Digestive System
Overview
The Digestive system is a great place to look see how form meets function in the body. From esophagus to anus the epithelia and underlying structures change to suit the job that portion is carrying out. It has many accessory glands that will aid in digestion
Mouth
Lip
Dense Irregular Connective tissue
Oral Surface
Mucous Gland
The Inside surface of the lip has an oral mucosa stratified squamous non keratinized epithelia. There are also mucous glands that empty into the mouth on the oral surface. The glands themselves are lined with simple cuboidal epithelia.
Glandular Epithelia
Skin Surface
Hair follicle
Sebaceous Gland
The skin surface of the lip is stratified squamous keratinized epithelia. This surface has all the regular integumentary structures; hair follicles, sebaceous glands, etc.
Muscle
There is a thick core of muscle in the lip, there is no skeletal elements in the lip so a thick core is required. This is skeletal muscle for voluntary movement.
Tongue
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The tongue has several layers, the oral mucosa is stratified squamous epithelia with lamina propria and muscular mucosa, then a massive block of striated muscle that controls the tongues movement. In the tongue we find Glands of Von Ebner (G) that secrete mucous into the oral cavity.
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the tongue has surface specializations called papillae that have a special function each. The filiform papillae are cone shaped and highly keratinized. they do not have taste buds but are there to scrape food.
The fungiform papillae are mushroom shaped and less numerous. these are non keratinized and found on the tip of the tongue. they have taste buds on the dorsal surface.
The foliate papillae are located on the dorsal surface of the tongue and have ventral furrows. they have functional taste buds until age 2.
Circumvallate papillae are v shaped and submerged in surface of the tongue. have salivary glands that empty in the furrows.
Teeth
Esophagus
The esophagus has the essential layers of the digestive tract. The Mucosa which is the epithelia, the lamina propria, and the muscular mucosa. The submucosa is the connective tissue connecting the mucosa to the muscularis externa which is skeletal and smooth muscle in the case of the esophagus. The serosa/ adventitia is loose connective tissue and mesothelium or peritoneum.
Here in the esophagus the epithelia is stratified squamous non keratinized to prevent abrasion from swallowing. The muscularis externa changes from skeletal muscle to smooth muscle in the esophagus as you move down the tube. There are also esophageal glands found in the submucosa "glands of von ebner"
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Stomach Esophageal Junction
Stomach
Esophagus
Some changes to make note of as you move from esophagus to stomach is the transition from stratified squamous to simple columnar epithelia of the stomach. Also the transition to a thicker three layered muscularis externa. The mucosa is now thicker and will form folds and crypts to increase surface area.
Rugae
Stomach
The stomach is divided into areas depending on the type of cells present in the epithelia, and the depth of gastric pits and glands. There is the fundic stomach superior to the entrance of the esophagus, where the cardiac stomach is found, followed by the pyloric stomach at the sphincter to the intestines. The stomach's mucosa and submucosa are folded into rugae which allow for expansion of the stomach when consuming food. There are different cells found in the gastric pits and glands.
Mucous cells : columnar cells lining the pit of gastric pits.
Parietal cells (P) : acidophilic, pump HCl into the lumen
Chief cells (C) : basophilic, secrete pepsinogen
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Cardiac stomach
GP
GG
Layers of the stomach
Mucosa
- Epithelia (simple cuboidal)
- Lamina Propria
- Muscularis mucosa
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Submucosa
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Muscularis externa
- circular
- oblique
- longitudinal
In the cardiac stomach the pits are shallower and the glands are coiled.
Lumen
Mucous cells
Gastric glands
P
P
C
Fundic stomach
Gastric pits
Gastric Glands
Muscularis mucosa
Submucosa
Muscularis Externa
Mucus cells in epithelia near lumen
P
Pyloric Stomach
The pyloric stomach has the deepest pits and glands in the stomach.
In the pyloric stomach the pits are the deepest with the shortest glands.
Stomach duodenal junction
From the stomach to the duodenum we see a change in the muscular externa with only 2 layers in the intestines. the epithelia is the same as the stomach (simple columnar) except now with microvilli on the surface to make a brush border. In the stomach the rugae provide an increase in surface area while in the intestine we see valves of kierkring, crypts of Lieberkuhn, and villi. There are also glands in the submucosa.
Small Intestine
The small Intestine is a place for absorption of nutrients from the chyme. To increase absorptive capabilities we see many features, valves of kerckring (folds of the submucosa), villi found on the valves (folds of he lamina propria), covered in simple columnar cells with microvilli to make the brush border. the villi are separated by crypts of Lieberkuhn. There are many more cell types in the intestines as well
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Goblet cells : produce mucous, function in protection and digestion (increase in number as you go through the small intestine)
Absorptive cells : columnar cells with brush border
Regenerative cells : source of the rest of the cells
Argentaffin cells : enteroendocrine cells
Paneth cells : located at bottom of glands, has a large number of secretory granules, they secrete lysozyme.
Duodenum
Brunners glands in the submucosa secrete an alkaline mucous to neutralize the acidic chyme leaving the stomach to enter the intestines.
intestinal villi are extensions of the lamina propria and epithelia to increase surface area.
Jejunum
Villi
Crypts of Lieberkuhn
The jejunum is characterized by having no or very little brunners glands, an increase in the number of goblet cells and a decrease in the number of argentaffin cells
Goblet cells
Brush Border
Absorptive cells
Ileum
The ileum is the last segment of the small intestine and we see an increase in the number of goblet cells, a decrease in the number of argentaffin cells, and we see peyers patches in the mucosa and submucosa. specialized m cells in the epithelia facilitate transport of antigens across the epithelium to the peters patch.
Mucosa
-Epithelia
-Lamina Propria
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-Mucularis
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Submucosa
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Muscular
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Paneth cells
Goblet Cells
Peyers Patch
Large Intestine
The large intestine absorbs water and ions. It is divided into the cecum, colon, rectum and anus.
The surface area is modified like the small intestine except now there is no valves of kerckring, no villi but do have crypts of Lieberkuhn. There are short microvilli present.
There is another layer of muscle present it is the Tania coli.
Rectum
The rectum also only has crypts present. Has a heavy investment of goblet cells
Rectoanal Junction
Can see the change from simple columnar to stratified squamous epithelia
Accessory Glands
Liver
The liver is a major accessory organ to digestion but also an important organ for the body. The liver is fed by several blood vessels; the hepatic portal vein from the GI tract that is rich with nutrients and toxins but is poor in oxygen, hepatic artery bringing freshly oxygenated blood to the liver, and fro the spleen carrying degraded red blood cells.
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The Liver is divided into lobules characterized by a central vein with portal triads surrounding the periphery.
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HA
HV
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The Portal triad is home to two blood vessels; Hepatic Portal vein (HV) and the Hepatic Artery (HA), as well as the Bile duct.
The liver has many functions
Metabolism
-glycogenesis and glycogenolysis
-lipoprotein synthesis
-synthesis of plasma protein (fibrinogen, prothrombin)
-detoxification
Storage
-glycogen
-fat and oil
-vitamins A, D, K and some B
Secretion of Bile
- bile salts, emulsifying agents, bile pigments,
Blood from the portal triad flows through the sinusoids in the liver lobule and is collected in the central vein.
The Liver lobule is made of radiating plates of hepatocytes. the hepatic plates are never more than 2 layers of hepatocytes thick surrounding the sinusoids where the blood flows.
The sinusoids are lined with the endothelial cells that create a small space called the space of disse where blood particles can flow into to be taken up by hepatocytes or filtered by cells of ito.
There are 3 ways to view the liver from a functional point of view. ​​
The Classic Lobule View, which focuses on the flow of blood from the portal triad to the central vein through the sinusoids. Blood from the hepatic portal vein and the hepatic artery mixes once it leaves the portal triad, mixing deoxygenated high nutrient blood with low nutrient high oxygen blood.
The Portal Lobule view, focuses on the bile drainage of the liver. Bile is created in the bile cannaliculi formed between two adjacent hepatocytes, the bile canaliculi will then empty into a shared bile duct in the portal triad. Bile ducts int eh portal triad are shared between the three lobules connected to it. The drainage of bile to the triad forms a triangular shape
The Hepatic Acinus view focuses on the hepatocyte activity in the lobule based on oxygen and nutrient availability. These are called the zones of Rappaport, there is zone 1 on the periphery, zone 3 closest to the central vein and zone 2 is in-between these areas. Blood is flowing from the periphery to the centre so the hepatocytes near the periphery have the greatest oxygen and nutrient availability, as you reach the centre of the lobule those hepatocytes have the lowest oxygen availability. The cells on the periphery do the most of the oxidative metabolism because of the readily available oxygen, zone 3...
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There are several cell types found in the liver
1) Hepatocytes : The main cell type in the liver that carries out many of the important functions
2) Sinusoidal endothelial cells : line the sinusoids that carry the blood, these also create fenestrations that allow plasma to flow into the space of disse
3) Kupffer cells : found in the sinusoids and are the macrophages of the liver
4) Cells of Ito :
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Pancreas
The pancreas is a lobulated organ formed of many exocrine acini that secrete digestive enzymes, as well as an endocrine portion that will secrete glucagon and insulin among other hormones. In this section we will examine the exocrine portion only and the endocrine portion will be discussed on another page.
The exocrine portion of the pancreas are compound acinar glands like clusters of grapes, these acini enter into interlobular ducts, then inter lobular ducts, main pancreatic duct, then the common bile duct. Acini cells are low cuboidal cells that are very polarized, the apical end has infoldings and granules while the nuclei sits basally. these cells are actively secreting digestive enzymes like lipase and trypsin via merocrine secretion (exocytosis of zymogen granules) so the cells are packed with rough ER and Golgi.
Centroacinar cells line the beginning of the intercalated duct, these cells secrete fluid that contain mainly bicarbonate that will neutralize stomach acid in response to secretin.
Ducts in the pancreas carrying digestive enzymes can range from small ducts with a simple endothelium to large muscular ducts with a simple cuboidal ep.
Salivary Glands
The salivary glands are located in the oral cavity tissues and secrete serous (watery) or mucous or a mixture to aid in digestion of food.
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D
The Parotid Gland is mainly serous secretions, these glands are located in each cheek near the ear. They consist of serous cells surrounding small lumens. Each lobe (L) has ducts (D) that will empty into the oral cavity.
The Sublingual Gland is where mucous secretions predominate with some serous demilunes. Majro salivary product is mucous secretion but serous demilunes also secrete lysozyme and amylase.
Mucous secretory cells appear white and frothy
SD
D
M
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The Submandibular Gland is mixed secretions. Most secretory units are serous acing, with about 10% consisting of mucous tubules capped with serous demilunes. The mucous cells (M) capped with serous demilunes (SD).
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Gallbladder
Lumen
The Gallbladder functions as an extra hepatic bile duct to store and conenctrate bin secretions. It has many layers similar to the GI tract.
Mucosa
- Simple columnar epithelia
- Lamina Propria
Muscularis
- Bundles of smooth muscle fibres oriented in all directions to facilitate emptying
Subserosa
- irregular connective tissue
Serosa
- Simple squamous epithelia
Columnar Epithelia