Small Intestine Structure and Function PDF

Interstitial cells of Cajal (pacemakers of the gut) —> they’re cells that lay between the longitudinal and circular layer of the muscolaris propria. They generate electrical slow waves in the smooth muscle cells of the stomach, small intestine and colon which organise basic motor patterns such as pe...

Interstitial cells of Cajal (pacemakers of the gut) —> they’re cells that lay between the longitudinal and circular layer of the muscolaris propria. They generate electrical slow waves in the smooth muscle cells of the stomach, small intestine and colon which organise basic motor patterns such as peristalsis and segmentation in the GI tract. They mediate communication between autonomic nervous system and smooth muscle. If these cells fail to develop or if they malfunction then digestion is slowed, nutrients absorption is impaired and a wide range of gastrointestinal disorders can result. EEnftiokgthiestesrsafhiff.ee THE SMALL INTESTINE Functions: Continue the digestive process started in the stomach Absorption of nutrients and water coming from the digested food Devices: Macroscopic devices (plicae and villi and criptae or glands, PERMANENT) Microscopic devices (microvilli on the apical surface of enterocytes) Biochemical devices —> pancreatic enzymes and bile secreted by the gallbladder into the duodenum (they contribute to the luminal digestion, the enzymes are released in the lumen and mixed with the food) and enzymes of the microvilli of enterocytes (membrane digestion, the process happens at the level of the membrane of the mucosa) Mechanical devices —> contraction and relaxation of intestinal wall for mixing (segmentation) and propulsion (peristalsis) Processed food coming out of the stomach is called chyme and after it comes out of the duodenum is called chyle (after exposure to pancreatic duodenal and biliary secretion) The small intestine is made of —> DUODENUM (secondarily retroperitoneal), JEJUNUM and ILEUM (also called mesenterial intestine because the peritoneum has a very long mesentery that attaches the ileum and jejunum to the posterior abdominal wall giving it a lot of mobility, intraperitoneal). It is 4 to 7 meters long DUODENUM —> divided into 4 parts (superior, mostly intraperitoneal because it is in continuity with the stomach, descending, horizontal, ascending). It is 25-30cm long  Year gestated agent 0 The JEJUNUM and ILEUM form the mesenterial intestine because they have a mesentery that attaches them to the posterior abdominal wall. This portion of intestine is on average 6m and the diameter becomes narrower towards the end. The vessels branching from the aorta reach the jejunum and ileum passing through the dorsal mesentery and forming anastomotic arcades with vasa recta The small intestine presents villi protruding on the folds. The folds of the stomach are called rugae and are transient while the folds of the intestine are called plicae circulares and are permanent. These folds are located transversely with respect to the longitudinal axes (horizontal). Folds have a different appearance and density (distance between one another) in the three portions of the small intestine (in the duodenum they are not as high as in the jejunum, in the ileum they’re almost flat). In the ampullary region of the duodenum (first) there are no folds, which start appearing in the descending portion. Then folds increase in the jejunum and decrease again in the ileum. In the duodenum there are lots of submucosal glands while in the jejunum and ileum there aren’t The initial part of the duodenum is smooth and then it starts to present plicae. When looking at the descending portion of the duodenum there is a longitudinal fold that presents two tiny openings called minor duodenal papilla (rostral) and major duodenal papilla (caudal). The major is always there while the minor might not be present (and it is normal). At the level of the major duodenal papilla there is the communication of the bile duct and of the main pancreatic duct with the duodenum ies a E i ii ii E.EE Bile is an emulsifying agent for lipids while pancreatic secretion has a high pH (neutral, to balance the acidity coming form the stomach) and enzymes that digest proteins, carbohydrates, lipids and nucleic acids. These enzymes are secreted in their inactive form and they are then activated by enzymes of the duodenal mucosa. Since bile and pancreatic secretions shouldn’t be secreted continuously there is a sphincter that regulates it. In between the villi of the intestine the mucosa invaginate forming crypts of Lieberkühn. In the duodenum at the bottom of the crypts of Lieberkühn Brunner’s glands originate (important, they’re tubulo-acinar glands that produce alkaline-secretion, very basic, to counteract the acidity of the stomach with which the duodenum is in continuum). There shouldn’t be a too thick layer of mucous because that would reduce the digestion and absorption. Only in the ileum there are Peyer’s patches (aggregations of lymphoid tissue in the submucosa and lamina propria) to protect the body against microorganisms that might have entered with food. INTESTINAL VILLI and CRYPTS Outside of the villi there is a layer of epithelium, at the core there is the lamina propria and the muscolaris mucosa (contribute to a gentle movement). The muscolaris mucosa also lines the crypts (promotes the passage of what is inside the crypt to the outside). VERY IMPORTANT! At the core of the villi there is also a net of capillaries and lacteals for absorption and to provide oxygen. Lacteals are specialised lymphatic vessels of the intestine able to absorb fats (too big to be absorbed by blood) Cell types of the epithelium lining villi and crypts: Enterocytes (presenting microvilli, important for digestion and absorption) Goblet cells (unicellular mucous-secreting cells) Stem cells (for cell renewal, 3-5 days enterocytes and 4 weeks Paneth and enteroendocrine cells) Enteroendocrine cells (and enterochromaffin cells) Paneth cells (release protective substances, part of the immune response) Enterocytes —> in the glycocalix there are enzymes important for the process of digestion and absorption (peptidases that reduce the size of polypeptides, and transporters). Each enterocytes has around 3.000 microvilli, which increase by 30 times the available surface. The brush border and the filamentous glycocalix present enzymes such as enterokinase (converts trypsinogen into active trypsin, which in turns leads to the activation of pancreatic zymogens), aminopeptidase (catalyses the cleavage of amino acids), oligosaccharidases (digest carbohydrates into monosaccharides). In the cytoplasm there are also cytoplasmic peptidases (to dissemble proteins). In enterocytes the resynthesis of triglycerides takes place —> the newly assembled TAG will form the hydrophobic core of chylomicrons (that will be absorbed by lacteals after the contribution of bile) ROTAVIRUS AND DIARRHOEA EffectsofRotavirus Enterocytes can be damaged in many ways (for example gluten can damage the enterocytes of celiac people, or gastrointestinal viruses) —> nearly every child in the world will have a rotavirus infection before the age of three. Rotavirus is the leading cause of life- threatening diarrhoeal disease among infants and young children in many countries. Now some vaccines have been licensed. Enteroendocrine cells —> the duodenum has enteroendocrine cells that produce gastrin, secretin and cholecystokinin among other peptide hormones to regulate digestion, intestinal motility and food intake theysensethe content the of stomach Erypts  Enterochromaffin cells are mechanosensors able to release 5HT (serotonin), which stimulates visceral sensation, intestinal motility and permeability Paneth cells —> they’re at the bottom of the crypts and release proteolytic enzymes (like lysozymes) and anti microbial peptides (like defensins). Related to the innate immune response Tuft cells —> sentinels of the GI tube and present also in the mucosa of the respiratory tract. They respond to noxious (damaging) organisms and generate a defensive response LL IInes won bacteria digest of free man iiiiiisiiit.to Edition eeatiE IE ais eet on Peyer’s patches are the main components of the gut-associated lymphoid tissue (GALT) and are more abundant in the ileum. In Peyer’s patches we can recognise a dome (mainly B cells), a germinal centre and a follicle associated epithelium (Peyer’s patches are organised in follicles). M cells and dendritic cells associated to the epithelium of the patches are antigen sampling and presenting cells. Lymphocytes reach Peyer’s patches through HEVs and leave through efferent lymphatic vessels Intestinal motility —> two types of movement: Segmentation (mixing movements) —> contraction with no relaxation in the segment below Peristalsis (propulsive movements) —> aims to move the content more and more caudal —> contraction and relaxation in the segment below Propulsive peristalsis is a basic movement that occurs in the oesophagus, 2 thirds of stomach, small and large intestine and rectum Peristaltic rush —> irritation of the small intestine that results in powerful peristalsis that moves the food along the whole intestine in minutes to sweep it out (es: diarrhea, liquid because there’s no time to absorb water) THE LARGE INTESTINE (caecum, appendix, ascending/transverse/descending/sigmoid colon, rectum, anal canal): Reabsorption of water and electrolytes Propulsion of increasingly solid feces towards the rectum before defecation Action of microbiota (largest population of microorganism that live with us) During fetal life we are sterile and then we start to acquire a symbiotic relationship with microbiota when we are born (as soon as we pass through the birth canal) NUMBERANDHETEROGENEITYOFMICROBIOTA action Iaf t.tw fdhsemicrobiote FITS let w.tt The maturation of the GI tube after birth is in part related to the colonisation of microbiota. The small intestine presents a smooth surface and a reduced size compared to that of the large intestine. The large intestine presents bulges on its surface and some bands of smooth muscles (thickening). The bulges (sacculations) are called haustra and in between the sacculations there are indentations called semilunar folds. The longitudinal muscle layer is concentrated in the three bands (teniae coli) called free tenia, mesocolic tenia and omental tenia while the circular muscle layer is continuous. The teniae begin where the appendix communicates with the caecum and can be observed all throughout the ascending colon, transverse colon, descending colon, sigmoid colon and upper part of the rectum. The large intestine is around 1.5 to 1.8 meters and it has on average a diameter of 7cm. In the region where the greater omentum is close to the teniae there are pockets full of lipids called omental (or epiploic) appendices. In the large intestine the folds depend on the fact that here and there there is a constriction of the smooth muscle layer that causes a groove, they’re not actual folds, they’re transient folds. The sacculations change content forty continuously because they’re given by the contraction of the muscolaris propria (like an accordion) Why are there Haustra? The contraction of the circular muscle layer causes the formation of tight rings The contraction of the longitudinal muscle layer (teniae) causes the bulging of the wall between rings (haustration) Purpose —> mixing and slowly sending the intestinal content forward through peristalsis 1/3 times per day mass movements lead to the filling of the rectal ampulla —> defecation Diarrhoea is an increase in the volume of stool or frequency of defecation. It is one of the most common clinical signs of gastrointestinal disease, but it can also reflect primary disorders outside of the digestive system. Disorders affecting either the small or large bowel can lead to diarrhoea. For many people, diarrhoea represents an occasional inconvenience or annoyance, yet at least 2 million people in the world, mostly children, die from the consequences of diarrhoea each year. There are numerous causes of diarrhoea, but in almost all cases, this disorder is a manifestation of one of the four basic mechanisms: osmotic diarrhoea (increased intestinal osmotic pressure), secretory diarrhoea (increase in intestinal secretions), inflammatory and infectious diarrhoea (due to inflammations) or diarrhoea associated to deranged motility (increased or decreased intestinal peristalsis outside groove Looking at the inside of the large intestine there are evaginations called semilunar plicae or folds that are not true folds (they’re not made of mucosa and submucosa, they’re caused by the invagination of the whole intestinal wall) and are not permanent (they depend on the state of contraction of the muscle layer). There are no plicae circulares or villi like in the small intestine Ileocaecal valve —> superior ileocolic lip + inferior ileocolic lip —> it is not a sphincter, there are just two flappy pieces (lips) that open and close according to the distension of the ileum and colon as is.in ea ieskEe metiemsn.ae ORIFICE APPENDICEAL 811mEurfethppendix DIVERTICULOSIS Evagination of the mucosa that form diverticula. They’re more common in the large intestine because where the longitudinal muscle layer is organised in teniae the circular muscle layer is left on its own and it is thus more fragile (thinner). Usually diverticula are located there or where blood vessels penetrate the wall. Diverticulosis can lead to gastrointestinal bleeding (inflammation due to stagnation of feces or rupture of the wall). When diverticula are inflamed they cause diverticulitis The mucosa of the large intestine lacks true folds and villi, however the epithelium invaginates to form tubular glands called glands of Lieberkühn. In the crypts of Lieberkühn there are goblet cells (increasing towards the end of the large intestine) to lubricate, enterocytes (fewer towards the end) for absorption of water, vitamins and electrolytes, enteroendocrine cells (at the bottom), Paneth cells (defence), transit amplifying cells (at a stage between that of stem cells and that of differentiated cells) and stem cells. Enterocytes present less and stubbier microvilli Jenial thine aiminner ᵗʰffÉÉe's yati.ge iffainae teniainco muscarise APPENDIX VERMIFORMIS The appendix presents lots of lymphocytes organised in follicles in the mucosa and submucosa. Some of the content of the intestinal tube my end up in the appendix (since it is a diverticulum) and this can cause an irritation or an inflammation of the wall (appendicitis). The peritoneum can be irritated too as the appendix is intraperitoneal (peritonitis). A perforated appendicitis and a consequent peritonitis can be very bad Lymphatic follicle Gastrointestinal bleeding can also be caused by polyposis (usually in the large intestine). Polyps are abnormal proliferation of the cells of the mucosa that can peafeated either be sessile (lie flat on the surface) or pedunculated (attached to a stalk). If the polyps grow too much they can occlude the gut tube or they may bleed. Polyps can remain benign or can become cancerous (and can digest the basal sessile lamina to enter blood vessels and migrate) Epithelial cells of the GI tract undergo rapid turnover. The replacement takes place thanks to the Wnt/β-catenin pathway. If Wnt binds to the frizzled receptors then β-catenin can enter the nucleus and promote gene transcription (proliferation), while if it doesn’t β-catenin gets destroyed. A disruption of this pathway can lead to familial polypsosis and colorectal cancer chroniclugommation otcon adf.EE RECTUM AND ANAL CANAL The rectum is around 15cm long and has a sort of sacculation towards the end called ampulla rectalis (where faeces are stored). The anal canal is around 4cm long. Both the rectum and anal canal lack haustra, teniae, appendices epiploicae and semilunar folds. The rectum presents three constant transverse folds (Houston’s folds) —> the middle rectal fold is the strongest and is located 7cm from the anus. Tumours below this fold may be palpable during rectal examination The mucosa presents mainly goblet cells and very few enterocytes longitudinalat g cut transverse ANAL CANAL The upper portion presents 8-10 longitudinal folds of the mucosa called anal columns that are separated by shallow sulci (invaginations of the mucosa). The columns are connected at the base by small horizontal plicae called anal valves (forming the pectinate line, it separates the portion of the anal canal derived from the hindgut from that derived from the proctodeum). It is called pectinate because it resembles a comb. Behind anal valves there are anal sinuses or crypts with mucous secreting glands called glands of Morgagni. When feces pass through the anal canal they squeeze mucous out of the glands of Morgagni as to lubricate the surface of the canal. The most internal muscle (which forms the internal anal sphincter) is made of smooth muscle cells, while the outermost muscle (which forms the external anal sphincter) is made of striated muscle cells (voluntary control). A child isn’t able to control the external anal sphincter and so as soon as the ampulla is full defecation happens. Autonomy is acquired during the early years of life The white line or anocutaneous line represents the transition point from non-keratinised stratified squamous epithelium of the anal canal to keratinised stratified squamous epithelium of the anus and perianal skin. The color of the line is white, hence the alternative name. Between the columnar epithelium and the squamous stratified epithelium (at the level of the pectinate line) there is a transition zone that presents a stratified cuboidal epithelium RECTAL VENOUS PLEXUS OR HAEMORROIDAL PLEXUS There are an internal and external haemorroidal plexuses. These venous plexuses help maintaining feces in the last part of the anal canal (continence). They are like vascular cushions in the anal canal and are present in all healthy individuals NORMAL If haemorrhoids become swollen and distended they are referred to as pathological haemorroids. Pathological haemorrhoids are observed in people who suffer from constipation, prolonged straining when defecating or raised intra-abdominal pressure (es Norm 04N pregnancy or ascites). Haemorrhoids can cause bleeding and itchiness —> gastrointestinal bleeding.