structure and function of small and large intestine Flashcards
Small Intestine: Overview
The small intestine digests food material an absorbs end products of the digestive process
the small intestine is divided into 3 regions: duodenum, jejunum, and ileum (these 3 regions are similar histologically; minor differences permit their identification)
Modifications to aid absorption in the small intestine
The luminal surface of the small intestine has modifications that increase its surface area to aid absorption
Pilicae circulares- permanent mucosal folds present in the duodenum, jejunum, and proximal ileum that increase surface area 2-3 fold
Villi- finger like projections of epithelium-covered lamina propria that increase surface area by 10 fold- light M
Microvilli- extensions of the epithelial cell apical plasma membrane that increase surface are 20 fold – EM
increase intestinal SA by 500x
Common histological features of the small intestine:
Villi and crypts
The villi are extension of the lamina propria that are covered in epithelial cells and contain a central lymphatic channel (lacteal) and several capillaries.
Smooth muscle fibers, loose connective tissue and lymphoid cells can also be seen in the center of the villus. lymphocytes are often present within the epithelium
The epithelial covering of the vili are composed of absorptive cells (enterocytes), goblet cells, DNES (enteroendocrine) cells
Crypts of lieberkuhn extend from the base of the villi into the lamina propria: contain DNES (enteroendocrine) cells, stem cells, paneth cells
absorptive cells/enterocytes are the most abundant cells of the small intestine
the enterocytes are columnar epithelial cells with an elongated nucleus
The principle function of enterocytes are terminal digestion and absorption of water and nutrients. These cells re-esterify fatty acids into TGs–> chylomicrons, and transport the bulk of the absorbed nutrients into the lamina propria for distribution to the rest of the body
The luminal surface of each enterocyte has a brush (striated) border (3000/cell), the microvilli are covered in a glycocalyx coat made of glycosylated membrane, The glycocalyx layer has protective roles and is involved in the digestion of disaccharides and dipeptides into monomers for adsorption thu peptide hydrolases and disaccharidases that are abundant on the apical surface
goblet cells
secrete mucin
easily recognizable light micro, full of carbs and are unstained. The carbs in the goblet cells identified by PAS stain
Goblet cells act as unicellular glands that secrete that secrete copious amounts of mucin that consists of high MW glycoprotein macromolecules that have 20% proteins and 80% carbs. The mucin is released at reasonably constant levels by exocytosis just after release the mucin granules become hydrated and expand several hundred fold in volume, the mucus lubricates and protects the intestine
Crypts of lieberkhun
the crypts of lieberkhun extend from the base of the villi into the lamina propria
In addition to enteroendocrine cells, the crypts contain Paneth and stem cells
the base of each villus is surrounded by the openings of several crypts, surface absorptive and goblet cells occupy the upper half of the crypt, the basal half of the crypt has no surface absorptive cells and only a few goblet cells, most cells are regenerative DNES, and Paneth
stem cells and cell turnover
the epithelial lining of the intestine is continuously being renewed by stem cells that proliferate in the crypts
New cells are born from stem cells in the crypts and migrate up the villus and are finally exfoliated from the tips of the villus
The intestinal epithelium is replaced every few days in humans
Stem cells can be IDed in the crypts as mitotic figures
paneth cells
secrete lysozyme and defensins
Paneth cells are long lived and do not participate in upward migration and just stay in the base of crypts, paneth cells are full of secretory granules and they continuously secrete lysozym in addition they secrete defensins that help neutralize bacterial and viral infections, they stain intensely with eosin and also with pt –> scarlet
Lamina propria and muscularis mucosae
lamina propria forms the core of the villi and extends down to the muscularis layer
lamina propria is highly vascularized to accept the products of absorption, it also has a lot of lymphoid cells that protect the intestinal lining from microbial invasion
The muscularis mucosae is present as an inner circular and out longitudinal layer of smooth muscle fibers
Some fibers enter the villus and extend to the tip, during digestion the fibers contract causing shortening of the villus, improtant to emptying the lacteal
immunological protection of the mucosa
the gut lumen is exposed to an immense variety of microbial flora that are potentially harmful
The lamina propria maintains an extensive immune defense system in the form of lymphoid tissue- Gut associated lymphoid tissue (GALT)
this exists as individual lymphocytes within the mucosa and eithelium as well as dense aggregates called lymphoid nodules- in some regions multiple lymphoid nodules coaelesce to form Peyers Patches
30-40 permanent peyers patches are found throughout the ileum with some more in the jejunum, nodules are least numerous in the duodenum
submucosa
Brunners glands and meissners plexus
the submucosa of the small intestine is a moderately dense connective tissue with small numbers of adipose cells
The duodenal submucosa differs from the rest of the GI due to the presence of large numbers of glands called brunners glands
Mucous acini like, and ascend thru the muscularis mucosae layer and enter the crypts to deposit their secretions into the lumen
Secretion- alkaline mucus that neutralizes the pH of the gastric chyme and protects the duodenum, they also contain epidermal growth factor that may allow stem cell proliferation in the crypts, the small intestine secretes 2 liters of fluid/day, mainly under neural control from the submucosal (Meissner’s plexus and hormones by enteroendocrine cells
muscularis externa is required to move chyme thru the intestine
The muscularis externa is composed of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle
Between the smooth muscle layers lies the myenteric plexus (Auerbachs)- part of the enteric nervous system, the myenteric plexis provides motor innervation to the muscularis externa and secretomotor innervation to the mucosa, the enteric NS is a part of the nervous system which directly controls the GI system
Two types of movement are associated with the small intestine: mixing contraction are localized and and expse chyme to digestive juices
Propulsive contractions are peristaltic waves of muscle movement that transports the chym along the intestine at about 2 cm/min
Control of peristaltic movement is mediated by the myenteric plexus in response to gastric distension
absorption
The main function of the small intestine are to complete digestion and absorb the products into the bloodstream and lymphatic system- 7 L of fluid, 30 g of na, .5 kg of carbs and protein, and 1 kg of fat are absorbed by the intestine daily
Luminal digestion of most foodstuffs results in individual molecules that can be absorbed by the enterocytes
the secretions of the liver and pancreas containing digestive enzymes and emulsifiers are deposited into the duodenum and are central to complete the digestive process, enterocytes also generate enzymes that are incorporated into the brush border that digest carbohydrate and polypeptides to monomeric units
large intestine
1.5 m long, composed of the cecum, appendix, colon (ascending, transverse, descending, and sigmoid), rectum and anus
The large intestine abssorbs most of the water from chyme and compacts the remaining material into feces for excretion
Cecum and colon are histologiccally indistinguishable, the ilium enters enters the medial side of cecal-colonic boundary and is closed by the ileocecal valve that prevents reflux of cecal/colonic content back into the ileum
The colon is not folded into plicae circularis and villi are absent from the mucosa giving it a relatively smooth surface
The colonic epithelium consist of cells similar to the small intestine without Paneth cells
large intestine crypts
although villi are absent the colonic mucosa is full of crypts of lieberkhun that are made up of cell types similar to those of the small intestine
The crypts in the colon are longer than in the small intestine and are highly glandular with an abundance of goblet cells, the crypts open into the lumen at the surface of the colon and can be seen as a regular array of holes across the mucosa
As with the small intestine the mucosal cells are born from stem cells at the base of the crypts and migrate up the walls of the crypts until they are sloughed off, colonic mucosa is replaced every week
The lamina propria is similar to the small intestine but the muscalaris mucosae is better developed with clear circular and longitudinal fiber, the muscularis externa outer longitudinal muscle are arranged in 3 distinct bands called taenia coli that remain partially contracted andcasue the large intestine to be puckered into sacculations– Haustra
