during transport of food

Muscularis Externa- (Esophagus wall)

* Inner circular and outer longitudinal
* Upper third- *striated*, middle third- *mixture of striated and smooth* and lower third- *smooth muscles*

Tunica Adventitia- (Esophagus wall)

Loose CT, not serosa