Case 11- Microanatomy Flashcards
The layers of the GI tract from the outside in
- Advenitia/serosa
- Submucosa
- Muscularis externa (longitudinal)
- Submucosa and the Myocentric plexus
- Muscularis externa (circular)
- Submucosa and submucosal plexus
- Mucosa= Muscularis mucosae, the Lamina propria and then the Epithelium
- The Lumen
GI tract layers- The Mucosa
The epithelium varies with region and function. The lamina propria contains glands, lymphatics and the capillary plexus. The Muscularis mucosa is made of smooth muscle and agitates the food.
GI tract layers- The Submucosa
Contains glands and the submucosal (Meissner’s) nerve plexus which innervates the secretory glands. Contains the Lymphatics.
GI tract layers- The Muscularis externa
Has a functional role in peristalsis. Made up of inner circular muscles and outer longitudinal muscles. Contains the Myenteric plexus which innervates the muscles, this is below the circular muscles.
GI tract layers- Adventitia / Serosa
The outermost layer of the GI tract. The adventitia is made of loose connective tissue and allows for binding, you get adventitia within the Oesophagus and colon. The Serosa is a Mesothelial layer which allows for movement. It is found in the stomach and small intestine.
The Oesophagus epithelium
Adapted to deal with friction. It is stratified squamous epithelium which is not keratinised. It gets sloughed off by food.
Types of Oesophagus muscle
- Upper 1/3= striated skeletal muscle (voluntary movement)
- Middle 1/3= transition
- Lower 1/3= smooth muscle (involuntary)
Adaption of stomach wall
1) The stomach has an additional layer of muscularis externa smooth muscle for churning (oblique), this lies just below the muscularis externa (circular).
2) Gastric pits- these are formed from simple columnar mucosal epithelium. Gastric glands in the lamina propria secrete into the pits.
3) No goblet cells or brush border cells.
How the epithelium/mucosa varies in the different regions of the stomach
- Cardiac regions: longer pits and small glands
- Body/fundus: short pits and long, branched tubular glands
- Pyloric region: large pits with mucus producing glands only
Adaptions of the small intestine
1) Contains villi for absorption. The villi are short and finger like in the ileum and tall and leaf shaped in the duodenum.
2) Microvilli make up the brush border cells.
3) The small intestine contains intestinal glands called the crytpts of Kierberkuhn.
4) Contains GALT (gut associated lymphoid tissue) which works as part of the immune system to protect the body from invasion in the gut. The GALT is arranged as a Peyers patch.
Adaptions of the epithelium of the small intestine
1) Simple columnar epithelium with brush border and goblet cells.
2) The epithelium (columnar absorptive cells) have microvilli which contain actin bundles and are coated with glycocalyx.
3) These cells secrete enzymes into the glycocalyx for terminal digestion i.e. dipiptidases.
4) Within the absorptive cells there is a junctional complex at the apex which contain zonula adherens, zonula occludens, demosome and gap junctions.
Cell types in the small intestine
- Paneth cells: Lysozyme and Defensins, they have anti-microbial properties and are used in the hosts immune system
- Enteroendocrine cells: APUD cells (E) and Argentaffin cells, they have endocrine functions. APUD cells synthesis, store and secrete catecholamines in response to sympathetic stimulation. Argentaffin is any hormone secreting cell in the Pancreas.
Large intestine adaptions
No villi or enzymes. Contains lymphoid tissue within the lamina propria. Contains intestinal glands and is made of simple columnar epithelium, it is absorptive with brush border cells due to the microvilli. There are a large number of goblet/mucous cells which secrete large volumes of mucin. The mucin is diluted in water to form mucus. It aids the passage of faeces.
Exocrine glands in the GI tract
Sublingual gland, Submandibular gland, Parotid gland and the Pancreas
Multicellular exocrine glands- Tubular glands
Can lie along ducts. You can have simple tubular glands (intestinal and sweat gland) and compound tubular glands (gastric glands).
Multicellular exocrine glands- Acinar glands
Cells within a sac at the end of the duct. You can have simple acinar glands (sebaceous) and compound acinar glands (pancreas and parotid). The compound acinar glands are a branched duct system.
Multicellular exocrine glands-Compound tubular
A type of acinar gland, for example the submandibular gland which releases saliva.
Embryology- Endoderm specification and internalisation
1) During gastrulation the endoderm layer forms from the epiblast.
2) The endoderm primarily forms the GI tract. (12-16 days).
3) At 16 days the gut tube forms, the endoderm is continuous with the yolk sac.
4) At 18 days internalisation begins and continues alongside embryonic folding.
5) The endoderm folds over its self in order to form a tube, the foregut and hindgut form first then the midgut. It goes from the outside in
Embryology- Establishment and differentiation
1) Embryo folding and complete internalisation of the gut tube (week 4).
2) Between weeks 4 and 5 the gut tube differentiates into the foregut, midgut and hindgut along the cranial caudal axis of the embryo.
3) The straight gut tube is established during week 5 and differentiation of the foregut, midgut and hindgut is complete.
4) The yolk sac shrivels up and forms the vitelline duct which is connected to the midgut. The vitelline duct is obliterated by week 8 and replaced with the umbilical tube.
Embryology- Growth, rotation and herniation
1) Growth, rotation and herniation- weeks 5-8
2) The foregut rotates 90 degrees clockwise, this causes the stomach to form and rotate. Creates lesser sac and foregut loop.
3) The growth of the liver causes the midgut to herniate outside of the embryo.
4) The midgut rotates 90 degrees anticlockwise around the superior mesenteric artery to create a midgut loop.
5) Between weeks 5-8 the arterial supply, Peritoneum and organs develop.
Embryology- Retraction, growth and further rotation
1) At 10 weeks the gut tube has the morphology of an adult gut tube.
2) The midgut retracts into the abdominal cavity of the embryo and the midgut rotates a further 180 degrees anticlockwise.
3) The foregut and midgut rotate in opposite directions
4) The gut tube fuses to the posterior abdominal wall at 5 weeks
Meckel’s (ileal) diverticulum
Caused by failure of the viteline duct to obliterate. The diverticulum forms from the tissue that remains. They are about 2 inches in size and originate 2 feet from the ileocaecal junction. Double the number of men then females get it.
Omphalocoele and umbilical hernia
An omphalocoele hernia is when the gut persists outside of the abdominal cavity. It is the persistent herniation of abdominal contents into the proximal umbilical cord. This is due to a failure in retracting the gut. The same happens in an umbilical hernia but the gut is covered in skin. An umbilical hernia develops after 10 weeks of development so is post midgut retraction. This is most likely due to a weakness in the abdominal wall
Gastrointestinal atresia and stenosis
Recanalization is when vacuoles are formed in order to create a larger lumen. Stenosis (incomplete occlusion) is due to insufficient recanalization. Atresia (complete occlusion) is due to a complete lack of recanalization. Can occur anywhere along the GI tract but is the most common in the duodenum
Complications of gastrointestinal atresia and stenosis
Can cause polyhydramnios in a pregnant women as the foetus is unable to swallow the excess amniotic fluid. They baby can regurgitate and drool once they are born
Oesophageal stenosis
Incomplete recanalization and vascularisation in the oesophagus. This can lead to Oesophageal atresia which is displacement of the tracheoesophageal septum.
Hypertrophic pyloric stenosis
Due to abnormal thickening of the pylorus wall. This constricts the pyloric canal restricting the passage of food. The stomach becomes distended and causes projectile vomiting. There won’t be any bile in the vomit. Stenosis occurs proximal to the bile duct so doesn’t effect bile production.
Duodenal atresia and stenosis
It occurs at the level of the bile and pancreatic duct junction (major duodenal papilla). It is the point of transition between foregut and midgut. Causes vomiting, the vomit contains bile when the blockage is distal to the bile duct. Polyhydramnios occurs.
Malrotation, non-rotation and volvulus
Malrotation is incomplete rotation of the midgut. Non-rotation is when the midgut does not rotate when retracting into the abdomen. Midgut volvulus is twisting of the small intestine, this may obstruct the SMA (superior mesenteric artery).
Congenital Liver abnormalities
In 5% of the population you have accessory hepatic ducts where ducts go from the right lobe of the liver to the gallbladder. You can also have extrahepatic biliary atresia where the bile duct is obliterated, this causes jaundice.
Annular pancreas
The ventral bud of the pancreas splits and forms a ring around the duodenum instead of binding to the dorsal pancreatic duct
Saliva secreted by the Parotid gland
Serous
25% of total secretions
Saliva secreted by the Sublingual gland
Mucus
5% of total secretions
Saliva secreted by the Submandibular gland
Serous/mucus
70% of total secretions
Function of Saliva- Lubrication
Mastication, swallowing and facilitating speech. This is the function of the fluid, mucus and proline rich proteins in the mucus
Function of Saliva- Digestion
Alpha-amylase initiates starch digestion. Lingual lipase initiates lipid digestion. R-protein (haptocorrin) absorbs vitamin B12.
Function of Saliva- Solubilisation
For taste sensation
Function of Saliva- Moistness
For thirst sensation
Function of Saliva- Oral and dental health protection
Lysozyme, thiocyanate, immunoglobulins, proline-rich proteins, mucus (binds pathogens), fluid and HCO3- (washes and neutralises food) provide immunological protection. The mucosa provides mechanical protection.
Structure of salivary glands
Made of secretory acini and ducts
Salivary gland- Acini
The acini can either be serous, mucous or a mixture of serous and mucous. A serous acinus secretes proteins in an isotonic watery fluid. A mucous acinus secretes mucin which is a lubricant.
The two stage process of salivary water and electrolyte secretion
- Primary secretion in the Acini= the acini secrete isotonic primary saliva into the ‘terminal lumen’ of the salivary ductal system.
- Secondary modification in the duct= the primary saliva travels along the ducts where its ionic composition is modified by duct cells to produce secondary saliva which is secreted into the oral cavity.
Primary saliva secretion in the Acini
- Parasympathetic activation causes ACh to bind to Muscarinic acetycholine receptors (M3) on Acini cells. Ca+2 is released from the ER
- Increased Ca+2 activates and opens the Ca+2 dependent Cl- channels on the apical membrane.
- Cl- flows out of the cell, down its electrochemical potential gradient into the luminal space depolarising the cell.
- Simultaneous opening of the Ca+2 dependent K+ channels on the basolateral membrane maintains the membrane potential. The membrane potential allows for the continued flow of Cl- out of the cell.
- Na+ follows Cl- into the luminal space most likely through an extracellular pathway. Water then follows via osmosis. Results in primary saliva which has an isotonic, plasma like (equal amounts of Na+ and Cl-) composition.
Secondary saliva production
1) Regulated by duct cells producing a hypotonic solution.
2) NaCl is reabsorbed from the saliva (lumen) back into the interstitial space.
3) KHCO3 is moved from the interstitial space and into the saliva (lumen).
4) Sodium ions get actively reabsorbed whilst chloride ions are passively reabsorbed.
5) Potassium is actively secreted into the saliva whilst bicarbonate is passively secreted.
6) Duct cells are impermeable to water
Saliva control- Parasympathetic signalling
Cranial nerve 7 innervates sublingual and submandibular salivary glands. Cranial nerve 9 innervates parotid salivary glands.
Saliva control- PNS acetylcholine neurotransmitter
Acetylcholine will activate the muscarinic receptor M3, this will then activate protein kinase C (PKC) which stimulates an increase in intracellular calcium levels. The rise in calcium causes an increase in primary salvia production by acinar cells.
Saliva control- PNS: VIP neurotransmitter
VIP will bind and activate VPAC receptors. This leads to an increase in intracellular calcium levels and therefore an increase in primary salvia production and protein secretion. Although the exact molecular mechanism of action is not fully understood it is thought to be mediated by PKC.
Saliva control- Net effect of PNS
Increased Parasympathetic stimulation results in an increased flow of saliva that is more watery in composition
Saliva control- Sympathetic stimulation
1) When noradrenaline binds to alpha 1 adrenergic receptors there is an increase in saliva flow.
2) When noradrenaline binds to alpha 2 adrenergic receptors there is a decrease in saliva flow.
3) Beta adrenergic signalling is primarily responsible for the control of protein secretion by salivary cells with activation leading to an increase in protein secretion and amylase output.
4) Sympathetic nervous system stimulation causes a reduction in saliva volume with an increase in salivary protein composition.
Cephalic phase response (CRP’s)
Stimulation of unconditioned and conditioned neuronal responses serve to activate the parasympathetic nervous system, stimulating saliva production.
Simple (unconditioned) CRP’s
Chemo/pressure receptors in the mouth are activated by the presence of food (or other stimulus). The impulse is sent via afferent nerves to the salivary centre in the medulla. The impulses are then sent via extrinsic autonomic nerves, then through both sympathetic and parasympathetic stimulation. Salivary glands increase production.
Acquired (unconditioned) CRP’s
When you think about, smell, see or prepare an appetising dish signals are sent to the cerebral cortex. Impulses are then sent to the to the salivary centre in the medulla. The impulses are then sent via extrinsic autonomic nerves, this through both sympathetic and parasympathetic stimulation. Salivary glands increase production.
Sphincters
A circular band of muscle that separates organs from each other. Allows gradual release of substances between different organs
Sphincter roles
1) Relaxation of the upper and then the lower sphincters in the oesophagus so that the food can smoothly pass through the oesophagus to get to the stomach
2) Contraction of the lower oesophagus sphincter prevents stomach acid entering the oesophagus in acidic reflux
3) Choose when we defecate, social and hygiene role
