Digestive Tract Courseware2

Digestive system Components: Digestive tract and Digestive glands. Functions: Obtain molecules necessary for the body from ingested food. A protective barrier for the body. Dr. Wang yanwu 5-9/12/2005

Part One

Oral Cavity and Related structures

Oral cavity  The oral cavity extend from lips to pharynx.  The surface of the oral cavity is lined by nonkeratinized stratified squamous epi., except the gum and hard palate (keratinized).  The wall of the oral cavity is made up of CT, muscles and bone.  The oral cavity can ingest, fragment and moisten the food.  The oral cavity is involved in speech, facial expression, sensory reception and breathing.  The oral cavity contains the lips, the teeth, the tongue and salivary glands.

1. Lip  The lips is muscular structure, skeletal muscle.  Internal surface is lined by mucous membrane and External surface is lined by hairy skin.  Transition zone refer to vermilion for its pink color.

2. Tooth Crown - The visible part of a tooth. Root - The anchor of a tooth that extends into the jawbone. The number of roots ranges from 1-3. Gum - The soft tissue that surrounds the neck of the tooth. Pulp - The soft center of the tooth. The pulp contains blood vessels and nerves; it nourishes the dentin.

Dentin – Tooth is made up of dentin. The hard but porous tissue surrounded the pulp. Dentin is harder than bone.

2. Tooth Enamel

Enamel - The tough, shiny,

Dentin

white outer surface of the tooth

Pulp

covered on the surface of the crown. Cementum - A layer of tough, yellowish, bone-like tissue that

Cementum Periodontal ligament

covers the surface of dentin in the root. It helps holding the tooth in the socket. Periodontal ligament / Periodontium : The fleshy tissue between tooth and the tooth socket; it holds the tooth in place. The fibers of the periodontal ligament are embedded within the cementum.

3. Tongue  A mass of skeletal muscle covered by mucosa, whose structure varies according to the region.  The dorsal surface of tongue is irregular, which covered by keratinized stratified squamous epithelium.  The ventral surface is smooth and lined by non-keratinized just Dorsal like oral cavity. surface

tip Ventral surface

 V-shaped boundary, which divided the dorsal surface into anterior 2/3 and posterior 1/3.  The posterior 1/3 show small bulges which composed of two types of lymphatic tissue, small collections of lymphoid nodules and lingual tonsils.  The anterior 2/3 of dorsal surface is covered by numerous projections or papillae(?).

V-shaped boundary Posterior

Anterior

Papillae

CV Foliate



Filiform papillae



Fungiform papillae



Circumvallate papillae



Foliate papillae

Fg

FL

a). Filiform papillae: The most numerous papillae. Present over entire surface. It is small and conical in shape. The epithelium at the tips of papillae is keratinized without taste bud. (White) b). Fungiform papillae: Irregularly interspersed among filiform papillae. These papillae resemble mushrooms, which contains a narrow stalk and a dilated upper part. The epithelium is not keratinized with taste buds. (RED)

c). Circumvallate papillae: The largest papillae.  7-12 CV arranged in a row just anterior to V-shaped boundary.  CV is encircled by a deep cleft.  The epthelium contains numerous of taste buds.  Underlying of the mucosa, there are serous glands, which open into the base of the clefts. d). Foliate papillae are poorly developed in human, which lies on dorsalateral surface of tongue. The epithelium contains many taste buds(?). CV

Foliate

Fg

FL

Taste bud There are 10,000 TB in your mouth, which present in the epi. of fungiform papillae, circumvallate papillae, foliate papillae, soft palate, etc. 

Each taste bud is a onion-shaped structure made up of modified epithelial cells. Each bud has a small cavity that opens to the surface through a taste pore.

The cells in TB are of 3 basic types. Receptor cells or taste cells, supporting cells, which have microvilli on the tip of the cells and basal cells. 

The same taste bud can respond to 4 types of taste (sweet, sour, salty and bitter). The sweet and salty is less sensitive, but bitter is most sensitive. 

The taste is a complicated sensation depending on the responses from taste buds all over the tongue. 

Brief summary 

The general structure: the oral cavity, the lips, the teeth and the tongue.



The papillae in the tongue

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The taste bud: structures and functions.

Part two

Esophagus, Stomach and Intestine

Dr. Wang yanwu

General structure 1. Components of digestive tract

2.

adventitia

Except oral cavity and pharynx, all organs share a similar histological plan: a hollow tube composed of a lumen surrounded by a wall. The wall includes: mucosa, submucosa, muscularis and adventitia.

3. General structure in details 1)

M

Mucosa (mucous membrane)

Epithelium : S: stratified squamous epi. (E, R, A) simple columnar epi. (S, SI, LI) F: A selectively permeable barrier.

A

• Facilitate the transport and digestion of food. • Promote the absorption of products of digestion. • Produce the hormone to affect the activity of DS. • Secret mucus for lubrication and protection. Lamina propria: LCT rich in BV, LV and nerves, Muscularis mucosa : Except esophagus, 2 sublayers (Inner circular and outer longitudinal of smooth muscle). Function: Promote the movements of the mucosa and increase its contact with the food. Separate mucosa from submucosa.

2) Submucosa: Dense CT with abundant lymphoid nodules. Submucosa nerve plexus. Protect the organism from bacterial invasion 3) Muscularis : Smooth muscle cells and very little skeletal muscle cells. inner circular and outer longitudinal sublayer Myenteric nerve plexus Function: The contraction of muscularis propel and mix the food in digestive tract.

M

4) Adventitia Serosa: LCT + mesothelium (mesentery) Fibrosa: LCT Folds: mucosa and part of submucosa A

Esophagus

Mucosa

1) Mucosa: epithelium: stratified squamous epi. lamina propria: loose CT muscularis mucosa:

LP MM

SM

only a layer of longitudinal arranged SM 2) Submucosa: CT esophageal gland: mucous gland

M SM

Mucularis

F

Epi

3) Muscularis: inner circular and outer longitudinal 

Proximal end: only skeletal M

Middle portion: mixture of skeletal and smooth M 



Distal end: only smooth M

4) Adventitia: fibrosa Functions:  Transport food from the mouth to the stomach.  Prevent the retrograde flow of gastric contents.

Stomach Functions: 

store food temporarily

 digest protein primarily  absorb part of water and Ion

Structures: The stomach includes 4 portions: fundus, cardia, body, pylorus Folds or rugae. The wall is composed of 4 layers: Mucosa, submucosa, muscularis and adventitia. Mucosa and muscularis have some special features .

I. Mucosa of stomach Mucosa is composed of 3 sublayers. ① Epithelium: simple columnar epi Surface mucous cell: Structures: LM: Tall columnar, ovoid and basallylocated nucleus. The apical parts are filled with mucin granules. Cells look empty with HE staining. EM: Tight junction Functions: Secreting alkaline mucus, which form Mucous barrier to protect mucosa against the damage of the gastric acids and enzymes in the lumen.

② lamina propria:  The epithelium dips into the lamina propria to form gastric pits.  Deep to gastric pits, the lamina propria is packed with numerous gastric glands and scanty CT.  Here’re 3 types of gastric gland: Main gastric gland, cardiac gland and pyloric gland. Gastric pit

Main gastric glands

Distribution: Most of the stomach (present in the body and fundus) except pylorus and cardia. Structures:  Simple or branched tubular glands.  The glands open into gastric pits. Each pit can receive the opening of several glands.  The epithelium of the glands includes chief cells, parietal cells, neck mucous cells, undifferentiated cells and endocrine cells.

1. Chief cell, peptic cell or zymogen cell Distribution: numerous in the basal part of the glands. Structures: LM: Low columnar, A round and basally-located Nucleus. Basal cytoplasm is basophilic and apical is light-stained EM: RER, Golgi complex and apical granules (pepsinogen). Functions: secrete pepsinogen (activated by gastric acids to form pepsin). Pepsin can break down proteins into peptides and solid food become liquefied.

Chief cells

2. Parietal cell or oxyntic cell Distribution: more numerous in the upper half of the gland. Structures: LM: large, pyramidal with round and centrally-located nucleus. The cytoplasm is strongly acidophilic. EM: Intracellular canaliculus (IC), which bear microvilli in the wall. Tubulovesicles (TV) and Abundant Mitochondria (acidophilic). Functions: secrete hydrochloric acid and produce intrinsic factor.

Active cell

Parietal cell

Resting cell

Function of Hydrochloric acids Pepsinogen (chief cells) hydrochloric acids

Proteins Pepsin

pH0.9-1.5

Peptides

Function of intrinsic factor

Intrinsic factor Complex Vitamin B12

Formation of erythrocytes

3. Mucous neck cell Distribution: neck part of the gland. Structures: Columnar cells, A flattened and basally-located Nucleus The cytoplasm is pale-stained in HE stain. Functions: mucus secretion.

4. undifferentiated cell : can multiply to replace other cells and play an important role in healing.

5. Endocrine cell:

Cardiac glands and pyloric glands Distribution: The mucosa of cardia and pylorus. Function: Mucous secreting glands. Structure: The majority of cells is mucous cells. Occasionally, parietal cells, chief cells and endocrine cells is present.

③ muscularis mucosa:  Well developed in stomach  2 sublayers of SM: inner circular and outer longitudinal

II Muscularis Well developed in stomach. Three sublayers: oblique, circular and longitudinal (from inside out).

Small intestines Small intestine is a tube, 5 meters long. It is composed of 3 parts:

 Duodenum: 25 cm long  Jejunum: 2 meters long  Ileum: 3 meters long

Functions:  The small intestine is the site of terminal food digestion, nutrient absorption and endocrine secretion.  The processes of the digestion are completed in the small intestine.  The nutrients are absorbed by the epithelial cells.

Structure: The wall of the small intestine includes 4 layers:  Serosa correspond exactly to the description  Muscularis  Submucosa: be also typical, except duodenum (Brunner)  Mucosa exhibit several special features, will be described below. Villi Mucosa

Lamina propria Submucosa Muscularis Crypts

Mucosa The surface area is extensive to allow adequate absorption of food. This is achieved by virtue of the following:

The length →1.folds →2. villi →3. crypts → Microvilli

1.Circular folds: also called valves of Kerkring. Structure : Mucosa and part of submucosa project into the lumen, which is so large that be readily seen with naked eye. Distribution: Be absent first 1-2 inch of duodenum and Be prominent in the rest of duodenum and the whole of the jejunum. Become fewer and less in the ileum. Function: increase the surface area and slow down the passage of the contents to facilitate absorption.

2. Villi: Structure: finger like projections, consisting of a core of connective tissue covered by a surface epithelium (described below). The CT core contains 3 important structures:  Numerous fenestrated cap. forming a plexus, which allow rapid absorption of nutrients into the blood.  A central lymphatic cap. called a lacteal. Sometime be double. Distal ends blindly and proximal ends in a plexus of lymphatic V.  Some smooth muscle cells derived from muscularis mucosa.

A electron microphotograph under SEM Finger like villi.

Distribution: most numerous in the duodenum, decrease along SI. Function : villi can increase the surface areas 7 times.

3. Crypts:  The invagination of epithelium into lamina propria.  Really simple tubular glands.  They are lined by epithelium and the epithelium is supported on the outside by a basal membrane.

Villi

Crypts

The epithelium of Mucosa The epithelium of mucosa of small intestine is simple columnar epi. Epi. of villi contains predominant columnar cells, specialized for absorption and mucous secreting goblet cells. Epi. of Crypts are also lined by undifferentiated cells and Paneth cells. Endocrine cells are present (described below)

1. Absorptive columnar cells Function: absorption of amino acids, carbohydrates, lipids. Structure: general characteristics of simple columnar epi. LM: An oval nucleus located in lower part. Free surface have striations. EM: Striated border is produced by microvilli arranged in regular manner. Usual organelles, such as lysosomes, smooth ER, and mitochondria.

& Free surface of cells: microvilli. Each microvillus is composed of a wall of cell membrane and center cytoplasm with filaments. The filaments are continuous with terminal web. The surface of microvilli is covered by fine fibrils and glycocalyx. & Lateral sides: junctional complexes and interdigitating cell membrane

2. Goblet cells What is a goblet? A drinking glass, broad above and narrow stem attach to a base. Structure LM: It is unstained by HE staining and bright with PAS reactions. The nucleus is flattened or triangular and located near the base of cells. EM: An expanded upper part with mucin granules. well developed Golgi complex and abundant RER. Function: mucous secreting cells.

3. Paneth cell Distribution: Deep part of crypts. Structure : LM: Pyramidal and Present in groups. And in the apical of the cell contains acidophilic granules. EM: considerable RER, usual organelles, and large granules. Function: not well known. They are known to relate with immune function and produce lysozyme.

4. Undifferentiated cells LM: Columnar cells in the epithelium of the crypts. The structure is similar to absorptive cells. In mitosis. EM: Cytoplasm contains secretory granules. Function: Proliferate by mitosis to form new cells and migrate to reach the wall of villi. Here they differentiate into typical epithelial cells.

5. Endocrine cells (described later) It is difficult to identify with HE staining.

The lymphatic tissue of small intestine (GALT)  Lymphoid nodules • Solitary (jejunum) and aggregated (ileum) lymphoid nodules. • Most prominent in the terminal ileum, these characteristic arrangements are called Peyer’s patches.

 Immune cells in lamina propria Eosinophil, Lymphocytes, Macrophages, Mast cells and Plasma cells

 M (microfolds)-cells: is specialized epithelial cells overlying Peyer’s patches. The cells can be identified by few or missing villi and containing lymphocytes. M-cells can take up antigen and transport to immune cells.

Distinguish features of small intestine 

Villi are an important characterization for identification of small intestine.



Duodenum have glands in the submucosa.

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There is not hard and fast line of distribution between jejunum and ileum.



Jejunum have fewer solitary lymphoid nodules and greater vascularity.



Ileum have abundant aggregated lymphoid nodules.

Large intestines Structures: 

Mucosa: numerous crescent-

shaped folds, no villus and no circular folds. Well-developed large intestinal glands in lamina propria. 

Submucosa: CT with fat cells.



Muscularis: 2 sublayers SM. Inner circular layer is thick,

Mucosa

Fold Submucosa

Muscularis

Outer longitudinal layer is thin, and form 3 thick bands—taenia coli

 Adventitia: Serosa, except posterior ascending and descending colon.

Mucosa Lined by simple columnar epi. and the lamina propria is filled with tubular glands. The cells in the epithelium and glands: 4 types of cells.  Columnar cells: absorb water and Ions, secrete mucus  Goblet cells: secrete mucus.  Undifferetiated cells: multiply and renew other cells.  Endocrine cells (describe below)

Functions:

 absorb water and ions  secrete mucus

Appendix The structure of the appendix resembles that of the colon with following differences. 1. The appendix is the narrowest part of the gut and is a blindended tubule. 2. The crypts are poorly formed. 3. The longitudinal muscle is complete and equally thick all round. Taenia coli are not present. 4. The mucosa and submucosa contains abundant lymphoid tissue.

Acute appendicitis A inflammation of appendix, is the most common surgical disease of the abdomen. Note the blood and inflammation debris within the lumen. Mucosa is mostly destroyed by the inflammation.

The endocrine cells of the guts 

The epithelium contains scatter cells that have an endocrine function.



The cells can be identified by the presence of granules that blackened with silver stain--- argentaffin cells and have a positive chromaffin reaction---enterochromaffin cells.

With new methods, some biologically active substances (amines or polypeptides) have been located in these cell, which be found in nerve system. 



Gastro-entero-pancreatic endocrine system

The action is local on neighboring cells (paracrine effect) and on cells at distant sites through the blood (endocrine). 

Brief Summary Structures

Functions Digestive tract is involved in breaking down food for absorption into the body. This process occurs in five main phases: ingestion, fragmentation, digestion, absorption, elimination of waste products. Oral cavity

Ingestion and Initial fragmentation

esophagus

Conduct food from oral to stomach

Stomach

Complete fragmentation and initial digestion

Duodenum

Get enzymes from pancreas and liver

Jejunum and Ileum Large intestine

Complete digestion and main absorption Absorption of Water and ions

Elimination of waste products