Class / Digestive System 2, Digestive duct. Lab Class. Histology of Organs and Systems

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CATEGORY: Life Sciences > Human & Animal Histology > Organs & Systems > Digestive duct > Large Intestine

Article 36: Large intestine, Colon, Myenteric plexus of Auerbach

Valentin MartínWesapiens/Natura 10 / 31 / 2011
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Abstract

Welcome to the lab class in histology of organs and systems. 47 Introducing lab class (in spanish)
The aim of these lab class is to provide students with the educational resources necessary to acquire the basic practical skills of each subject, i.e. to recognize, locate and describe the types of cells, tissues and structures of each organ.
Each of these practical sessions are structured around three main elements:
- Definition of learning objectives.
- HandBook: Description of structures, tissues and cell types of the system itself.
- Exercises: location of structures / cell types in virtual slides

LEARNING OBJECTIVES
- Identify the general structure of the digestive duct layers.
- Identify the layers of the esophageal wall.
- Identify the type of epithelium of the esophageal mucosa.
- Locate the submucosal glands of the esophagus.
- Differentiate the layers forming the wall of the stomach.
- Identify the epithelium, lamina propria, the muscularis mucosa of the stomach.
- Locate the pits (foveae) and the gastric glands.
- Differentiate the mucous cells, parietal and chief cells of the gastric glands and their distribution according to the depth of the gland.
- Differentiate the different types of mucous cells by the nature of its secretion using specific staining techniques such as PAS-Alcian blue.
- Identify the sub-layers of the muscularis propria of the stomach.
- Characterize the structure of the wall of the small intestine.
- Differentiate between folds (plicae or valves of Kerking) and intestinal villi.
- Identify which layers form the intestinal villi.
- Identify the cell types that form the intestinal epithelium: enterocytes and goblet cells.
- Characterize the lamina propria of the villi, identify the cell types and lacteals.
- Locate the crypts of Lieberkühn, Panneth cells and enteroendocrine cells.
- Identify the two sub-layers (inner circular and outer longitudinal) of the muscularis layer of the small intestine.
- Differentiate the characteristics of each segment of small intestine: duodenum, jejunum and ileum.
- Identify the glands of Brunner in the duodenum.
- Identify the layers of the large intestine wall and characterize the differences between their structure and those of the small intestine.
- Characterize the proportion of goblet cells in the epithelium of the large intestine and compare with that of the small intestine.
- Identify the two sub-layers (inner circular and outer longitudinal) muscularis layer of the large intestine.
- Identify the myenteric plexus of Auerbach.

HANDBOOK
GENERAL ISSUES
The digestive duct's main mission is food digestion and subsequent absorption of nutrients that may be useful, and the removal of unusable waste.
Each of the features of this process is done in a certain portion of the digestive duct, thus:
- The esophagus is responsible for driving the bolus from the mouth to the stomach.
- The stomach is carried out most of the digestion process, generating a slurry (chyme) a mixture of the bolus with gastric juices. Digestion is the process by which ingested food are broken down to its basic components (monosaccharides and disaccharides, fatty acids, amino acids and small peptides and nucleic acids).
- In the small intestine completes digestion and absorbs 90% of the nutrients.
- The main function of the large intestine is the water recovery, packing and preparing feces for his expulsion.

The digestive duct wall has a structure common to all sections, although each section has its peculiarities.
The wall of all sections shows four layers, from the lumen are: 1 General structure of the digestive duct

- Mucosa:
It consists of a lining epithelium, below, a lamina propria of loose connective tissue and a thin layer of smooth muscle, which is called "muscularis mucosa".
In much of the digestive duct, the lining epithelium (and glands that form) is responsible for the secretion of substances for digestion and is in this epithelium where nutrient absorption occurs. 15 Gastric pits (foveola)
The connective tissue of the lamina propria is highly cellular loose connective tissue where there are large numbers of cells for the defense of the body, known as MALT (Mucose Associated Lymphatic Tissue), which also known as Peyer's patches. 37 Large intestine, structure of the appendix In the lamina propria also can find many blood vessels and lymphatic vessels that is where you collect the nutrients absorbed.
- Submucosa:
This layer consists of dense connective tissue highly vascularized. 20 Stomach, structure of the submucosa
The submucosa serves as a structural base of the mucosa
- Muscularis:
This layer consists of different sub-layers of smooth muscle in different directions. 33 General structure of large intestine
Thus in most of the digestive duct can be located two sublayers: an internal sublayer circulary oriented (parallel to the light) that contract strangle the lumen of the duct. Another external sublayer, longitudinally oriented (parallel to the longitudinal axis of the duct), which shortens the length of the duct. By combining the two movements is achieved by pushing forward the bolus along the duct even against the direction of gravity. This movement is called peristalsis.
- Adventitia:
This is the outermost layer and consists of areolar connective tissue and an outer mesothelium lining, these two tissues form the serous. The serosa is the visceral peritoneum. 23 General structure of the small intestine

Innervation of the digestive duct is carried out by the enteric nervous system which is composed by two nerve plexuses: the myenteric plexus (located between the sublayers of the muscularis, also called Auerbach's plexus) 23 General structure of the small intestine ((36) ) and submucous plexus (located in the submucosa and also known as Meissner's plexus). 20 Stomach, structure of the submucosa
Into these plexuses are found motor neurons, sensory neurons and interneurons. Motor neurons of the plexus of Auerbach (myenteric) innervate the muscle cells of the muscularis, controlling the peristaltic movement. Motor neurons of Meissner plexus (submucosal) controls secretion, innervating the secretory cells of the mucosa, as well as the control the movement of the muscularis mucosa. The myenteric plexus sensory cells act as mechano receptors in smooth muscle of the muscularis, while the sensory cells of the submucosal plexus mainly function as chemoreceptors. Interneurons connect both plexuses.
The enteric nervous system is regulated by the autonomic nervous system (sympathetic and parasympathetic).

The arteries that supply the digestive tract wall enter the duct wall by the serous layer, cross the muscularis up to the submucosa where they form a plexus. from which leave branches to the mucosa, where they form a capillary network in the lamina propria. These capillaries drain into a venous plexus located in the deeper part of the lamina propria, from which departs veins towards the submucosa, where they form another venous plexus, from which emerge major veins that leave the digestive duct wall by the same route used by the incoming arteries.
In the lamina propria of the mucosa are often found blind lymphatic capillaries that drain into a lymphatic plexus in the deepest part of the lamina propria (close to the muscularis mucosa). From this plexus emerge lymphatic vessels that cross the muscularis mucosa towards the submucosa where they are again forming another plexus from which depart the major lymphatic vessels which follow a paralel route to the blood vessels.

ESOPHAGUS
The esophagus is a straight tube that connects the mouth to the stomach and its only mission is to transport the food bolus into the stomach. In the esophagus produces no digestion or absorption.
At the microscope, in a cross section, the esophagus is seen as a thick-walled duct in which two of the three layers: 7 General structure of the esophagus
- The mucosa, in which the epithelial lining is stratified squamous type, 8 Esophageal wall 9 Structure of the esophageal wall 10 Esophagus, structure of the mucosa as in the oral cavity and unlike the rest of the digestive tract (which is simple columnar). Depending on the type of food we can observe differences in the epithelial lining, so hard feeding species (eg rodents) its epithelium is keratinized, 9 Structure of the esophageal wall whereas in other species with food not so hard (as humans) its epithelium is not keratinized. 10 Esophagus, structure of the mucosa
- The muscularis layer, which is quite wide 7 General structure of the esophagus and that serves to propel the bolus towards the stomach by peristaltic waves.
Also worth mentioning is the presence of mucous glands. At the bottom of the esophagus can be seen in the esophageal mucosa glands, known as glands of the cardias (given their similarity and proximity to the glands of this stomach area). Can also be seen glands inside the submucosa layer. 11 Esophagus, submucosal glands All glands of the esophagus are of mucous secretion, this mucous is tasked to lubricate the bolus transit.
The transition between the esophagus and stomach is abrupt and not gradual, so it is easy to recognize under the microscope. 12 Esophageal - gastric junction

STOMACH
The stomach has the function of the chemical breakdown of food. (digestion) To carry out this function has a number of specialized structures.
First, and as you can see in a low magnification image at the microscope, 13 General structure of the stomach wall the submucosa forms folds that increase the secreting surface.
The lining epithelium shows gastric pits (gastric foevoles) where gastric glands flowing. This epithelium, which is simple columnar type, consists almost entirely of mucus cells, which under the microscope show clusters of mucin in their apical portions. 15 Gastric pits (foveola) The mucin (mucus) that these cells secrete is very dense and forms a thick layer lining the epithelium which protects from substances secreted by the stomach for digestion.
Then you must include the gastric glands. The gastric glands are found in the mucosa and secrete the substances needed for digestion (hydrochloric acid and pepsin). The gastric glands are simple tubular glands 14 Gastric glands and show slight differences in different parts of the stomach, so we can distinguish three types of gastric glands:
- Glands of the Cardia. These glands are located in the cardia, close to the esophagus. They are simple tubular glands, consisting mainly of mucus cells and are very similar to the glands of the lower esophagus. The mucus secreted by mucous cells of these glands is more fluid that is secreted by the epithelial lining, but performs the same function of protection.
- Corpo-fundic glands. These glands are found in the body and fundus of the stomach. They are simple straight tubular type, although two or three of these glands can lead to the same gastric pit and may also be branched in their distal ends. 14 Gastric glands . In these glands we can differentiate several areas: - Neck (the area adjacent to the pit) and - The body, which in turn can be divided into an upper and lower regions.
On the neck of these glands can find several types of cells: - mucous neck cells - parietal cells (oxyntic) - Cells in mitosis (stem cells) and occasionally - enteroendocrine cells. 16 Stomach, cell types in the neck of the gastric glands
In the body of these glands can be seen: - chief cells (zymogenic) - parietal cells (oxyntic) and - enteroendocrine cells. 16 Stomach, cell types in the neck of the gastric glands 17 Stomach, cell types in body of the gastric glands
The mucous neck cells are similar to those of the glands in the cardia and like them, they secrete fluid protective mucus. 16 Stomach, cell types in the neck of the gastric glands The mucous cells is slightly acidic, unlike secreted by cells in the epithelial lining, which is neutral. This is easily seen in a sample stained with PAS-Alcian blue, while PAS reagent stains the neutral mucin, but the Alcian Blue stains the acidic mucin. 22 Stomach, secretion of mucous substances
Parietal cells (oxyntic) are easily distinguished under the microscope, as it has a large ovoide cytoplasm, slightly stained with hematoxylin-eosin. Its nucleus is round and is usually located in a central position. These cells secrete HCl (hydrochloric acid). 16 Stomach, cell types in the neck of the gastric glands 17 Stomach, cell types in body of the gastric glands
The stem cells are pluripotent, are often seen in mitosis, they can differentiate into different cell types found in the gastric glands. 16 Stomach, cell types in the neck of the gastric glands
The chief cells, or zymogenic, are easily distinguished under the microscope in slides stained with hematoxylin-eosin, where you can see numerous white granules in the apical area of an elongated cytoplasm. The nucleus is usually located in basal position. These cells secrete the enzyme pepsin (in the form of a precursor called pepsinogen). 17 Stomach, cell types in body of the gastric glands
Among the other cell types of the gastric glands can be distinguished endocrine cells (enteroendocrine). 16 Stomach, cell types in the neck of the gastric glands 17 Stomach, cell types in body of the gastric glands These cells secrete different hormones (i.e. gastrin, somatostatin) related to different processes of digestion, but to distinguish different subtypes is necessary the use of immunohistochemical techniques.

- Pyloric glands. These glands are found in the pyloric region and are also tubular, but have more ramifications than Corpo-fundic ones. They tend to be rolled. In these glands can be located mucous secreting cells (which secrete, fluid mucin, like in the glands of the cardia and mucous neck cells of the Corpo-fundic glands). You can also find endocrine cells (enteroendocrine).

The third specialization of the stomach is found in the muscularis layer, which show three sub-layers (from submucosa to adventita are: inner oblique, middle circular and external longitudinal). 21 Stomach, structure of the muscularis The reason of three muscle layers with different orientation is the ability to make a movement, not to forward the chyme, but to mix food and secretory products (enzymes, hydrochloric acid) and thus promote digestion. Between these sublayers are able to distinguish the nervous plexus of Auerbach. 21 Stomach, structure of the muscularis

Although not exclusive to the stomach, include the large presence of defense-related cells in the lamina propria of the gastric mucosa 18 Stomach, cell types in the lamina propria (plasma cells, mast cells, macrophages, lymphocytes).
The muscularis mucosa of the stomach is formed (as in other parts of the digestive tract) of smooth muscle, but in the stomach is thicker than elsewhere. 19 Stomach, Muscularis mucosae
Submucosa layer shows large amount of collagen fibers (dense connective tissue) from which you can see some defense cells, but not as many as in the lamina propria. Are abundant blood vessels and can locate the nervous plexus of Meissner, although not as easily distinguished as that of Auerbach (nervous plexuses weakly stained with hematoxylin-eosin, so very easily to distinguish from muscle bundles that stain very intensely, whereas they are difficult to distinguish from the collagen fibers that stain with similar intensity). 20 Stomach, structure of the submucosa

SMALL INTESTINE
Bowel function is to complete the process of digestion, but especially the absorption of nutrients.
To carry out this function, the small intestine has a number of specialties, among which include the substantial increase in the absorption surface.
To achieve a significant increase in the absorption surface (without increasing the volume) small intestine performs folds, this folding process is performed at three levels.
- The first level is related to the submucosa, which folds into the intestinal folds (also known as circular valve or valves of Kerkring). 23 General structure of the small intestine 24 Small intestine, structure of intestinal plica and villi
- The second level occurs when the mucosal layer folds, in what is known as villi. 24 Small intestine, structure of intestinal plica and villi 32 Small intestine, Ileum, villi and lacteal
- The third level of folding is observed in the apical surface of epithelial absorptive lining cells (enterocytes), they are know as the microvilli. At the optical microscope the microvilli appear as a continuous band in the apical part of epithelium called brush border. 2 Digestive duct, Goblet cells 1 If we study it at higher magnification with the electron microscope, we see that the brush border is formed by numerous evaginations, thin and parallel, of the apical surface of enterocytes (microvilli) 5 Digestive duct, Brush border (microvilli) 6 Digestive duct, Brush border (microvilli), transverse sectioned
Some authors consider an additional level of folding, formed by invaginations of the basal area of the epithelial lining, which form a well or crypts, which are known as crypts of Lieberkühn. 24 Small intestine, structure of intestinal plica and villi 25 Small intestine, structure of intestinal mucosa
The lining epithelium of the small intestine is simple columnar type in which most cells are absorptive (enterocytes). 2 Digestive duct, Goblet cells 1 3 Digestive duct, Goblet cells 2 can also be found, in small numbers, goblet cells, which have the characteristic shape of a cup, with the mucous secretion in the apical part 2 Digestive duct, Goblet cells 1 3 Digestive duct, Goblet cells 2 . In electron microscopy shows that the secretion is formed by numerous low density granules. 4 Digestive duct, Goblet cells, Electron microscopy
As mentioned at the base of the villi are the crypts of Lieberkühn. In these crypts, predominantly secretory cells called Paneth cells, can be seen. 26 Small intestine, structure of crypts of Lieberkühn, Paneth cells Paneth cells are easily distinguished under the microscope as its apical part shows a large number of granules that stain intensely with hematoxylin-eosin. 26 Small intestine, structure of crypts of Lieberkühn, Paneth cells Paneth cells secrete substances that are used to protect the intestine from microbial pathogens, among these substances include lysozyme.
In addition to the Paneth cells in the crypts of Lieberkühn, enteroendocrine cells are seen. 26 Small intestine, structure of crypts of Lieberkühn, Paneth cells with general staining techniques, as hematoxylin-eosin, two different morphologies of these cells can be distinguished: - the open type, with elongated cytoplasm extending from the base to the lumen and - the closed type , rounded appearance and located near the basement membrane. 26 Small intestine, structure of crypts of Lieberkühn, Paneth cells Functionally more types can be distinguished because they secrete a large number of hormones, which include: serotonin, somatostatin, secretin, cholecystokinin, etc ...
In the crypts of Lieberkühn, as in the intestinal villi, you can also see goblet cells.
In the central part of the villi is common to see a lymphatic vessel called lacteal. 32 Small intestine, Ileum, villi and lacteal This lymphatic vessel starts on a blind terminal in the apical part of the villi and drains into a lymphatic plexus near the muscularis mucosa. This lymphatic vessel, as well as for the lymphatic circulation, is used as a vehicle for transport of absorbed nutrients, especially lipids, which are released in the lumen of lacteals (from enterocytes) to be drained to the venous system. The remaining nutrients are transported by the blood vessels of the villus microcirculation system.
You can also see smooth muscle cells oriented in the longitudinal axis of the villi, that can contract and shorten the villi help drain the vessels. 25 Small intestine, structure of intestinal mucosa 32 Small intestine, Ileum, villi and lacteal
In the lamina propria can be seen numerous defense cells, particularly lymphocytes, which are often grouped in clusters of diffuse lymphatic tissue, known as Peyer's patches. Peyer's patches are part of the MALT (Mucose Associated Lymphatic Tissue). Peyer's patches are located close to the muscularis mucosa.
The submucosa consists of dense connective tissue 25 Small intestine, structure of intestinal mucosa with lots of blood vessels and the plexus of Meissner.
Throughout the small intestine the muscularis layer is composed of two sublayers (an internal circular and an outer longitudinal) between these two sublayers is located Auerbach's myenteric plexus. 29 Small intestine, Duodenum, Muscularis
Anatomically there are three regions or segments of the small intestine - duodenum - jejunum and - Ileum. These regions also show some histological differences.
- Duodenum. The most significant feature of the duodenum is the presence of mucous secreting glands in the submucosal layer. 27 Small intestine, structure of duodenum These glands are known as Brunner's glands. 28 Small intestine, Duodenum, Brunner's glands Brunner's glands drain into the crypts of Lieberkühn at the base of the villi. Also noteworthy is that the muscularis mucosa does not form a continuous layer and is limited in the best case, of some scattered smooth muscle fibers. 28 Small intestine, Duodenum, Brunner's glands
- Jejunum. The jejunum shows the basic structural pattern of the small intestine, with the particularity that the villi are thinner and longer than in other intestinal regions. 30 Small intestine, general structure of the Jejunum
- Ileum. It shows all the features described for the small intestine. 31 Small intestine, Structure of the ileum 32 Small intestine, Ileum, villi and lacteal
- Jejunum. The jejunum shows the basic structural pattern of the small intestine, with the particularity that the villi are thinner and longer than in other intestinal regions. 30 Small intestine, general structure of the Jejunum
- Ileum. It shows all the features described for the small intestine. 31 Small intestine, Structure of the ileum 32 Small intestine, Ileum, villi and lacteal

LARGE INTESTINE
The function of the large intestine is to absorb most of the water and inorganic salts of the chyme, compacting the chyme to be eliminated as stool.
The surface epithelium of the intestine is quite smooth and has no villi (but you can see circular folds). 34 Large intestine, structure of the Colon But it shows many deep crypts of Lieberkühn. 33 General structure of large intestine 34 Large intestine, structure of the Colon 35 Large intestine, structure of the Colon mucosa
The lining epithelium is simple columnar type and it is composed of enterocytes and a considerable number of goblet cells. 35 Large intestine, structure of the Colon mucosa
In the large intestine can see lots of lymph nodes (Peyer's patches) that sometimes invade also the submucosa. 34 Large intestine, structure of the Colon 37 Large intestine, structure of the appendix
The submucosal layer is formed by dense connective tissue 35 Large intestine, structure of the Colon mucosa which shows large amount of blood vessels and nerve plexus of Meissner.
Compacting stool, its transit through the tube becomes more difficult, so you can see many goblet cells that secrete mucus as a lubricant. Also to make this transition the muscularislayer (which consists of two sub-layers: inner circular and outer longitudinal) is considerably thicker than in the small intestine. 33 General structure of large intestine 34 Large intestine, structure of the Colon Obviously you can see, between the two muscle sublayers, the myenteric plexus of Auerbach. 36 Large intestine, Colon, Myenteric plexus of Auerbach
Anatomically the large intestine is divided into different regions: - The cecum, the appendix 37 Large intestine, structure of the appendix , - the colon 34 Large intestine, structure of the Colon and the rectum.

EXERCISES
- Find the different layers of the esophagus ((38)) 39 Esophagus mucosa H-E 1,5 um
- Distinguish the gastroesophageal junction 40 Esophagus-Stomach junction PAS-H 7 um
- Find the gastric pit and neck of the gastric glands ((41.1))
- Locate mucosal cells of the lining epithelium and from the gastric pits and distínguish them of mucous neck cells ((41.1))
- Locate stem cells (mitosis), parietal cells, chief cells and enteroendocrine cells in the gastric glands ((41.1)) ((41.2))
- Distinguish between mucus cells of the lining epithelium and from the gastric pits and distínguish them of mucous neck cells using the technique of PAS-Alcian blue 40 Esophagus-Stomach junction PAS-H 7 um
- Distinguish villi 42 Small intestine (ileum) PAS-H 7 um
- Distinguish Lieberkühn crypts and lacetal in the small intestine 42 Small intestine (ileum) PAS-H 7 um ((43)) 44 Small intestine (ileum) H-E 7 um
- Distinguish Paneth cells and enteroendocrine cells in the small intestine ((43))
- Find the Brunner's glands in the duodenum 45 Small intestine (duodenum) H-E 1,5 um
- Locate goblet cells in the lining epithelium in the large intestine ((46.1))
- Locate the Auerbach's myenteric plexus of the colon ((46.2))

Article

Original article

Species / Animal group Human
Microscopy Method Light field optical microscopy
Embedding method Plastic
Staining method Hematoxylin-Eosin
Section thickness 1-2 um

This high magnification interactive image shows a close-up of a nervous plexus of Auerbach between the two sub-layers of the colon's muscularis.
We can distinguish neurons as well as satellite cells.

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