Lecture 1: Microanatomy Of GI Tract Flashcards
What is the Function of the GI Tract ?
- mechanical and chemical breakdown of food
- absorption of resulting nutrients
Name the Layers of the GI Wall
- Mucosa
- Submucosa
- Muscularis Externa
- Serosa / Adventitia
List and Describe the Layers of the Mucosa - Epithelium
- secretion, absorption, protection
- changes depending on region located
- replaced every few days
Describe the Submucosa
- collagenous and elastic CT (can appear loose or dense)
- many small blood vessels that supply mucosa
- Meissner’s nerve plexus: controls motility, secretion rate, blood flow
- glands in some regions (duodenum, oesophagus)
Describe the Muscularis Externa
- 2 smooth muscle layers: inner circular + outer longitudinal
(3 layers in stomach; oblique) - peristaltic movements
- innervated by (myenteric)Auerbach’s nerve plexus
- inner circular layer modified in some regions to form sphincters
Describe the Serosa / Adventitia
- loose CT, adipose tissue, blood vessels
Serosa - suspended by mesentery and covered by peritoneum
Adventitia - no mesentery, retroperitoneal (e.g. in duodenum - less mobile organ)
Describe the Function of the Stomach
- storage tank for ingested food, distendable (thick folds; rugae)
- chemical breakdown of food -> preliminary digestion; enzymes and acids)
- mechanical breakdown of food -> food mixed with gastric juices to form chyme
4 Regions = Cardia, Fundus, Body, Pylorus
List and Describe the Layers of the Mucosa - Lamina Propria
- loose fibroelastic CT
- blood vessels
- lymphoid tissue -> immune surveillance
- most gut glands
Describe the Histological Features of Gastric Pits and Glands
- mucous cell -> lubrication, protection
- parietal (oxyntic) cell -> stains pink, produces HCl; activates certain enzymes
- enteroendocrine cell (G cell) -> local hormonal control, produces gastrin; for gut motility (blood flow, secretion, contraction)
- chief (zymogenic) cell -> stains blue/purple due to lots of rER, granules contain pepsinogen
- HCl comes in contact with pepsinogen
- Pepsinogen activated to pepsin
- Pepsin responsible for breakdown of protein
Describe the Histological Features of the Principal Epithelium Cell Types found in the Stomach
- rugae (temporary folds of mucosa)
- mucosa -> stains quite darkly due to lots of epi cells w/ lots of nuclei
- simple columnar epi arranged into downward invaginations
- branched tubular glands (gastric pits and glands)
- muscularis mucosae
- submucosa -> CT so stains lighter
- muscularis externa (3 smooth muscle layers; oblique)
Describe the Function of the Oesophagus
- transport of masticated material from pharynx to stomach
Describe the Functions of the Small and Large Intestine
Small - completes digestion and absorbs nutrients (large SA to max absorption)
Large - reabsorption of water and electrolytes (dehydration of luminal contents)
- absorption of vits liberated by bacterial action
- compaction of chyme into faeces
- prod mucus to facilitate passage of faeces
Describe the Sections of the Small Intestine
Duodenum - shortest
- receives bile from gall bladder + digestive enzymes from pancreas
- Brunner’s glands in submucosa
(secrete alkaline mucous to counteract acidity of chyme)
Jejunum - “fasting gut”; always found empty when dissected
- most nutrient absorption
- prominent plicae circulares and tallest villi
Ileum - longest
- many lymphoid follicles (Peyer’s patches) in submucosa
- immune surveillance
Describe the Epithelial Specialisations of the Oesophagus
- mucosa
- epithelium SSNK
- muscularis mucosae -> well developed, folded to allow for stretch, follows line of epi
- submucosa -> loose collagenous elastic CT w/ mucous glands; lubrication
- muscularis externa -> IC + OL smooth muscle
- upper 3rd = skeletal -> voluntary
- middle 3rd = mixed
- lower 3rd = smooth -> involuntary
List and Describe the Layers of the Mucosa - Muscularis Mucosae
- 2 smooth muscle layers
- independent movement of the mucosa
