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