Gastrointestinal tract Flashcards
Learning outcomes
Learning Outcomes
- Describe the epithelial specialisationsof the oral cavity and oesophagus.
- Describe the histological features of gastric pits and glands and the principal epithelial cell types found in the stomach.
- Describe the epithelial specialisationsof the small and large intestinesand their principal epithelial cells.
- List and describe the layers of the mucosa of the alimentary tract.
- Relate the mucosa of the alimentary tract to the layers lying deep to it,i.e.submucosa; muscularis externa; serosa or adventitia.
What is the GI tract and what does it consist of?
-Responsible for digestion, the mechanical breakdown of food and absorption of resulting nutrients
- 9m long from oral cavity to anus
- Accessory cells secrete substances used in process of digestion in alimentary canal
Oral cavity, pharynx, oesophagus, stomach, small intensine (duodenum, jujunum, ileum), large intestine (caecum, ascending/descending colon, sigmoid colon, rectum, anus)
Layers of gut wall- 1. Mucosa
Epithelium
• Responsible for secretion, absorption and protection
• Changes depending on what region it is located Lamina Propria
• Loose fibroelastic CT
• Blood vessels & lymphoid tissue (variable)
• Contains most of the gut glands
Muscularis Mucosae
• 2 thin layers of smooth muscle
• Responsible for independent movement of mucosa
e.g gastric glands
- Submucosa
•Collagenous & elastic connective tissue
•Many small blood vessels that supply the mucosa
•Plexus of nerves (referred to as Meissner’s nerve plexus) - controls motility, secretion rates and blood flow
•May contain glands in some regions e.g., duodenum, oesophagus
e.g Brunner’s glands of duodenum
- Muscularis externa
- Typically, two layers of smooth muscle -inner circular and outer longitudinal
- Perform the peristaltic movements of gut
- Innervated by the myenteric nerve plexus (referred to as Auerbach’s nerve plexus)
- Inner circular layer may be modified in some regions to form sphincters
- Serosa/ adventitia
•Loose CT, adipose tissue & blood vessels
•Serosa – if suspended by a mesentery and covered by peritoneum
•Adventitia – if retroperitoneal, with no mesentery
***Mesentery and peritoneum will be covered in Gross Anatomy
Oral cavity and microanatomy of regions
Hard palate
•Anterior 2/3
•Lined with SSK epithelium
•Underlying bone
Tongue
•Dorsum –specialised epithelium (taste buds and papillae)
•Ventrum– SSNK epithelium
Gums (Gingiva) •SSK epithelium Soft palate •Movable posterior 1/3 of palate •Lined with SSNK epithelium but no bony skeleton
Cheeks
•SSNK epithelium
Oesophagus- function and components
Function: Transport of masticated material from the pharynx to the stomach
•M –Mucosa
•E – Epithelium (Stratified Squamous Non-Keratinising)
•MM – Muscularis mucosae (well developed)
•SM –Submucosa (loose collagenous and elastic CT containing mucous glands)
•IC and OL – inner circular and outer longitudinal layers of the muscularis externa
•Upper third – skeletal
•Middle third – mixed
•Lower third – smooth
Stomach- function and components
Functions:
•Storage tank for ingested food that is distendable - Thick folds called rugae are present when stomach is empty and disappear as stomach distends
•Chemical breakdown of food - preliminary digestion via secretion of digestive enzymes
•Mechanical breakdown of food - mixes food with gastric juices to form chyme
•Four anatomical regions (the microanatomy changes depending on the region):
- Cardia
- Fundus
- Body
- Pylorus
Stomach- stomach layers
- R – Rugae (temporary folds of mucosa)
- M – Mucosa
- Simple columnar epithelium arranged into downward invaginations
- Branched tubular glands called gastric pits and glands
- MM – Muscularis mucosae
- SM – Submucosa
- ME – Muscularis externa
- Only part of gastrointestinal tract to contain three layers of smooth muscle
Gastric pits and glands
- Mucous cell - Lubrication; Protection of stomach wall
- Parietal cell (Oxyntic cell) Stains pink; Produces hydrochloric acid (HCl)
- Enteroendocrine cell - Produces gastrin
- Chief cell (Zymogenic cell)-Stains blue/purple due to lots of rER; granules contain pepsinogen
Small intestines- functions and components
Functions:
•Completes digestion and absorbs nutrients
•Large surface area (SA) required to maximise absorption – entire small intestine has SA of 250m2
Plicae circulares- circular ridges projecting into gut lumen
Villi- Finger-like structures projecting from plicae circulares
• Lined with simple columnar epithelium• Crypts of Lieberkühn at base of villi
• Four cell types – enterocytes, goblet cells, Paneth cells and enteroendocrine cells
Microvilli- Small Intestine• Circular ridges projecting into the gut lumenPlicae Circulares (Singular –plica circularis)• Finger-like structures projecting from plicae circulares• Lined with simple columnar epithelium• Crypts of Lieberkühn at base of villi• Four cell types – enterocytes, goblet cells, Paneth cells and enteroendocrine cells
Villi (Singular – villus)• Microscopic finger-like structures projecting from apical surface of absorptive enterocytes
Duodenum- components
Shortest portion – approx 25cm (“12 fingerbreadths in length”)
•Receives bile from gall bladder and digestive enzymes from pancreas
•Brunner’s glands in submucosa –secrete alkaline mucous that counteracts acidity of chyme from stomach
Jejunum and ileum- components and functions
Jejunum & Ileum
Jejunum
•About 2.5m in length
•“Fasting gut” – when dissected it is always found empty
•Majority of nutrient absorption occurs here
•Prominent plicae circulares and tallest villi
Ileum
•Longest segment -3.6m in length
•Numerous lymphoid follicles (Peyer’s patches) in submucosa•Immune surveillance of intestinal lumen
Crypts of lieberkuhn- components and functions
- Tubular glands located between the bases of villi
- Epithelial cell factories
- Stem cells produce all 4 cell types:
- Enterocytes (absorptive)
- Goblet cells (mucus)
- Paneth cells (P)
- Enteroendocrine cells (E)
•Stem cells located at base of crypt at cell position 4
