Digestive Microanatomy

Oral cavity/salivary microanatomy

 Layers of GI tube:
- mucosa consisting of epithelium, lamina propria and muscularis mucosae
- submucosa
- muscularis externa
- adventitia (in thoracic cavity) or serosa (in abdominal cavity)
 Oral cavity: the mouth, covered with keratinized stratified squamous epithelium. Salivary glands exists outside the tube empty their secretion into it.
 Salivary gland: innervated by the autonomic nervous system (parasympathetic)
- parotid: located on each side of the mandible near the ear. This gland secretes only watery serous secretion rich with protein and enzyme through the parotid duct into the mouth
- submandibular: located at the back of the mandible and its secretion is both serous and mucous (viscous and less fluid) through the submandibular duct
- sublinguinal: located underneath the tongue and secrete both serous and mucous secretion into the sublinguinal duct.
 Serous acinus:
- triangular cells surrounding a small lumen
- large nucleus and lots of rough ER for active protein secretion
 Mucous acinus:
- foamy cytoplasmic cells filled with mucus containing vesicles
- flattened nucleus around the edge
 Mixed acinus:
- contains both serous and mucous cells
- serous cells arranged in an half moon shape (demi-lune) externally to the mucus acinus
 Myoepithelial cells: flat thin cells that surround acini and muscle cells and contracts to help squeeze and transport the secretion out. Occur in ducts too.
 Intercalated duct: surrounded by cuboid cells and conduct the secretion from the acini
 Striated ducts: cuboidal to columnar epithelial lined duct that is continuous from intercalated duct. Here the cells have an conspicuous folded basal membrane filled with mitochondria to supply energy for active transport of ion and water from lumen to interstitial fluid (used to modify/concentrate the secretion)
 Interlobular ducts: ducts that run between lobule of the secretory gland and is lined with stratified cuboidal epithelium. It is formed from the branches of striated duct.
 Oesophagus special structure:
- epithelium: thick sacrificial stratified squamous epithelium of 6 – 8 layers. In human it is non-keratinized. The base of the stratified epithelium contains undifferentiated generative cells for replacement
- muscularis externa: inner circular and outer longitudinal muscle. Top third of oesophagus, muscles are striated skeletal, middle third is a mixed of skeletal and smooth and lower third is all smooth muscle
- glands: mucus gland in the mucosal and submucosal layer of lower oesophagus secrete mucus to aid food movement and protect epithelium from acid reflex from stomach.
- Covering: covered with adventitia (loose connective tissue )
- Sphincters: the physiologically based UES and LES controlled by nerves and no thickening of muscle (hence unable to be identified morphologically during relaxation)

Stomach

 Parts of stomach:
- cardia: the junction orifice of the transition from esophagus to stomach. Contains the LES and mostly mucus glands and few parietal
- fundus: the top (roof) of the stomach with similar cell type to the body
- body: the major part of the stomach for mixing and contains parietal and chief cells mainly
- pylorus: region of the stomach that connects to the duodenum and contains mostly mucus and endocrine hormones. Separated into two parts: the antrum and canal
 Wall of the stomach:
- rugae: folds of the stomach wall made up of mucosa and submucosa. This are not permanent as when the stomach is full, the folds disappear but appear when it’s empty, so to adapt to the amount of food.
- Muscularis externa: has an additional layer of oblique muscle inner to that of the circular muscle layer, hence allowing tri-directional contraction
- Sphincter: LES to control entry of food into stomach and prevent acid reflux. Pyloric sphincter consists of thickening of the circular muscle to control exit of material and is innervated by autonomic nerves (hormone and stimuli sensitive)
 Gastric pit: small sunken glands in the stomach wall made up of 4 -5 branches that extend into the lamina propria. Lined by:
- surface mucus cells which is replaced every 5-6 days
- neck mucus cells
- parietal cells
- chief cells
- enterendocrine cell
- undifferentiated cells
 Surface mucus cells: simple columnar cells that coats the surface of the stomach and pits containing an elongated basal nucleus and globules of mucus in the cytoplasm. Mucus secreted are insoluble, alkaline for protection against acid. Too much acid from aspirin, alcohol, or acid can cause ulceration
 Mucus neck cells: cells at the neck of the gastric pit (below and at point of branching) with an expanded apex and basal nucleus. Contains globules of soluble acidic mucus that is secreted when food is present.
 Parietal cells: oxyntic cells interspersed within the surface layer of the deeper gastric pit, with a large nucleus (resembles fried egg). The membrane dips down forming convoluted canaliculi. The membrane system becomes more extensive when stimulated to increase surfaces area for HCl secretion, i.e. increase H+ and Cl- pumps on the surface.
- The cytoplasm of the cell is neutral but extracellular pH is 0.8 hence contains a lot of mitochondria for active pumping.
- Also produces the intrinsic factor and help B12 absorption and bicarbonate into bloodstream.
 Chief cells: cells located at the base of the gastric base containing zymogen granules. These store the inactive precursor of pepsin (main protein secreted in stomach), pepsinogen. Upon secretion pepsinogen is activated to pepsin only in acidic environment.
 Enteroendocrine cells: hormone secreting cells located deep within the gastric pits containing many secretory granules. Hormones are released through basal lamina into the capillaries of lamina propria. Examples include APUD (amine precursor uptake and decarboxylation), argentaffin, chromaffin cells.

Small and Large Intestine Microanatomy

 Distribution of gastrointestinal endocrine cells:
- Parietal cells: producing gastrin located at the pyloric antrum in major amounts and decreases in number to duodenum to jejunum
- Pancreas HCO3-: producing secretin and distributed from duodenum to ileum in decreasing levels
- Pancreas zymogen and gall bladder: producing cholecystokinin and distributed from duodenum to ileum in decreasing amounts
- Liver glycolysis: producing glucagon and spreads from fundus to colon in increasing amounts
- Inhibitory hormone release: producing somatostatin and located entirely way from fundus to colon
- Smooth muscle stimulant: producing serotonin and distributed from fundus to colon in equal amounts
 Hormone secretion: enteroendocrine cell located in the mucosal layer at the base of the cells. Hormone is liberated through the basal lamina into capillaries of lamina propria
 Small intestinal lining: folding of the intestinal wall prolong contact between food and enzyme and epithelial surface
- Enterocytes: columnar epithelials with tight junctions in between to prevent material from going between the cell)
- Goblet cell
 Small intestinal folding:
- Plicae: circular folding including mucosa and submucosa (provides 3 times the surface area)
- Villi: finger projection into the lumen and folding of lamina propria with no submucosa (provides 10 times the surface area)
- Microvilli: folds of the membrane of the epithelial cells (provide 20 times the surface area)
 Brunner’s glands: acini cells located in the submucosa that may secrete bicarbonate to protect against acid. These are only found in duodenum
 Crypt of Lieberkuhn: pits between villi that act as glands. Paneth cells are found at the bottom of the gland with granules to secrete lysozymes.
 Content of the villi:
- Smooth muscles cells in the lamina propria of villi contract rhythmically to stir content of the lumen. Muscle fibers runs up the core of connective tissue from muscularis mucosa
- Microvilli move using actin within their center to increase local mixing at surface
- Lacteals runs up the villus with a blind end and transports lymphatics
- Lymph tissue and nodules can break down to submucosa. In the ileum, they form very extensive areas called Peyer’s patches
- Blood vessels that carry away nutrients absorbed
 Over the surface of the nodules, villi are absent. M cells covers here and there is ample antigen.
 Large intestine lining: mucosa is smooth with no plicae and villi and crypts of Lieberkuhn are numerous
- columnar absorptive cells: similar to enterocytes of the small intestine and use used to absorb water and electrolytes
- goblet cells: secrete mucus for lubrication
 Features of large intestine:
- Epithelial replacement every 6 days and 4 weeks for enteroendocrine cells. If cell division is prevented, GI upset can occur. Response to hormonal to divide and differentiate.
- Large amount of bacteria in the lumen
 Content of lamina propria of large intestine:
- Dense layers of collagen immediately beneath surface of epithelium
- Numerous lymphatic nodule (GALT – gut-associated lymphatic tissue) extending down into the submucosa but nodes does not start draining until the muscularis mucosae. This explain why colonic cancer metastasis is slow
 Longitudinal muscle: longitudinal muscles of the large intestine form three bands of teniae coli
 Renewal cells of the gastrointestinal tube:
- Esophagus: located at the basal layers and divides into upper cells which is replaced 2-3 days
- Stomach: differentiate up the neck of the gland upwards to the line the cell (last several days) while at the base of the gland, differentiate down to form parietal and chief cells (last months).
- Small intestine: located in the lower part of the crypts and develop into enterocytes and goblet cells (last only several days) and paneth cells (last for a month)

Liver, Gallbladder and Pancreas Microanatomy

 Organization of blood with hepatocytes: blood derived from gastric and intestinal capillary passes into the hepatic portal system and into the liver in which composition of blood is modified. Thus the structural organization of the liver hepatocytes must allow maximum close contact with blood.
 Exocrine pancreas: a tubular-acinar gland with highly branched ducts
- Acini: an acinus consist of 5-8 pyramid shaped surrounding a tiny lumen with no myoepithelial cell
- Acinar cells: the cells have large spherical nucleus in the basal position and mitochondria oriented perpendicular to the basal surface. Protein secretory products are synthesized in the basal rER and packaged and accumulated as zymogen granules in the apical cytoplasm.
- Duct: begins with the smallest intercalated ducts inserted into the acini lined with centro-acinar cells. The intercalated ducts lead into intralobular then interlobular ducts. The duct cells produce alkaline fluid to neutralize acid.
 Blood delivery to the liver:
- Portal vein: provide 80% of the blood to liver that is nutrient rich but deoxygenated
- Hepatic artery: provides 20% of the blood that is oxygenated
 Lobule organization: hexagonal shape centered by a central vein and portal space distributed around the corners of the lobules. Each portal space contains the portal triad but hepatic artery, portal vein and bile duct.
- Vessels distributed around the edge of the lobule branches off and perforates the lobule to supply the hepatocytes
 Functionality of lobule: blood flows from the portal space to central so cells closer to the portal space will receive most nutrients and oxygen and central cells receives more toxins
 Vein supply: portal vein branches forming distributing vein that circulates the periphery of the lobule with further branching of venule inlet penetrating into the lobule itself
- Central vein: thin wall consist of only endothelium and connective tissue. Receives converging blood from the sinusoids
- Central vein leads on to sub-lobular vein and then hepatic vein and finally IVC
 Sinusoids: large capillary that goes between spaces of the hepatocytes and transmit blood from portal space to central vein. Hepatocytes arranged in singular rows hence exposed to sinusoid on either sides.
 Organization around hepatocytes: hepatocytes are lined with thin layer of endothelium before contacting with sinusoid. The space between hepatocytes and endothelium is called the space of Disse.
- Hepatocytes are polyploidy with more than one nucleus with suggest rapid regenerative ability
- Microvilli of the hepatocyte maximize nutrient uptake in the space of Disse
- Bile canaliculi running between hepatocytes transfer bile into the bile ducts
- Endothelial fenestration filters red blood cell out

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