(Transcribed from Dr. Kalliecharan’s lecture, 8-9 Mar 2000 by Brian Buschman)
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This set of notes encompasses the path taken for food from the time it enters the body until it is excreted. Other GI organs such as the liver will be in the GI organ notes.
The mouth is lined with non-keratinized stratified squamous epithelia over it’s entire inner surface. The non-keratinized epithelium continues outside to cover the lips and ends where the lips meet the skin of the face. As discussed in the chapter regarding skin the face is covered with keratinized stratified squamous epithelia. The epithelia of the roof of the mouth is supported by bony tissue in the front where the hard palate exists and by skeletal muscle and mucous glands where the soft palate lies in the back of the mouth.
The tongue is made of skeletal muscle arranged in three planes. The tongue is covered by non-keratinized stratified squamous epithelia and on it’s dorsal surface it has three four types of papillae.
1) Filiform are found on the anterior two-thirds of the tongue and have a non-keratinized tip and no taste bud.
2) Fungiform are also found on the anterior two-thirds of the tongue and have one or two taste buds located in the crypts created between two papillae.
3) Foliate are rudimentary in humans and are found on the lateral surfaces of the tongue.
4) Circumvallate are 7-12 papillae that sit immediately behind the sulcus terminalis. They have many taste buds near their base. They have von Ebner’s glands that secrete lingual lipase into the groove where the taste buds lie which will provide a flow of liquid to move food particles away so they can process new taste information and to prevent the formation of a hydrophobic layer over the taste buds. The lingual lipase will also function to break down triglycerides in the stomach.
Taste buds are made of taste pores which are made of four different types of cells:
1) Type I cells which support the taste bud and have apical granules.
2) Type II cells which support the taste buds but do not have granules.
3) Type III cells have vesicles they release to transfer the stimulus underlying nerve fibers.
4) Basal cells only touch the basal end of the taste bud and can differentiate into any of the other three cell types.
Taste buds are arranged in the following from the front to the lateral side as sweet, salty, sour and the taste of bitter on the top middle.
Saliva is composed of:
1) Water
2) Proteins
3) Glycoproteins
4) Enzymes. The primary enzyme being lysozyme to break down bacterial cell walls and a-amylase to begin carbohydrate digestion.
5) Antibodies. IgA since it is the secretory immunoglobulin.
6) Electrolytes. Some of which are used for tooth development.
Salivary glands secrete into intercalated ducts that are the ducts of an individual secretory unit. The intercalated ducts join to give straight ducts which join to form interlobular ducts which then form interlobar ducts.
The dentin of teeth are made of 70% hydroxyapatite crystal with the matrix made of collagen type I. The dentin contains odontoblasts which secrete the organic matrix called predentin. The odontoblasts are similar to osteoblasts. They also have odontoblast processes that reach through the dentin as far as the enamel to supply and build the dentin. They also have matrix vesicles that they small hydroxyapatite release which contain crystals which serve as the sites for future hydroxyapatite crystallization.
Enamel is the strongest substance in the human body being made of 95% hydroxyapatite crystals. It has fibers called amelogenins and enamalins that are secreted by ameloblast cells which are of ectodermal origin.
Pulp is located in the middle of the tooth and lies deep to enamel. It is made of loose connective tissue and contains the nervous and vascular supple of the teeth. It has myelinated nerve fibers that come in and the unmyelinated nerve ends reach through the dentin with the odontoblast processes which sense pain.
Cementum covers the dentin and is similar in composition to bone yet it is not arranged into haversion systems. It’s job is to help anchor dentin to the root and it is secreted by cementocytes which have canaliculi which reach throughout the cementum like the canaliculi of osteocytes in bone.
The periodontal ligament is made of dense connective tissue that functions to bind the cementum to the underlying alveolar bone. It’s collagen has a very high turnover rate which makes it the first tissue in the body affected by vitamin C deficiency (scurvy).
The bone sockets that hold the teeth are made of primary, alveolar (spongy) bone. The alveoli provide channels that the nerves and vessels are able to pass through to reach the teeth. Being primary bone it is easier for the teeth to change locations such as when a person has braces.
The organs in the GI tract all have a similar structure with slight differences. They are all composed of the following four layers:
1) The mucosa is the layer immediately surrounding the lumen. It itself has three sub-layers:
a) An epithelial lining who’s type varies per location in the tract.
b) A lamina propria made of loose connective tissue that contains blood and lymph vessels as well as some diffuse lymph tissue.
c) Muscularis mucosae which is a band of smooth muscle cells in the mucosa that is usually in two layers, and inner circumfrencial layer and an outer longitudinal layer.
2) The submucosa is next and composed of dense irregular connective tissue with blood and lymph vessels. It may contain glands. If there is a muscularis mucosae it will contain a submucosal (Meissner’s) nerve plexus which is the parasympathetic nerve plexus for the muscularis mucosae.
3) The muscularis externa is another muscular layer which is also made up of an inner circumfrencial layer and an outer longitudinal layer. Between the layers of muscle lies the myenteric (Auerbach’s) nerve plexus which is the parasympathetic nerve plexus for the muscularis externa.
4) The outer layer is either a serosa or an adventitia depending on the layer. Either one is a layer of loose connective tissue, vessels and adipose tissue. The difference is that a serosa has a simple squamous epithelium called mesothelium whereas an adventitia does not.
The mucosa in the GI tract functions as:
1) A selectively permeable barrier which allows more or less material to cross depending on the point of the tract in which it lies.
2) A secretory and synthetic unit for digestive enzymes, hormones, mucus, IgA and so forth.
3) An absorptive layer.
4) A protective barrier against bacterial infection. Largely because of the diffuse lymphatic tissue, lymphatic nodules, macrophages and eosinophils located in the submucosa.
All organs below have the same arrangement as listed immediately above unless specified below.
It is lined with a non-keratinized stratified squamous epithelia.
Near the stomach the mucosal layer has esophageal-cardiac glands which secrete mucus.
The submucosa has esophageal glands located in the submucosa which also secrete mucus.
The muscularis externa had three regions with different muscle types. The superior third is made of skeletal muscle with is under somatic control. The middle third is a mix of skeletal and smooth muscle. The inferior third is smooth muscle under involuntary control.
Most of the esophagus has an adventitia (the part in the abdomen) but the small section in the thorax has a serosa.
In anatomy we will study that the stomach has four regions, cardiac region, fundic region, body and pyloric region. Histologically the stomach as three regions as the body and fundus have the same structure.
The stomach is lined with simple columnar epithelia that is arranged with gastric pits that each have five to seven tubular glands emptying into them. There is a lamina propria between the glands and the pits. The glands secrete directly into ducts which empty into the gastric pits and vary depending on the region in which they lie.
The stomach has internal folds in the mucosa known as rugae that are longitudinal folds that stretch out when the stomach fills up.
The muscularis external has three layers rather then the normal two. It has an inner oblique, middle circular and outer longitudinal layers. The middle layer is responsible for the pyloric sphincter.
A serosa surrounds the stomach, like most of the GI tract.
The cardiac region of the stomach is a narrow circular band at the junction between the stomach and the esophagus. It has glands that secrete mucus and lysozyme and has very few parietal cells.
The fundic region has gastric glands which open into gastric pits. There are two sets of glands, those that open into the neck of the pits and those that open into the bottom of the pit. The cell types located near the surface are the undifferentiated cells, mucus neck cells and parietal cells. The undifferentiated cells will differentiate into one of the other cell types. The mucus neck cells secrete mucus but the mucus is less soluble then the mucus secreted by the mucus glands on the stomach’s surface.
The cells of the base of the gastric pits have enteroendocrine cells that secrete GI hormones, chief (zymogenic) cells that secrete pepsinogen and parietal cells. The pepsinogen that is secreted by the chief cells is automatically activated when it comes into contact with the low pH of the stomach.
Parietal cells are round or pyramidal and have infoldings on the cell membrane called intracellular canaliculi and tubulovesicles. Tubulovesicles are tube shaped pieces of plasma membrane. When the cells are stimulated by gastrin and become active the tubulovesicles bind to the plasma membrane and they form more microvilli on the intracellular canaliculi to make more intracellular canaliculi which increase the cell’s surface area. They have lots of mitochondria just inside such folds. Parietal cells secrete HCl and intrinsic factor (IF). IF is essential for the uptake of vitamin B12 and a shortage of IF will cause pernicious anemia. Parietal cells are stimulated by gastrin to secrete HCl.
The pyloric region has deep gastric pits with short coiled glands that secrete mucus and lysozyme. It also has a number of enteroendocrine cells including gastrin (G) cells which secrete gastrin that stimulates the parietal cells to secrete HCl. It also has D cells that secrete somatostatin hormone that inhibits the release of gastric hormones, including gastrin.
The small intestine has a mucosa that is made of tall, simple columnar cells with a striated boarder. The striated boarder is caused by the attachment of about 3000 microvilli per cell. The luminal boarder is lined with disaccharidases and peptidases that are used to break down disaccharides and amino acids respectively. The cells have a junctional complex that is used to hold them together. The mucosa is arranged so that it has infoldings of the entire mucosa for the sake of increasing the intestinal surface area called villi. The whole small intestine has folds called plicae circularis that exist to increase the absorptive surface area.
The lamina propria is made of loose connective tissue, vessels, nerves and smooth muscle cells that are responsible for the rhythmic movement of the villi.
The submucosa has Brunner’s glands in the duodenum (see below).
The lamina propria and submucosa together have a collection of lymphatic nodules called Peyer’s of which more can be found in the ileum.
The muscularis externa has the normal two layers but the serosa outside of it only exists in some areas.
Lipid digestion is mostly taken care of by pancreatic lipase and bile and absorbed in the jejunum. As discussed in biochemistry they are then carried by chylomicrons into the lymphatic system therefore bypassing portal circulation.
Amino acids and monosaccharides are absorbed by active transport in the small intestine.
Goblet cells produce acid glycoprotein that helps protect and lubricate the lining of the intestine. There are a few in the duodenum and they increase as you approach the ileum.
Paneth cells can be found in the basal part of glands. They secrete lysozyme to control the normal bacterial flora.
M-cells form pits over Peyer’s patches and help transport antigens to them.
Enteroendocrine cells secrete:
1) Cholecystokinin (CCK) that triggers the release of bile from the gallbladder.
2) Secretin causes the flow of pancreatic and biliary bicarbonate and water.
3) Gastric inhibitory peptide that inhibits gastric secretions.
Brunner’s (duodenal) glands are located only in the early duodenum for the sake of raising the pH from that of the very acidic stomach to the near neutral pH of the intestinal tract.
The large intestine does not have villi or plicae circularis since it does not play an absorptive role in the way the small intestine does. The mucosa does have lots of interstitial glands, many goblet cells ad enterocytes (epithelial absorptive cells) but very few enteroendocrine cells. The colon’s function in to reabsorb excess water, form fecal material and produce mucus.
The lamina propria contains lymphoid cells and lymphatic nodules.
The muscularis externa is not the normal layers but is three bands of smooth muscle evenly spaced around the colon called teniae coli.
The serosa has fatty projections called appendices epiploicae.
The mucosa forms longitudinal folds called anal columns of Morgagni. (Why would you name anal columns after yourself?) The external portion of the anal canal will become stratified squamous epithelia which is of ectodermal origin. That being immediately below the anal columns and at that spot are many veins which enlarge to become hemorrhoids in the situation of portal hypertension.
The appendix is a small evagination of the cecum and has lots of lymphoid tissue. It has very few glands. It is so small that the teniae coli join together to give a normal muscularis externa.
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