Gastrointestinal Flashcards
How doe diarrhoea kill children?
Fluid and electrolyte imbalance - immediate
Malnutrition - delayed
What is the association between diarrhoea and malnutrition?
Increased energy loss - Diarrhoea and vomiting - Increased metabolic needs Reduced energy intake - Malabsorption - Withholding of food
How do the proportions of infective causes of diarrhoea vary between developing and developed countries?
More bacterial and parasitic diarrhoea in developing countries
More viral diarrhoea in developed countries
What is dysentery and what is it caused by?
Presence of blood, pus, and mucus in faeces
Caused by
- Shigella = enteroinvasive E coli (EIEC)
- Amoeba
- Non-infectious sources
What are the aetiological agents of foodborne diarrhoea (food poisoning)?
Staphylococcus aureus - pre-formed toxin ingested Salmonella Clostridium perfringens Bacillus Vibrio cholerae Listeria - associated with soft cheese Viruses - Rotavirus - Norovirus Ciguatoxin
What organism causes antibiotic-associated colitis?
Clostridium difficile
How does haemorrhagic colitis differ from dysentery? What is it caused by?
Blood present, but no pus in faeces
Caused by enterohaemorrhagic E coli (EHEC)
What is the cause of cholera-like diarrhoea which is not caused by V cholerae?
Enterotoxic E coli (ETEC)
What are the types of diarrhoea producing E coli, and what diseases do they produce?
ETEC = watery diarrhoea
- Colony factor Ags allow for adhesion to brush border
- Produces enterotoxins
Enteropathogenic E coli (EPEC) = non-specific gastroenteritis
- Adhesins: intimin, Bfp
- Produces T3S effectors
EHEC = blood diarrhoea = haemorrhagic colitis
- Causes haemolytic uraemic syndrome (HUS)
- Evolved in EPEC
- Adhesins: intimin, Efa
- Produces Shiga toxins
EIEC = dysentery
- Doesn’t cause HUS because doesn’t produce Shiga toxin
- Adhesin: IpaC
- Produces Sen toxin
Enteroaggregative E coli (EAEC) = watery diarrhoea
- Adhesin: AAF
- Produces Pet and EAST
Which diarrhoea-causing bacteria are adhesive enterotoxigenic?
Cholera
ETEC
Which diarrhoea-causing bacteria are adhesive with brush border damage?
EPEC
Which diarrhoea-causing bacteria have invasion restricted to the mucosa?
Shigella
Which diarrhoea-causing bacteria invade the submucosa?
Salmonella
Campylobacter
Which diarrrhoea-causing bacteria systemically invade?
Salmonella
Why can Salmonella typhi cause enteric fever?
Can survive in macrophages, especially in immunocompromised people
What are the virulence determinants of diarrhoea causing agents?
Adhesins
Invasive ability
Exotoxins
Ability to resist killing
How is laboratory diagnosis of diarrhoea causing agents made?
Macroscopic appearance
Miscroscopy
Culture for bacteria only
Ag detection used mainly for viruses and parasites
Detection of nucleic acid for viruses, bacteria, and protozoa
What is the treatment of diarrhoea?
Replace fluid and electrolytes Reduce fluid loss - Anti-diarrhoeals - Anti-motility agents - Anti-secretory agents - Binding agents - Antibiotics
What are the common structural features of the gastrointestinal tract (GIT)?
Mucosa
Submucosa
Muscularis externa
Serosa/adventitia
Describe the subcomponents of the mucosa
Mucosa = innermost layer Sensing and responding to contents Epithelium - Columnar enterocytes/stratified squamous epithelium - May also include endocrine cells - Renewed every 5-6 days Lamina propria - loose connective tissue containing - Nerves - Blood vessels - Immune cells Muscularis mucosae - Thin layer of smooth muscle - Forms boundary of mucosa - Facilitates mixing Most diverse layer of GIT Varies from region to region
Describe the subcomponents of the submucosa
Dense irregular connective tissue - gives structural strength and elasticity Contains - nerves - Ganglia - Blood vessels Sometimes contains - Glands - Immune cells
Describe the subcomponents of the muscularis externa
Responsible for gut movement Usually has 2 layers of smooth muscle - except for stomach - Inner circular - Myenteric plexus between 2 layers - Outer longitudinal
What is the difference between serosa and adventitia?
Serosa secretes fluid from simple squamous epithelium > allows organ movement
- Contains thin layer of connective tissue
Adventitia = connective tissue joining GIT with surrounding structures
- Present in parts of oesophagus and rectum
- Doesn’t secrete fluid
What does the enteric nervous system regulate?
Absorption and secretion regulated by submucosa ganglia
Smooth muscle activity regulated by myenteric ganglia
Describe the histology of the oesophagus
Non-keratinising squamous epithelium Submucosal glands - Secrete mucus to lubricate movement 2 muscle types - Top = striated - Bottom = smooth
Describe the structure of the gastric glands
Vary between cardia, corpus, and pylorus Mucous cells - Secrete mucus - In isthmus and neck Parietal cells - Secrete HCl - In isthmus, neck, and base Chief cells - Secrete pepsinogen - In base Enteroendocrine cells - Secrete gastrin - In neck and base
Describe the histology of the small intestine
Villi = finger-like extensions of mucosa
- Covered by simple columnar epithelium
- Each epithelial cell has microvilli
Very folded surface = plicae circulares - maximise surface area for absorption
What are the absorptive and secretory zones of the small intestine?
At level of villus, tubular glands > secrete fluid and mucus into lumen between villi
Glands = crypts of Lieberkuhn
Absorption through microvilli on enterocyte surface of villi
What are the epithelial cells of the small intestine?
Enterocytes = fluid transport and absorption Goblet cells = mucus secretion Enteroendocrine cells = hormones Paneth cells = secrete anti-microbial peptides - deep in glands Stem cells = renew epithelium
What are the distinctive features of the duodenum?
Brunner’s glands in submucosa
- Release alkaline mucus to inactivate stomach enzymes and neutralise acid from stomach
Low plicae circulares
Long villi
What are the distinctive features of the ileum?
Peyer’s patches = massive lymphoid follicles
- Very basophilic
Short villi
More goblet cells
What is distinctive about the large intestine’s muscularis externa?
3 bundles of longitudinal muscle form taeniae coli
How do the large intestinal glands differ from those of the small intestine?
No villi
Straight tubular glands, compared to coiled Brunner’s glands
Columnar epithelium
Why do the large intestine and rectum contain more goblet cells?
Lubrication for increasingly solid material in lumen
How do the types of epithelium in the large intestine differ from the types in the small intestine?
In large intestine
- Paneth chells rare
- Higher proportion of goblet cells
What are the key regulatory functions of the GIT?
Control contractions of intestinal smooth muscle
Regulate secretion of digestive enzymes and solvents needed for their proper function
Control reabsorption of water from lumen to prevent dehydration
Coordinate widely separated regions to ensure proper function
What are the local control systems of the gut?
Enteric nervous system - Controls contractile activity - Controls secretion of water and salt - Control over mm-cm = local action Interstitial cells of Cajal - Pacemaker cells > produce rhythmic activity in muscle - Nervous system operates on top of this
Describe the endocrine control system of the gut
Signals from intestinal mucosa to ancillary organs like
- Brain
- Pancreas
- Gall bladder
Essential for secretion of enzymes and solvents into intestinal lumen, and regulating appetite
What do enterochromaffin cells release?
Serotonin
What are the mucosal enteroendocrine cells?
Enterochromaffin cells CCK-secreting cells Secretin-secreting cells Somatostatin-secreting cells GLP-secreting cells ECL cells release histamine
What are the reflex pathways involved in the GIT?
Vago-vagal reflex pathways coordinate movements in upper GIT
- Control of swallowing
- Regulates acid secretion in stomach
- Coordinates contractions of stomach and duodenum
Instestino-intestinal reflexes
- Some mediated by vagus
- Others via dorsal root ganglia and spinal cord
- Viscerofugal neurons have cell bodies in gut wall and project out to pre-vertebral sympathetic ganglia > reflex inhibition of enteric nervous system
CNS control relating to anticipation, mood, and activity
Describe the cephalic phase of digestion
Triggered by sight, smell, and taste of food
Causes
- Salivation
- Gastric acid and pepsin secretion
- Relaxation of gastric corpus and fundus
Operates via vagus nerve
Accounts for 30% of acid secretion in stomach
Sets up stomach to store food and begins barrier function of GIT
Describe the general control of acid and pepsin secretion
Acetylcholine from enteric neurons excited by vagal efferent neurons stimulates
- ECL cells to release histamine > histamine stimulates parietal cells
- Parietal cells to release H
- G cells to produce gastrin
Acetylcholine inhibits
- D cells from producing somatostain
How is peristalsis controlled in the oesophagus? How does this differ to the rest of the GIT?
Entirely under neural control via vagus nerve
As opposed to rest of GIT > under enteric nervous system control
Secondary peristalsis can be activated in oesophagus if vagus nerve fails
What happens when chewed food enters the stomach?
Fundus and corpus of stomach relax to accommodate volume of food swallowed > distension activates enteric and vago-vagal reflexes > more acid and pepsin secretion
Due to interstitial cells of Cajal: large pacemaker potentials > strong constrictions > propagate from corpus to pylorus > drive food towards pyloric sphincter
Food enters antrum > reflex inhibition of acid secretion in corpus > continued constriction driving food towards closed sphincter > acid, protease, and mechanical activity separate components of food > liberates fat - floats to top of stomach
What happens to food in the antrum of the stomach?
Continual grinding up in presence of protease and water > food becomes dilute paste
Peptide bonds broken
Amylase breaks carbohydrates up
Pylorus opens briefly by relaxing pyloric sphincter due to enteric nervous activity > squirt of food mixed with acid and pepsin enters duodenum
What is the response to acid in the duodenum?
Activates D cells in mucosa of duodenum to release somatostatin
- Enters portal circulation to reach stomach
Excites terminals of vagal afferent neurons > triggers vago-vagal reflex > Brunner’s glands release mucus and bicarbonate
Vago-vagal reflex inhibits gastric emptying
Duodenal-pyloro-antral reflex closes pylorus > inhibits gastric emptying
What does the presence of fatty acids and amino acids in the duodenum activate? What is the consequence of this?
Release of CCK from I cells
Leads to
- Excitation of terminals of vagal afferent neurons > activates many gastric reflexes > inhibits appetite
- Excitation of intrinsic neurons > stimulates mixing behaviours of bolus of food and enzymes
- Release of digestive enzymes from pancreas
- Contraction of gallbladder > forces bile down common bile duct > duodenum
Amino acids trigger release of secretin from S cells
What is the role of secretin in the duodenum?
Triggers secretion of bicarbonate-rich solution from pancreas > neutralises acid in duodenal lumen
What does neutralising acid in the duodenum do?
Inactivates pepsin
Stops somatostatin secretion from duodenal D cells
Removes inhibition of gastric emptying by somatostatin-stimulated reflexes
Describe sensing in the intestinal mucosa
Sensory information from mechanical stimuli
- Distension stretches mechanoreceptors > activates vagal afferents
- Mucosal deformation
Sensory information from chemical stimuli acting via enterochromaffin and enteroendocrine cells
- Nutrients and other stimuli act on apical surface receptors > mediator release from basolateral surface
How do mucosal cells “taste” the lumen?
Enterochromaffin and enteroendocrine cells in intestinal crypts express same taste receptors as those on tongue for bitter and savoury flavours
Tastants cause release of serotonin
L cells express components of sweet taste receptors
What hormones do L cells release? What are their functions?
Release in duodenum and jejunum - GLP-1 - GLP-2 - PYY GLP-1 and GLP-2 = carbohydrate absorption and insulin resistance PYY = appetite regulation
What are the types of motor patterns of the GIT?
Retropulsion = constrictions running towards pylorus mixes pancreatic juices and bile with food > activates and facilitates digestion Segmentation = local constrictions alternating with relaxation mix food with digestive enzymes and bile > brings nutrients to epithelium Peristalsis = propels contents into new regions of intestine
Describe what happens to food as it passes from the jejunum to the colon
Absorption of water makes content more viscous > increases resistance to flow
High levels of anaerobic bacteria in colon > feed off and ferment colonic contents > release short-chain fatty acids > stimulate enteric reflexes
What causes defecation?
Distension of rectum triggers urge to defecate via sacral primary afferent neurons
Mass movement contractions move faecal matter from colon to rectum
Conscious neural activity relaxes anal sphincter and contracts abdominal muscles for normal defecation
What is the migrating motor complex?
Wave of constriction initiated in antrum/upper duodenum
Propagating slowly to ilio-colonic junction
Constriction clears bacterial and cellular debris from otherwise empty lumen
What are the gastric pits lined by?
Foveolar epithelium
True or false: there are normally lymphocytes in the gastric lamina propria
False
Why does the stomach not digest itself?
Gastric mucosal barrier
Contains bicarbonate to neutralise acid
Phospholipid monolayer in direct contact with lumen > prevent water from entering
How may the gastric mucosal barrier be broken down?
Organisms that can survive acid
NSAIDs which block prostaglandin synthesis
Bile - refluxes from duodenum and antrum > disrupts surface hydrophobic layer
Alcohol - breaks down permeability barrier
How do prostaglandis prevent and reverse mucosal injury in the stomach?
Inhibit acid secretion
Stimulate bicarbonate and mucus secretions
Increase mucosal blood flow
Modify local inflammation
How is inflammation initiated when there is breakdown of the mucosal barrier?
HCl can gain access to lamina propria and stimulate mast cells to produce histamine > start inflammatory process
What are the common causes of acute gastritis?
Chemical injury Alcohol/drugs Stress Shock Burns Head injury Septiceamia Staphylococcal food poisoning
How does acute gastritis heal?
Regeneration in 24-48 hrs due to rapid cell turnover in stomach
What is the difference between gastric erosion, acute and chronic peptic ulcers?
Erosion = defect above muscularis mucosae
Acute ulcer = defect penetrating muscularis mucosae and submucosa
Chronic ulcer = defect penetrating deeper than submucosa with dense fibrosis at base
What are the main types of causes of chronic gastritis?
Autoimmune
Helicobacter-associated
Chemical
Describe the autoimmune cause of chronic gastritis
Immune-mediated destruction of acid secreting tubules > atrophy > achlorhydria and loss of intrinsic factor > pernicious anaemia
- Caused by circulating autoAbs to parietal cell membrane H/K ATPase, IF receptor, and gastrin receptor
Confined to gastric corpus mucosa with total loss of parietal cells
Loss of HCl and parietal cells causes hypergastrinaemia > linear and nodular ECL hyperplasia and occasionally carcinoidosis
- Hypergastrinaemia due to lack of stimulation of D cells by HCl
Describe helicobacter-associated chronic gastritis
Helicobacter pylori uses its flagella to enter mucosal gel layer > colonises neutral area where it can survive
Secretes urease to increase pH
- Ammonium toxic to cells
Uses adhesins to attach to gastric surface epithelium and intercellular junctions
Neutrophilic gastritis as acute inflammatoryresponse
- Acute foveolitis = damage to gastric pits > replacement with intestinal mucosa = intestinal metaplasia > dysplasia and carcinogenesis
- Soluble Ags/chemoattractants
- IL-8
Infiltration with chronic inflammatory cells at 4 wks
Infection has low clearance rate and generally persists for life when established
Describe chemical chronic gastritis
Reflux of bile and alkaline duodenal juice due to
- Altered antro-duodenal motility
- Gastro-jejunostomy
- Long term use of aspirin/NSAIDs
Direct mucosal injury: disruption of mucous layer and gastric barrier > epithelial desquamation
Compensatory foveolar hyperplasia with
- Elongation and tortuosity of gastric pits
- Vasodilation
- Oedema
- Fibromuscular hyperplasia of lamina propria
- Mild inflammatory cell infiltration
Describe the progression from normal mucosa to adenocarcinoma due to chronic H pylori infection
Normal mucosa > H pylori infection > chronic gastritis > atrophic gastritis > intestinal metaplasia > dysplasia > adenocarcinoma
What are the longer-term outcomes of gastritis?
Chronic gastritis >
- Antral predominant gastritis > duodenal ulcer (age 20-40)
- Multifocal atrophic gastritis >
- Gastric ulcer (age 40-70)
- Gastric cancer (age >70)
What is another name for multifocal gastritis?
Pan-gastritis
What are the two main patterns of H pylori gastritis?
Antrum-predominant - Chronic inflammation - Neutrophils - Increased acid output - In duodenum - Gastric metaplasia - Active chronic inflammation - Duodenal ulcer risk Pan-gastritis - Chronic inflammation - Neutrophils - Atrophy - Intestinal metaplasia - Reduced acid output - Normal duodenum - Gastric ulcer risk
How does antrum-dominant H pylori gastritis lead to dudenal ulcers?
More acid in D1 section > duodenal mucosa comes to resemble gastric mucosa > H pylori colonises D1 > active chronic duodenitis > duodenal ulcer
What are the diseases associated with H pylori?
Peptic ulcer disease Gastric adenocarcinoma Gastric B-cell lymphoma of MALT Iron-deficiency anaemia Atrophic gastritis > increase in susceptibility to bacterial gastroenteritis and B12 deficiency
What are the most common sites for peptic ulcer disease?
Most common in D1 and antrum
Oesophagus at squamo-columnar junction with gastric cardia/Barrett’s mucosa
Gastro-enterostomy stoma
Meckel’s diverticulum
Describe the chronic ulceration found in peptic ulcer disease
Deep, sharply punched out
Destroying all layers through muscularis propria to subserosa
Scarring at base drawing in gastric folds to its margin
Scarring precludes restoration of submucosa and muscularis propria, leaving radial scar on healing with partial restitution of specialised gastric mucosa
- Replaced by intestinal and pyloric gland metaplasia
What are the four histological layers of an ulcer in peptic ulcer disease?
Exudate of fibrin, neutrophils, and necrotic debris
Narrow zone of fibrinoid necrosis
Zone of granulation tissue
Zone of fibrosis
What are the possible complications of peptic ulcer disease?
Perforation from anteriorly located ulcers > generalised peritonitis
Haemorrhage due to erosion of artery > haematemesis, melaena, anaemia
Penetration as ulcer erodes into adjacent organ > fistula
Stenosis due to contraction of fibrous scar > pyloric canal stenosis/lower end oesophageal stenosis
Development of malignancy
What is coeliac disease?
Immune mediated disease in genetically susceptible people, driven by gluten found in wheat, rye, and barley
Results in chronic inflammation of small bowel mucosa
Remission on gluten free diet = hallmark of disease
Describe the process of regeneration of the intestinal epithelium
Cells replaced every 2-3 days
Move from zone of proliferation > zone of maturation > incorporated into villous epithelium
Everyvillus surrounded by group of crypts
What happens if the cells of the intestinal epithelium are being sloughed off at a faster rate than normal?
Enlargement/elongation of zone of proliferation
Describe the microscopic changes associated with advanced coeliac disease
Total villous atrophy with crypt hyperplasia and intraepithelial lymphocytes
- Submucosa entirely normal
- Lymphocytes found in lamina propria and at surface - very few in crypts
What are the three stages of coeliac disease?
Infiltrative (type I): villus:crypt length normal (4:1) but increase in intraepithelial lymphocytes
Hyperplastic (type II): intraepithelial lymphocytosis, elongation and branching of crypts
Destructive (type III): villi shortened and blunted and villus:crypt ratio <1:4
What are the other causes, aside from coeliac disease, of intraepithelial lymphocytosis and villous atrophy with crypt hyperplasia?
Tropical sprue Small bowel bacterial overgrowth Common variable immunodeficiency = deficiency in IgA, IgG, and IgM Autoimmune enteropathy Various drugs
What are the typical clinical presentations of coeliac disease?
Gastrointestinal symptoms - Diarrhoea - Bloating - Abdominal cramps - Flatulence - Steatorrhoea Iron-deficiency anaemia Vitamin deficiency Malabsorption of nutrients Infants: failure to thrive Osteoporosis because of lack of vitamin D Lethargy Migraines Infertility Mouth ulcers
Outline the role of environment in the development of coeliac disease
Breast feeding protective
Timing/amount of gluten introduced to infant diet
- Too much gluten, too soon > increased risk
Infections increase risk
Outline the effect of T cells on coeliac disease
CD4 HLA-DQ2/8 restricted T cells - reactive to gluten-specific epitopes
Reside in small bowel mucosa
Cause damage by producing harmful cytokines; eg: IFN-gamma
CD8 T cells accumulate in epithelium and involved in immune response
Why do toxic gluten peptides survive digestion by the gut?
High content of proline confers resistance to digestion by proteases
Describe the consequence of gluten peptide deamidation
Deamidated gluten peptides bear negatively charged glutamate instead of glutamine Bind to HLA-DQ2 CD4 T cells recognise deamidated peptides presented by MHC class II > activated > produce cytokines
Outline the overall pathophysiology of coeliac disease
Ingestion of gluten peptides > gluten crosses mucosal epithelium > exposure to tTG > deamidated gluten/ross-links gluten > presentation on DC > presentation of deamidated gluten peptide and/or gluten-tTG complex to CD4 T cell
- Th1 response > IFN-gamma
- Secretion of MMPs by fibroblasts > villous flattenning and increased enterocyte death
- Increased cytotoxicty of CD8 T cells against enterocytes > villous flattenning and increased enterocyte death
- IL-15 released by CD8 T cells > promotes survival of CD8 T cells > predisposition to T cell lymphoma
- Th2 response > plasma cells > anti-tTG and anti-gliadin Abs > structural enterocyte change > epithelial damage
How is coeliac disease diagnosed?
Serological testing
- Ab to tTG
- Ab to deamidated gliadin peptide (DPG-IgG)
HLA-DQ haplotyping used to rule out diagnosis if HLA-DQ2/8 absent
Small bowel biopsy during gluten exposure = gold standard
What is EATL?
Enteropathy-associated T cell lymphoma
What is the predominant class of bacteria found in the body?
Gram negative rods
What are the main phyla associated with the human body?
Bacteriodetes
Firmicutes
Actinobacteria
Proteobacteria
What is the development of the microbiota?
Development finished at about 2.5 years
Usually stable after that but can be modified slightly by various events
What are the general roles of the microbiota?
Metabolism
Development
Protection against enteropathogens
What are the factors that influence the gut microbiota?
Mode of delivery Age Diet Antibiotics Genetics Environment Chronic inflammation
What is the effect of diet on the intestinal microbiota?
Short term changes in diet can have profound effect on gut microbial composition
Bacteriodes genus decreases when more Western diet introduced
What is the role of microbiota in nutrition?
Directly supply nutrients
- Carbohydrate breakdown
- Vitamin production
- Bile acid breakdown
- Amino acid metabolism
Alter metabolic machinery of host cells by changing host genes and maintaining enterocyte differentiation/function
- Bacterial degradation of host glucans > new glycan synthesis; eg: mucus
- Produce short chain fatty acids from indigestible carbohydrates
- Induce changes in host genes > promote angiogenesis
Describe the role of the mucosal immune system in immunity
Protects body surfaces
GIT immune system greatest site of Ag challenge as there’s large surface area of small intestine
2 main functions
- Protection from pathogens - IgA
- Tolerance to normal microbiota and food Ags
What are the immune roles of the cells of the villus?
Sites for induction of T and B cell activation
Enterocytes secrete TGF-beta, chemokines, and anti-microbial peptides
Goblet cells secrete mucins, lysozyme, and lactoferrin
Lamina propria lymphocytes
Paneth cells at base of crypts secrete defensins
What villous cells are imporant in inducing tolerance?
Enterocytes
Intraepithelial lymphocytes
What are the innate defences of the gut?
Peristaltic action Acid Mucous layer/glycocalyx = molecular sieve Enterocytes - Barrier - Antimicrobial factors to kill off cells that get through barrier - Cytokines and chemokines Innate leukocytes Mechanisms for controlled Ag access - M cells - DCs
What is the significance of IL-22 in the innate gut immune system?
Enhances antimicrobial defence and epithelial repair and barrier integrity
Produced by NK cells and intraepithelial lymphocytes
How are macrophages in the gut different from those in the rest of the body?
Express lower levels of TLR
Hypo-responsive to TLR signalling
Describe how M cells allow Ag access safely
M cells don’t have villi and don’t secrete mucus > microbial access easier
Located over sites of organised lymphoid aggregates and deliver Ags directly to cells in these aggregates
Ags immediately taken up into DCs and macrophages > present to T and B cells
What is the role of dendritic cells in the mucosa?
Direct sampling of Ag from intestinal mucosa
Indirect sampling of Ag from intestinal mucosa via goblet cells and M cells
Induce variety of T cell differentiation pathways
- Tregs and Th2 in steady state
- Th1 and Th17 during inflammation
Bias B cell isotype switching to secretory IgA
Describe the fate of T and B cells activated in the gut
Activated B cells produce secretory IgA
CD4 T cells have multiple roles depending on their subset
CD8 T cells protect against intracellular infections
Many persist as memory cells
What are the effects of intestinal microflora on the mucous layer and gut epithelium?
Block binding sites
Produce bacteriocins
Interact with PRRs on enterocytes
- Stimulates mucin production
- Stimulates proliferation of crypt enterocytes and Paneth cells
- Stimulates release of antimicrobial peptides
- Induces regulatory cytokines
Short chain fatty acids inhibit production of inflammatory cytokines
IL-22 produced after PAMP interaction promotes epithelial barrier integrity
How does the gut immune system respond differently to normal microbiota and pathogens?
Normal microbiota induce physiological inflammation via Tregs and Th2 cells
Pathogens induce pathological inflammation via Th1 and Th17 cells
What is the role of the gut microbiota in obesity?
High-fat diets/obesity associated with decrease in diversity of microbiota
Low microbial diversity > higher levels of
- Insulin resistane
- Serum triglycerides
- Cholesterol
- Insulin
How may the intestinal microbiota be associated with infectious disease?
Microbiota escapes GIT can cause infections - Urinary - Respiratory - Wound - Peritoneal - Bloodstream Needs abnormality in patient - Anatomical - Functional - Immunocompromised
How may alteration in the intestinal microbiota be a cause of GIT disease?
Susceptibility to infection by gastrointestinal pathogens and/or overgrowth of certain commensals
How is pseudomembranous colitis caused? Where is it commonly found and spread?
Overgrowth of C difficile, usually due to use of antibiotics/cytotoxic drugs
Commonly found and spread in hospitals
How is antibiotic-resistant pseudomembranous colitis treated?
Metronidazole and vancomycin
How is recurrent C difficile treated?
Faecal transplant
What is the role of salivary amylase?
Alpha-amylase hydrolyses alpha1-4 linkages between glucose molecules
How is salivary amylase inactivated?
By acid pH in stomach
How is salivary amylase reactivated?
In duodenum when pH returned to neutral
How are carbohydrates digested in the small intestine?
Pancreatic alpha-amylase secreted due to CCK release from duodenal mucosa
Mixes with luminal contents by segmentation and retropulsion
Pancreatic amylase can’t break alpha1-6 linkages, leaving some oligosaccharides intact
Remainder of digestion happens at brush border mebrane of mucosal enterocytes
Isomaltase breaks alpha1-6 linkages
How is maltase and sucrase activated?
Synthesised as single large glycoprotein
Separated and activated in brush border membrane by pancreatic proteases
How are glucose and galactose transported into intestinal enterocytes at the brush border?
Through Na-dependent glucose transporter (SGLT1)
How is fructose transported into intestinal enterocytes at the brush border?
Facilitated diffusion through GLUT5
How are monosaccharides absorbed into the bloodstream from intestinal enterocytes?
GLUT2
Describe protein digestion in the stomach
Pepsinogen secreted from chief cells in stomach
Pepsinogen > pepsin by gastric acid
Pepsin hydrolyses bonds between amino acids > polypeptides
Pepsin inactivated at neutral pH in stomach
Describe protein digestion in the duodenum and jejunum
CCK released triggered by amino acids in small intestine
Secretion of pancreatic proteaases as proenzymes
Enterokinase in brush border membrane activates cleavage of trypsinogen > trypsin
Trypsin cascades cleavage of other proenzymes
Peptidases break up polypeptides into mix of short peptides and free amino acids
How are proteins digested at the brush border membrane?
So di- and tri-peptides transported directly into enterocytes > broken to free amino acids by enteroyte peptidases
Free amino acids transported into enterocytes via various transport systems
How is lipase secreted?
In inactivated form
Activated by colipase
Colipase activated by trypsin
What is the role of gastric lipase?
Minor
Produces just enough fatty acid to stimulate duodenal fatty acid receptors to release CCK
How are lipids absorbed?
Inside micelles - brought to apical surface of epithelial cells at tips of villi
Contact enterocyte membrane > dissolve in membrane > enter cells
Inside enterocytes, reformed in smooth ER and coated with apolipoproteins > chylomicrons
Chylomicrons secreted into lymphatics by exocytosis
Where, apart from via digestion of fats, are short chain fatty acids produced in the body?
Proximal colon by fermentation of dietary fibre
Absorbed in distal small bowel and proximal part of colon via H dependent mechanism
Contribute significantly to total energy intake
Describe the early development of an embryo
Egg fertilised in Fallopian tube > moves into uterus, propelled by cilia > cell division continues > blastocyst formation > implants into uterine wall between 5 and 10 days
What is gastrulation?
Formation of 3 germ layers by epiblast
What are the derivatives of the ectoderm?
Nervous system
Epidermis
What are the derivatives of the mesoderm?
Blood Heart Kidneys Gonads Most - Bones - Muscles - Connective tissues
What are the derivatives of the endoderm?
Epithelium of gut and associated organs
Describe the formation of the nervous system
Notochord induces overlying ectoderm to form neural plate > neural plate folds in on itself to form neural tube > neural crest cells separate from neural tube
What is the most common congenital heart defect?
Ventricular septal defect
- From failure to divide single ventricle into left and right ventricles
What is the second most common congenital heart defect?
Atrial septal defect
- From failure to close foramen ovale
What is hypospadiasis?
Partial failure of fusion of urogenital folds to make urethra
Occurs in 1 in 300 males
Easily corrected with surgery
Describe the structure of H pylori
Gram negative curved rod
Describe the process of acid secretion in the stomach
Secreted by parietal cell by H/K ATPase
Acetylcholine and gastrin act to increase histamine production from enterochromaffin-like cells > stimulate H/K ATPase to secrete more acid
How is the gastric mucosa maintained at neutral pH whilst the stomach contents have a pH of 1-2?
Mucous layer containing
- Bicarbonate
PGE2 and PGI2 important in maintaining mucous layer
What are the common H/K ATPase inhibitors = proton pump inhibitors (PPIs)?
Omeprazole
Esomeprazole
- Better bioavailability
How are PPIs used?
For about 8 weeks for healing of peptic ulcer
Also used for
- Zollinger-Ellison syndrome
- Reflux oesophagitis
What are the common histamine H2 receptor antagonists?
Cimetidine
- Causes gynaecomastia in men
Ranitidine
What are the common antacids?
Magnesium hydroxide
Sodium bicarbonate
- Can interact with other prescription drugs
What are the common side effects associated with antacid use?
Magnesium based - Diarrhoea Sodium based - Systemic alkalosis - Hypertension Calcium based - Rebound acidity
What are cytoprotective agents?
Coat cells and protect from erosion
What effect does peptic ulcer disease have on motility? How is this treated?
Increased motility
Treated with spasmolytics
What are the different types of spasmolytics?
Muscarinic receptor antagonists
Direct spasmolytics = anti-spasmodic drugs without anti-cholinergic effect
What is the drug class of hyoscine butylbromide?
Muscarinic receptor antagonist
What is the drug class of mebeverine?
Direct spasmolytic
How do prostaglandin E analogues work?
Increase mucus secretion and mucosal blood flow
Decrease gastric acid secretion
What is the drug class of misoprostol?
Prostaglandin E analogue
Why are prostaglandin E analogues contraindicated in pregnancy?
PGE causes uterine contractions at high levels
Analogue may may raise levels high enough to cause miscarriage
What are the receptors involved in emesis?
Acts directly on vomiting centre - Histamine H1 receptors - Muscarinic receptors Acts on chemoreceptor trigger zone - Dopamine D2 reeptors - Serotonin 5HT3 receptors Neurokinin-1 (NK1) receptors
Where is the vomiting centre located?
In dorsolateral reticular formation in floor of medulla
Where is the chemoreceptor trigger zone?
In medulla but outside BBB
What are the common anti-emetic drugs?
Histamine H1 receptor antagonists - Act directly on vomiting centre Muscarinic receptor antagonists - Act directly on vomiting centre Dopamine D2 receptor antagonists - Act on chemoreceptor trigger zone Serotonin 5HT3 receptor antagonists - Act on chemoreceptor trigger zone NK1 receptor antagonists
What is the drug class of promethazine, and what is a major side effect?
Histamine H1 receptor antagonist
Sedative
What is the drug class of hyoscine hydrobromide, and what is a major side effect?
Muscarinic receptor antagonist
Sedative
What is the drug class of metoclopramide?
Dopamine D2 receptor antagonist
What is the drug class of prochlorperazine?
Dopamine D2 receptor antagonist
What is the the drug class of ondansetron?
Serotonin 5HT3 receptor antagonist
Other than at the chemoreceptor trigger zone, where else will ondansetron work?
In GIT, where some cytotoxic drugs can trigger release of serotonin
Which drug classes are very effective in treating motion sickness?
Histamine H1 receptor antagonists
Muscarinic receptor antagonists
What are the effects associated with blocking muscarinic receptors?
S > dry mouth
L > blurred vision
U > urinary retention
D > constipation
What effects are associated with prolonged D2 receptor antagonist use?
Tardive dyskinaesia
- Extra-pyramidal effects
- Parkinson’s like
- Repetitive, involuntary, purposeless movements
How does metoclopramide work in the gastrointestinal tract?
At higher doses acts on 5HT3 receptors in gut as pro-motility drug
- Stimulate gastric emptying
Acts on 5HT4 receptors as agonist
- Stimulates acetylcholine release
- Further stimulate gastric emptying
Doesn’t cause diarrhoea because
- Selective for upper gastrointestinal tract
Define a parasite
Plant/animal that lives on/in another living organism on which it’s metabolically dependent
Define a definitive host
Host in which parasite reaches sexual maturity
Define an intermediate host
Host in which development occurs but parasite doesn’t reach sexual maturity
Define a paratenic host
Host in which parasite enters body and doesn’t undergo development but remains infective
Define a reservoir host
Animal which can be normally infected with parasite that also infects people
What are ectoparasites?
Organisms that gain metabolic benefit and life cycle requirements from cutaneous surface interaction with host
What are the principle groups of arthropod parasites?
Insects - Flies - Mosquitoes - Fleas - Lice Arachnids - Mites - Ticks
What are the three main species of human louse?
Pediculus humanis - Body louse - Clothing spread Pediculus capitis - Head louse - Spread by contact Pediculus pubis - Pubic louse - Spread by contact
Describe the life cycle of lice
- Egg
- Diagnostic stage - 1st nymph
- 2nd nymph
- 3rd nymph
- Adults
- Infective stage
- Diagnostic stage
What are scabies?
Sarcoptes scabiei
Itch mites
Form and live in burrows in epidermis
How is scabies spread?
Female lays eggs around entrance to tunnel
Eggs spread by scratching
How can scabies be detected?
Under microscope of scrapings/biopsy
How is scabies treated?
Topical scabicides
Mites can live for short periods of time away from host, so clothes and bedding should be washed with hot water for sterilisation
What are the common symptoms of tick infection?
Rapid ascending paralysis caused by neurotoxin - starts with local paralysis
How is the incidence of disease caused by ticks controlled?
Clothing
Repellent
Examination of body surfaces after potential exposure
Where do ticks usually live, and how do they infect humans?
Usually in long grass
Infect humans via contact
Why are protozoa generally not true parasites?
Most can live outside host
What are the two general forms of a protozoan parasite?
Trophozzoate = active infective form Cyst = inactive, typically resistant to destruction
Describe the illness caused by Entamoeba histolytica
Invades tissues in colon
Causes persistent large-volume diarrhoea and/or extra-intestinal infection: liver/brain abscesses
What is the main form of disease control for amoebiasis?
Clean water and sewage treatment
Treatment of infection = metronidazole and drainage of abscesses if present
What is Giardia intestinalis, and how is it transmitted?
Flagellated primitive eukaryote
Faecal-oral transmission by zoonoses - commonly possums
What is the main disease control for G intestinalis?
Cleaning water and sewage treatment
How is a diagnosis of G intestinalis made?
Presence of cysts in faeces
What is Toxoplasma gondii?
Obligate intracellular parasite
Reproduces in cats
Describe toxoplasmosis
Infection with T gondii persists for life, but usually asymptomatic in immunocompetent people
In immunocompromised people - susceptible to more serious symptoms
- Brain lesions
- Disseminated disease
Infection during pregnancy
- Can transfer to foetus
What are the main groups of helminths?
Roundworms (nematodes)
Flukes
Tapeworms
Describe roundworms
Tube-like animal covered with cuticle
Most free living
Describe flukes
All parasitic
Cycle back and forth between human and animal reservoir phases
Describe tapeworms
All parasitic
Large and consume high amounts of energy
Where do helminths develop?
Generally eggs develop outside host
Except for strongyloides
Describe the life cycle of pinworms = Enterobius vermicularis
- Eggs on perianal folds
- Diagnostic stage - Embryonated eggs ingested by humans
- Infecteive stage - Larvae hatch in small intestine
- Adults in lumen of caecum
- Pregnant females migrate to perianal region at night to lay eggs
Describe transmission of pinworms
Highly contagious
Transmitted via perianal/vaginal itch
- Contaminates fingers and sheets
Frequent reinfection
What is the life cycle of Ascaris lumbricoides?
- Adult roundworms mature and live in lumen of small intestine
- Eggs passed into faeces
- Diagnostic stage - Unfertilised eggs can be ingested but not infective. Fertile eggs mature in soil and become infective
- Infective eggs swallowed
- Infective stage - Larvae hatch
- Invade intestinal mucosa > portal system > systemic system > lungs
- Mature in lungs > penetrate alveolar walls > ascend bronchial tree > swallowed
What illnesses can A lumbricoides cause?
Pneumonitis in lungs
Intestinall obstruction
- Pancreatitis
- Cholangitis
How is A lumbricoides diagnosed?
Presence of eggs in faeces
Imaging of obstructions
Can be ruled out if patient has been away from endemic areas for over 2 years
Describe the life cycle of Strongyloides stercoralis
- Rhabditiform larvae in intestine excreted in stool
- Diagnostic phase - Development into free-living adult worms
- Eggs produced by fertilised female worms
- Rhabditiform larvae hatch from embryonated eggs
- Rhabditiform larvae develop into infective filariform
- Infective filariform larvae penetrate intact skin
- Infective phase - Filariform larvae enter circulatory system > lungs > penetrate alveolar spaces > carried to trachea and pharynx > swallowed > reach small intestine > adults
- Adult female worm in intestine
- Eggs deposited in intestinal mucosa > hatch > migrate to lumen
- Autoinfection: rhabditiform larvae in large intestine > become filariform larvae > penetrate intestinal mucosa/perianal skin > follow normal infective cycle
Describe the life cycle of Schistosoma mansonii
- Eggs in faeces
- Diagnostic phase - Eggs hatch > miracidia
- Miracidia penetrate snail tissue
- Sporocysts in snails
- Cercariae released by snail in water and free swimming
- Infective stage - Penetrate skin
- Cercariae lose tails during penetration > schistosomulae
- Circulation
- Migrate to portal blood in liver > mature into adults
- Paired adult worms migrate to mesenteric venules of bowel/rectum > lay eggs > eggs circulate to liver > shed in stools
What are the three requirements for schistosomiasis?
Fresh water supply
Snails
Human contact
Where is schistosomiasis mainly found?
Africa
Tropical South America
What illnesses may be caused by re-infection with S mansonii?
Eggs deposit in various tissues Cause local inflammation - Acute hepatitis - Local itch - Pulmonary fibrosis
Describe the illness caused by Echinococcus granulosus
Zoonotic infection
Humans accidental intermediate hosts as they consume infected animals
Larval cysts = hydatid cysts
Cysts can enlarge and compress tissue > pain
Commonly deposit in
- Liver
- Lungs
What are the causes of taeniasis?
Taenia saginata
Taenia solium
What is taeniasis?
Humans intermediate host of cestode parasites
Can cause brain cysts
Tapeworm can be up to 10 m long
Infection in humans by ingesting raw/undercooked infected meat
What are the primary tissue types in the liveer?
Hepatocytes Blood vessels and lymphatics - Capillaries called sinusoids Connective tissue Fibrous connective tissue capsule Serous external surface
Outline the process of liver regeneration
Existing hepatocytes able to divide and replace lost tissue after injury/disease
Life span = 150 days
After extensive repair, regenerated liver tissue may not have same lobular structure > more fibrous
How is liver tissue organised?
Hepatocytes organised into lobules around blood vessels
Surrounded by connective tissue for support
- Amount of collagen not very significant
Describe the structure of portal triads
Branch of hepatic artery
Branch of hepatic portal vein
Bile duct = canaliculi
Lymph vessel
Describe the structure of the lobule central vein
All blood going away from liver empties into central vein
Carried into hepatic vein
Sinusoids surround each row of hepatocytes and converge on central vein
Outline the features of sinusoids
Direct, 2-way communication between hepatocytes and bloodstream
Larger than other capillaries with discontinuous lining
Gaps between endothelium and adjacent hepatocytes = spaces of Disse
Microvilli line surface of hepatocytes in contact with sinusoids
Kupffer cells = macrophages present on inner walls of sinusoids
What are the three methods of defining the organisation of hepatocytes?
Classic lobule model
Portal lobule model
Acinar lobule model
How is the classic lobule model organised?
Centre of lobule = central vein
Triads in periphery
Focus on direction of blood flow
How is the portal lobule model organised?
Central of lobule = portal triad
Central veins in periphery
Focus on direction of bile flow
How is the acinar lobule model organised?
Different zones have different oxygenation and metabolic function Zone 1 - High O2 - High in toxins - High in nutrients Zone 3 - Low O2 - Low in toxins - Low in metabolites
Describe the histology of the gall bladder
Simple columnar absorptive epithelium No - Goblet cells - Mucous cells - Endocrine cells
Describe the structure of the exocrine pancreas
Basal region of cells has high density of rough ER
Luminal surface has numerous zymogen granules containing inactive digestive enzymes
Pancreatic glands formed from acini
Describe the structure of the endocrine pancreas
Islets of Langerhans
Rich vascular supply because must secrete hormones into blood
What parameters are required for approximating energy expenditure?
Weight
Height
Age
Activity factor
Describe control of appetite from higher centres
Weight controlled in hypothalamus
How are weight-controlling centres peripherally modulated?
Higher cortical centres
Size of fat stores
Presence of food in gut
How is leptin trasported into the brain? What does it do there?
By receptor on choroid plexus
Signals size of fat stores
Suppresses appetite
What is the role of the brain insulin receptor in the control of body weight?
Insulin signals to brain that there’s a lot of fat present > appetite suppressed
How is glucose a critical physiological regulator of feeding?
Glucose metabolism produces acetyl-CoA > LCFA-CoA > inhibits food intake
What are the long term inhibitors of food intake?
Leptin
Insulin
Describe the progression of dysplasia in intraepithelial neoplasia
Benign neoplasm - orderly Dysplasia - Mild - Moderate - Severe Carcinoma - Invasion = malignant cells breach basement membrane to invade underlying stroma
What is the tropism of human papilloma virus (HPV) infections?
Low risk types - Major cause of genital warts - Mild squamous dysplasia High risk types = 16 and 18 - Moderate to severe squamous dysplasia - Major cause of squamous cell carcinoma
How does a HPV infection lead to cervical dysplasia?
Episomal viral replication
- Latent infection with low level viral replication
- Majority of infections transient with viral clearance
Integration with cellular genome in high risk HPV types
- Integration of viral genome into host DNA > E2 gene disruption
- Overexpression of E6 and E7 oncoproteins
- Loss of p53 and Rb function tumour suppressing function
- Loss of p53 apoptosis function
- Continued cell proliferation > high risk for malignancy
What is the key histological hallmark of HPV infection?
Koilocytosis = white clearing around nuclei
Describe the histological progression of HPV infection to squamous cell carcinoma
CIN 1 = increase ink variation in size and increase in mitotic activity of basal cells
CIN 2 = increase in thickness of dysplasia
CIN 3 = dysplasia seen full thickness
Squamous cell carcinoma = invasion into basement membrane
How do Pap smear screen for squamous dysplasia?
Scraping of cervical transformation zone to visualise cells and nuclei
In high-grade squamous intraepithelial lesions - crowding of cells with high amount of chromatin
What is the diagnostic criteria for Barrett’s oesophagus?
Endoscopic evidence of columnar lining in oesophagus above gastro-oesophageal junction
Histological evidence of intestinal metaplasia (goblet cells) in biopsies from columnar epithelium
Describe the pathogenesis of Barrett’s oesophagus
Chronic reflux oesophagitis > repetitive mucosal injury by gastric acid > cellular proliferation, re-epithelialisation by columnar epithelium (tries to secrete bicarbonate) > likely exposure to carcinogens > increased risk for oesophageal adenocarcinoma
What are the histological criteria for identifying dysplasia?
Surface maturation of glandular mucosa Architecture of glands - Crowding - Change in shape and complexity - Gland fusion Cytology of proliferating cells - Nuclear atypia - Loss of polarity Response to inflammation and erosion/ulcers - Reactive/regenerative changes vs dysplasia
What are the histological changes in reflux oesophagitis?
Barrett’s, negative for dysplasia > low dysplasia > high grade dysplasia > intramucosal carcinoma > deep invasive adenocarcinoma
What are the types of hiatus hernia?
Sliding - Common - Sometimes called reflux symptoms Rolling - Volvulus - involves twisting - May cause strangulation > purple colour
What is achalasia?
Lower oesophageal sphincter fails to relax > abnormal peristalsis
How are bleeding duodenal ulcers managed?
Local injection of adrenaline > local vasoconstriction
Cauterisation to aid process of haemostasis
What is the most common reason for small bowel operations?
Adhesions in small intestine > small bowel obstruction
- Arise from scar tissue from previous surgery
What are the types of herniae?
Ventral - Usually incisional - Underlying viscera involved Groin - Inguinal - especially in men - Direct - Indirect - Femoral - more common in women
Describe the hepatitis A virus (HAV)
Non enveloped (+) ssRNA Resistant to stomach acid Single serotype globally
Outline the life cycle of HAV
Can replicate in both liver and intestinal epithelial cells
Contaminated water/food > ingestion > replication in intestinal epithelia > blood > replication in liver > secreted in bile > excreted in faeces
What causes pathology in a HAV infection, and how long do symptoms last?
Immune mediated cytopathology
Viral clearance
Symptoms for 2-3 weeks
Describe the serological response to HAV and hepatitis E virus (HEV)
IgM rises then falls
Rising IgG titre confirms acute infection
ALT (liver enzyme) also rises with acute infection
What are the clinical features of a HAV infection?
Incubation period = 30 days Symptoms - Jaundice - Vomiting - Pale faeces - Dark urine Symptoms last for 2-3 weeks
What are the prevention and treatment options for HAV?
Sanitation Administration of Ig - Pre-exposure for travellers - Post-exposure - within 14 days - Intimate contacts - Within institutions Supportive rehydration and nutrition Inactivated vaccine
Describe the hepatitis E virus
Non-enveloped (+) ssRNA
More fragile than HAV
How does the transmission of HAV and HEV differ?
Both transmitted via faecal-oral route
HEV has minimal person-person transmission, whilst HAV highly infectious
For which groups of people do HEV infections have a higher risk of more severe outcomes?
Pregnant women
Increasing age
What are the clinical features of a HEV infection?
Jaundice Malaise Anorexia Abdominal pain Hepatomegaly Nausea and vomiting Fever Pruritis
Describe the pathogenesis of HEV
Poorly understood
Entry across intestinal mucosa
Secreted in faeces, 2 weeks before and 1 week after symptoms
Detected ins erum for 2 weeks after onset
Affects Kupffer cells and hepatocytes
Outline the typical serological course of a HEV infection
Incubation period of 40 days on average
Symptoms start to appear when Abs reach significant level
Diagnosis made on ELISA assays for IgM and IGG
Compare and contrast HAV and HEV prevention and treatment
Sanitation used for prevention of both
Supportive therapy used as treatment for both
In HEV, administration of serum Ig not effective
No vaccine for HEV
What are adenomatous polyps?
Dysplastic precursor lesions for colorectal carcinoma
What are the type of adenomatous polyps?
Tubular adenoma - Sessile/pedunculated Villous adenoma - Often large and sessile Tubulovillous adenoma - Mixed featured
What are predictors of increased malignant risk of adenomatous polyps?
Increassed polyp size
Villous morphology
High grade dysplasia
What are the syndromes which increase the risk of early onset colorectal cancer?
Lynch syndrome (HNPCC)
Familial adenomatous polyposis (FAP)
MUTYH-associated polyposis
Describe FAP
Autosomal dominant syndrome APC gene mutation More than 100 adenomatous polyps in large bowel Most progress to carcinoma by 30 Attenuated variant - Less than 100 polyps - Colorectal carcinoma by 55
Describe the histology of an adenomatous polyp
Abnormal crypt architecture
Dysplasia
- Crowded cells
- Enlarged, hyperchromatic, pseudostratified nuclei
- Abnormal complexity to glandular architecture
- Fusion of glands in high grade dysplasia
- Goblet cell depletion
- Increased mitotic count
No invasion beyond muscularis mucosae
- No lymphatics in lamina propria > lymphatic spread not possilbe
- Complete excision = curative
What are the genetic pathways involved in colorectal cancer?
Chromosomal instability; eg: FAP
Microsatellite instability; eg: Lynch sydrnome
CpG island methylator phenotype
What are the common genetic changes that occur in the dysplasia-carcinoma sequence?
Loss of APC function > decreased cell adhesion and increase cellular proliferation Chromosomal instability Accumulated mutations - Proto-oncogenes; eg: K-Ras - Activation of telomerase
What is Lynch syndrome?
Hereditary non-polyposis colorectal cancer
Most common familial colorectal cancer syndrome
What are the histological features of a sessile serrated adenoma?
Typically arise in proximal colon
Saw-tooth architecture
More complex branching than in hyperplastic polyp
Dilatation at base of crypts
Elongated, vesicular nuclei, prominent nucleoli
Increased atypia with dysplasia
Outline the staging of colorectal cancers
Depth of tumour invasion
- Tis (in situ) = carcinoma in situ = adenomatous polyp
- T1/stage 1/Dukes A = invades beyond musularis mucosae
- T2 = invades into muscularis propria
- T3/stage 2/Dukes B = invades beyond muscularis propria into subserosa/pericolic/perirectal fat
- T4 = invades other organs/structures/perforates visceral peritoneum
Lymph node metastases
- N0 = no lymph node metastases
- N1/stage 3/Dukes C = metastases in 1-3 lymph nodes
- N2 = metastases into 4+ lymph nodes
Distant metastases
- Mx = distant metastases can’t be assessed
- M0 = no distant metastases
- M1/stage 4/Dukes D = distant metastases
Describe the structure of the hepatitis B virus (HBV)
Double-walled structure with outer envelope and inner capsid = fullin infectious
- Incomplete dsDNA
- Viral DNA polymerase
- RNA primer
Incomplete particles containing only envelope proteins = non-infectious
Outline the process of HBV replication
HBV enters hepatocyte > dsDNA moves into nucleis > repairs gap in incomplete dsDNA > cccDNA transcribed to pre-genome RNA > pre-genomic RNA released into cytoplasm > combines with encoded polymerase > reverse transcription using encoded polymerase > core particle with viral DNA > core particle re-enters nucleus for further amplification of cccDNA/goes to ER to be coated with HbsAg > released as infectious virus
Outline the life cycle of HBV
Sex, close contact > penetration of mucosal epithelia > blood > replication in liver > blood, semen, secretions
- Sex, close contact
- Injection
What is the main mode of HBV transmission in endemic countries?
Perinatal transmission
Mothers who’re HBeAg psotive much more likely to transmit virus to their offspring then those who aren’t
How does age of HBV infection influence clinical outcome?
Infected at younger age > less clinical illness but higher likelihood of chronic infection
Infected at older age > increased likelihood of clinical illness but less chance of chronic infection
What is a chronic infection with HBV marked by?
Persistent high levels of HBsAg and lack of anti-HBs
Outline the progression of liver disease
Normal > infection > acute/chronic hepatitis B > cirrhosis > hepatocellular carcinoma
What is the most common cause of liver cancer?
HBV infection
What are the serological tests used for diagnosis of hepatitis B infection?
HBsAg = general marker of infection
Anti-HBs IgG = recovery and/or immunity to HBV infection, and successful vaccination
Anti-HBc IgM = marker of acute infection
Anti-HBc IgG = marker of past/chronic infection
HBeAg = active replication of virus
Anti-HBe IgG = virus no longer replicating
- May still be positive for HBsAg
HBV-DNA = active replication of virus
- More accurate than HBeAg
What are the current HBV antiviral drugs?
IFN-alpha
Nucleoside analogues
Nucleotide analogues
New generation drugs targetting cellular receptor NCTP
Describe the HBV vaccine
Surface protein made from yeast with Alum adjuvant
2-3 doses protects from HBV and HDV
Also can be used for post-exposure vaccination of healthcare workers
Describe the structure of the hepatitis D virus (HDV)
Depends on coating with HBsAg to be taken into hepatocu=ytes
ssRNA
What are the clinical features of HDV?
Only infects in conjunction with HBV
- Co-infection with HBV
- Severe acute disease
- Low risk of chronic infection
- Super-infection of HDV in HBV-positive patients
- Usually develops chronic HDV infection
- High risk of severe chronic liver disease
What is the association between hepatitis C virus (HCV) infection and intravenous drug use?
80% of infectious occur through injecting drug use
Why is there no vaccine for HCV?
HCV instigates poor immunity and carriers can be superinfected readily with a different strain of HCV
Describe the structure of HCV
(+) ssRNA
Linear
Enveloped
High mutation rate > international genotype diversity
Outline the replication of HCV
Associates with lipid receptors and envelope protein receptors > enters via endosome > uncoated, releases RNA into hepatocyte > goes to ER for translation to various viral proteins > production of (-) ssRNA > RNA-dependent RNA polymerase makes (+) ssRNA > translation of new proteins and new viral particles which assemble in membranous web > exocytosis of virus/cell-to-cell transmission to spread virus to other hepatocytes
How does HSV evade the immune system?
Viral enzymes highly error prone > high levels of mutation
What are the sequalae of HCV?
70-90% become chronic carriers
Liver fibrosis
Cirrhosis > liver failure
Primary hepatocellular carcinoma
What are the treatments for HCV?
IFN-alpha and ribavirin - Not effective for all - Significant side-effects IL-28 gene associated with HCV recovery New treatments target non-structural proteases - Viral entry inhibitors - HCV RNA translation inhibitors - Post-translational processing inhibitors - HCV replication inhibitors - Viral assembly and release inhibitors
How does constipation result from use of laxatives?
Laxatives empty out a large proportion of bowel
Takes time for faeces to fill descending colon and rectum > patient experiences constipation
How do bulking agents work?
Hydrophilic colloids containing indigestible vegetable fibre > greater faecal water retention > greater vvolume of intestinal contents > increased normal reflex bowel activity
What are common bulking agents?
Bran
Psyllium
What should bulking agents be taken with?
Water
How do faecal softeners work?
Act as detergents to enhance mixture of water into faeces
What are the main groups of osmotic laxatives?
Saline laxatives Disaccharide hydrophilic colloid Polyols Macrogols/polyethylene glycols Others - usually in suppository form
What is the drug class of magnesium sulphate (Epsom salks)?
Saline laxatives
What is the drug class of lactulose?
Disaccharide hydrophilic colloid
What is the drug class of sorbitol?
Polyols
What is the proposed mechanism of action of stimulant laxatives?
May
- Stimulate colonic myenteric nerve plexuses
- Irritate intestinal mucosa
- Direct sensory nerve ending irritation
What is the most common group of laxatives?
Stimulant laxatives
What is the drug class of bisacodyl?
Stimulant laxatives
What is the drug class of senna?
Stimulant laxatives
What are the common anti-diarrhoeal drugs?
Opioids
Musculotropic antispasmodics
Muscarinic receptor antagonists
What is the advantage of using loperamide over other opioids?
Doesn’t cross BBB > doesn’t produce CNS effects caused by other opioids
What is the drug class of mebeverine?
Musculotropic antispasmodic
What is the drug class of hyoscine hydrobromide?
Muscarinic receptor antagonist
What is simethicone used for?
Flatulence - defoaming polymer > removes gas by changing surface tension of air bubble in GIT
What are the three patterns of acute hepatitis?
Lobular disarray and apoptosis
Zonal coagulative necrosis
Acute hepatitis with Mallory bodies
What can cause lobular disarray and apoptosis in the liver?
Acute HAV and HBV
Describe the lobular disarray and apoptosis pattern of acute hepatitis
Inflammation of entire lobule and portal tracts
Presence of apoptotic hepatocytes = Councilman bodies
Describe the zonal coagulative necrosis pattern of acute hepatitis
Intrinsic liver toxin causes hepatocellular injury in predictable and dose-dependent way
Liver injury caused by toxic metabolite (NAPQI) which directly injures hepatocytes and causes depletion of glutathione
Coagulative necrosis most commonly seen in zone 3
No inflammatory response in acute phase > macrophages present later
What can cause zonal coagulative necrosis in the liver?
Paracetamol toxicity
Describe acute hepatitis with Mallory bodies
Presents with - Fever - Jaundice - Right upper quadrant tenderness Presence of fat vacuoles Presence of neutrophils Mallory bodies result from massive collapse of hepatocyte cytoskeleton containing keratin - Form C shaped structures around hepatocyte nucleus Extreme hepatocellular swelling Scarring around portal vein
What can cause acute hepatitis with Mallory bodies?
Alcohol - also called alcoholic hepatitis
How is cholestasis identified histologically?
Plugs of yellow bile located in dilated biliary canaliculi
How is chronic hepatitis defined?
Persistence of liver injury with raised serum aminotransferase for >6 months
Not all patients with chronic elevation of liver enzymes have chronic hepatitis
What are the main causes of chronic hepatitis?
Chronic HBV and HCV infectionss most common
Autoimmune hepatitis
Drug-induced hepatitis
How is chronic hepatitis diagnosed histologically?
Portal tract densely infiltrated by lymphocytes
Lymphocytes spilling across edge of portal tract into periportal tissue, disturbing interface between portal tract and hepatocellular parenchyma
- Interface not well defined due to spillage
- Degree of interface hepatitis = grade of interface hepatitis - determinant of rate at which fibrosis develops
Apoptotic bodies should be seen in interface and associated with lymphoplasmacytic inflammation
- Apoptosis hallmark feature of acute and chronic hepatitss
Fibrosis in septa
- Radiate in stellate fashion outwards from portal tract
- Stage of chronic hepatitis describes degree of fibrosis and how far liver is on way to cirrhosis
Describe the stages of chronic hepatitis
Stage 1 = enlarged portal tracts with no septa
Stage 2 = septa but not much linking between portal tracts
Stage 3 = portal-to-portal bridging
Stage 4 = cirrhosis
What is non-alcoholic fatty liver disease (NAFLD)?
Different categories
Steatosis = accumulation of abnormal amounts of lipid in hepatocytes
- Macrovesicular/large droplet steatosis
- Usually caused by increased triglyceride synthesis/decreased excretion
- Microvesicular/small droplet - very rare
Steatohepatitis and fibrosis (NASH)
- Steatosis = background on which steatohepatitis develops
- Macrovasicular steatosis accompanied by inflammation and hepatocyte injury
- Hallmark = ballooning degeneration
- NASH and alcoholic steatohepatitis can only be distinguished from each other clinically
What is the pathogenesis of non-alcoholic steatohepatitis (NASH)?
Steatosis = protective mechanism against factors causing excess fatty acid synthesis and levels in circulation
Excess fat stored in liver as lipid droplets
When storage pathway overladed, hepatocellular free fatty acids may be diverted into toxic metabolic pathway > lipotoxic metabolites
Metabolites trigger inflammatory response
Whatt substances are exported out of the liver by the bile?
Bilirubin Cholesterol in lipoproteins Drugs Heavy metal ions, especially Cu IgA Abs
How do bile acids act as detergents?
Amphipathic > form micelles
Fats aand lipids captured inside micelles
Outer surface of micelles hydrophilic > washed away with water
How does cholesterol contribute to gall stones?
Bile salts at limit of their ability to keep cholesterrol in micelles
If excess cholesterol, won’t be able to be taken up into micelles > precipitates to form gall stones
What are the functions of colipase?
Binds to lipase to activate it
Prevents bile acid inhibition of pancreatic lipase
What is the enterohepatic circulation?
Bile salts in gall bladder > released into duodenum > actively transported out of ileum > enter portal circulation > enters liver > actively transported into gall bladder
How does the pancreas protect itself from digestion if trypsinogen is accidentally cleaved to trypsin?
Pancreas also makes trypsin inhibitor > binds to any traces of active trypsin present before secreted into intestine
Overwhelmed in case of pancreatitis, causing inflammation and pain
What are the two groups of reasons that complicate drug use?
Unusual drug behaviour
- Drugs with small therapeutic index
- Need constant monitoring of plasma concentration and therapeutic effect
- Low bioavailability
- Slow distribution
- Peak of curve with slow distribution higher than what would be expected using its volume of distribution
- Drug high enough to saturate elimination process
Inter-patient variability
Why must the loading dose f digoxin be divided?
Has narrow therapeutic index
Also has long half-life so needed loading dose
If loading dose administered all at once, would produce toxic peak concentration
What is the effect of multiple dosing of zero-order elimination drug?
Steady state reached due to other elimination mechanisms
Steady state never expected to be reached in theory
Increasing dose rate > disproportionate increase in concentration as same amount eliminated per unit time, not same proportion
What are the effects of age on pharmacokinetics?
Renal excretion and hepatic metabolism reduced in neonates and elderly
Metabolism reduced in babies because they’re deficient in some drug metabolising enzymes - particularly phase II glucuronate conjugation enzymes
Metabolism reduced in elderly because reduced activity of cytochrome P450
What are the genetic factors affecting pharmacokinetics?
Polymorphism of metabolising enzymes > fast and slow metabolisers
What are the two categories of drug-drug interactions
Pharmacodynamic: drug A modifies effect of drug B without affecting its concentration
- Receptor antagonists
- Physiological effects
- May be beneficial
Pharmacokinetic: drug A modifies concentration of drug B at its receptor
- A = drugs affecting gastric emptying rate
- D = drugs displace other drugs from plasma protein binding sites
- M = drugs can induce metabolism of other drugs via induction/inhibition of cytochrome P450
- E = drug alters protein binding/tubular secretion/urine flow/pH
Where are the hepatocyte enzymes located?
Membrane - ALP - GGT Cytoplasmic - ALT - AST - LD Organelle - Mitochondrial ASL - Lysosomal SOD Nucleus - DNA synthase
Describe the mechanism of cell death
Disruption of cytoskeleton > disordered molecular transport and disrupted membrane integrity
Mild cell damage > hydropic cell swelling > bleb formation
Severe damage > lysis of cells
What are the factors affecting enzyme activity in plasma?
Liver enzymes usually released into plasma by blebbing apoptotic cells
- Being turned over physiologially
Enzymes usually cleared by macrophages and Kupffer cells
What causes the release of liver enzymes?
Liver necrosis - Viruses, toxins, anoxia - Elevations in ALT, AST, LD - Especially AST with toxins Biliary disease - Gall stones, cancer - ALP, GGT Inducing drugs - No cell damage - increased production of GGT and ALP > higher GGT and ALP plasma levels via normal cell turnover - Alcohol, anticonvulsants - GGT, ALP
Whys is ALT more specific than AST?
ALT found in cytoplasm of liver and bone, where it’s involved in Cori cycle
AST found in mitochondria and cytoplasm of any cell that has mitochondria
What is the consequence of AST levels being higher than ALT levels?
ALT removed at half rate of AST
Therefore higher AST levels indicate acute hepatocellular damage/damage affecting mitochondria
What is the consequence of ALT levels being higher than AST levels?
Indicates chronic/resolving infection
What are the common drugs causing hepatitis?
Flucloxacillin Amoxicilllin Statins Ethanols Paracetamol
Why does mild hepatitis A infection not cause right upper quadrant tenderness and pain?
ALT levels are around 250 - need to be >500 for symptoms like tenderness and pain
Only produce vague symptoms like
- Nausea
- Loss of appetite
Describe the normal action of GGT
Predominantly found in liver and biliary epithelium
Low levels in kidneys
Needed in production of glutathione
Describe the normal action of ALP
Adds phosphate groups, involved in transport Found in - Bone - Liver - Placenta
Describe the difference between extra-hepatic and intra-hepatic biliary obstructions
Extra-hepatic
- Entire biliary tree obstructed
- Jaundice
- Significant increases in GGT and ALP
Intra-hepatic
- Biliary obstructions only lead to mild obstructions to bile flow, not complete block
- Can be caused by liver enzyme-inducing drugs
What is the most common cause of liver enzyme elevation?
Obesity
Affects ALT more than AST
Caused by sugar
What is included in a liver function test?
Bilirubin Albumin PT/INR Vitamin K Liver enzymes marker of cell damage and not directly marker of liver function
