Histology of the upper GIT Flashcards
Describe the basic structure of the digestive tract
- It is a hollow tube made of varying dimeters
- The wall is made of four different layers
- Generally from the esophagus until the anal canal the structure remains the same except for the epithelium, glands and sphincters
What are the 4 layers of the digestive tract?
1) Mucosa (inner layer, which varies from a region to another)
2) Submucosa
3) Muscularis externa
4) Serosa/Adventitia
What is the mucosa and what are its different parts?
It is the innermost layer composed of:
1) Epithelium (changes across the GIT depending on the organ and function it serves)
2) Lamina propria
3) Muscularis mucosa
- Very thin smooth muscle that causes the local folding of the mucosal layer made of an inner circular layer and an outer longitudinal layer of smooth muscle, it extends into the mucosa but not into the villi
What is the epithelium of the mucosa of the esophagus?
Stratified squamous non-keratinized epithelium
What type of epithelium lines the mucosa of the stomach?
Simple columnar (perfect for gastric secretions)
What type of epithelium lines the mucosa of the anal canal?
Keratinized stratified squamous
What is the submucosa and what does it contain?
- It is the layer directly beneath the mucosa, where we will se glands, their ducts and lymphoid tissue like the MALT/GALT
- This layer is responsible to protect us from any pathogens that we possibly ingest
- This layer has elastic fibers, blood vessels, nerves, glands (in the esophagus and duodenum), and lymphoid tissue
- It has the submucosal plexus (Meissner’s plexus “parasympathetic ganglion”, it induces peristalsis and stimulates the mucosal glands)
What is the muscularis externa?
Two layer inner circular and an outer longitudinal muscle (there is also a muscularis externa layer)
What is the serosa/adventitia?
- In cases where the organ is intraperitoneal “surrounded by the peritoneum from all sides” then the final layer would be the serosa which is made from (simple squamous epithelium)
- On the other hand if the organ was retroperitoneal then the final layer would be the adventitia made of (loose connective tissue and nerves)
- FYI: The Esophagus has three parts: Cervical, Thoracic, and abdominal. Of these three parts first two is adventitia, and the the last one is Serosa (but the part of abdominal esophagus that is close to the diaphragm has both Serosa and adventitia)
Where is the adventitia found and what type of tissue forms it?
- it is found as the last layer of the retroperitoneal organs
- It is formed from loose connective tissue and nerve
Where is the serosa found and from what type of epithelium is it found?
- It is found as the last layer of the peritoneal organ
- It is made of simple squamous epithelium
- It secretes a slippery fluid and often has plenty adipose tissue
What forms the lamina propria (the second layer of three of the mucosa)?
- Collagen and reticular fibers
- Blood vessels
- Nerve endings
- Glands & MALT (which monitors and produces an immune response to pathogens passing with food through the GIT
What is the epithelial lining (mucosa) from the stomach to the colon?
Simple columnar
What is the epithelial lining in the other areas of the GIT (other from the stomach to the colon)?
Stratified squamous
Where are the postsynaptic autonomic neurons found?
1) Submucosa (Meissner’s plexus), where it induces peristalsis and stimulates the mucosal glands
2) Between the two layers of the muscularis externa(Myenteris plexus “relating to the muscles “between the inner circular and outer longitudinal layer of muscularis externa”” or Auerbach’s plexus)
3) Postganglionic fibers supply the surrounding of the smooth muscle of the muscularis externa
What is found within the muscularis externa?
1) Two layers of smooth muscles (inner circular and outer longitudinal)
2) Myenteric plexus (Auerbach’s plexus)
3) Some areas where there is thicker smooth muscle (sphincter area) and a circular muscle open and closes an opening
- We can find a inner oblique layer in addition to the two layers in the stomach
What are the muscularis externa layers of the stomach?
1) Inner oblique
2) Middle circular
3) Outer longitudinal
What are the different functions of the epithelium?
1) It acts as a permeability barrier between the gut lumen and blood
2) It transports and digest food
3) It secretes enzymes
4) It absorbs nutients
5) It produces and secrete hormones that are involved in digestion
What are the different function of the mucosa of the GIT?
1) Protective
2) Secretory
3) Absorptive
Where is the protective mucosa of the GIT found and what is made of?
- Found in the oral cavity, pharynx, esophagus, and the anal canal
- Made of stratified squamous surface epithelium
Where is the secretory mucosa found?
- Found in the stomach
- Tubular glands of the mucosa
Where is the absorptive mucosa of the GIT found and what does it contain?
- Found in the small intestine
- It contains villi and glands (crypts “In the junction between the two villi, there will be depressions that are called crypts and these crypts are spaces that lead to glands”)
What is a brunners gland?
The extension of some crypts (glands) through the muscularis mucosa in the duodenum
What are the different layers of the esophagus?
1) Mucosa
- Epithelium (stratified squamous non-keratinized epithelium)
- Lamina propria (contains papillae, mucous glands are many near the junction with the stomach)
- Muscularis mucosa (poorly developed in the upper part, and it is well-developed in the caudal part)
2) Submucosa (contains a compound tubulo-alveolar mucus gland)
3) Muscularis externa
- The upper esophagus (cervical) contains skeletal muscle only
- The middle esophagus (thoracic) contains a mixture of skeletal and smooth muscle
- The lower esophagus contains smooth muscle only
4) It is surrounded by adventitia (for the entire length except for the abdominal part which is surrounded by serosa)
Describe the histology of the gastro-esophageal junction
- Their is an abrupt transition of the mucosa from a protective stratified squamous epithelium to a glandular secretory mucosa
- The muscularis mucosa, submucosa and muscularis externa continues uninterrupted beneath the mucosal junction (note that the muscularis externa does not form a thick anatomical sphincter (but rather is a physiological sphincter mechanism)
- Submucosal glands are not seen in the stomach, instead glands of stomach will be seen in the lamina propria
- We can notice that Muscles are not thick here unlike the pyloric sphincter where muscles will be thick
Describe the histology of the stomach “interiorly”
- The stomach mucosa is thrown into a prominent longitudinal fold called a rugae, which permits great distention after eating
- The mucosa shows tiny grooves which is called “Gastric Pits”, which are depressions going down from the epithelium of the stomach
- The gastric glands opens into the bottom of the gastric pit
Describe the composition of the epithelium of the mucosa of the stomach
- Made of surface mucus cells/simple columnar, which invaginates into the lamina propria forming gastric pits
- It has no goblet cells, and gastric glands are found at the base of the pits
Describe the histology of the lamina propria of the mucosa of the stomach
- Made of loose connective tissue with smooth muscle and lymphocytes
- It contains gastric glands and opens into the bottom of the gastric pit (Histologically the glands found at the fundus and the body of the stomach will be the same, both very well-developed. At the two ends (cardiac and pyloric), the glands aren’t well-developed)
Describe the histology of the muscularis mucosa of the stomach
Made from an inner circular and outer longitudinal smooth muscle
Describe the histology of the gastric pit and glands
- Lined by simple columnar epithelium
- It has a oval basal nucleus
- It has many mucous granules in the apical cytoplasm (which neutralizes the acidity of the gastric secretions)
What are the gastric glands of the cardiac region?
- They are simple or branched tubular cardiac glands with a large lumen at the terminus, which opens into the gastric pit
- They are shallow and most of them are mucus cells, while there is few parietal cells
- The proportion of pit to gland is 1:1 (thus they are not very developed glands
- A clue to identifying this section is the esophageal part, if you see a sudden transition from stratified squamous to simple columnar with gastric glands, then know it’s a section from the cardiac region of the stomach
Describe the histology of the fundus and body of the stomach
- They have a narrow gastric pit with a proportion of pit to gland 1:5 (thus well-developed gland)
- The glands are long and straight
- It contains many parietal and chief cells
- The neck of the gland has undifferentiated cells and mucous cells, while the base has parietal, chief, and enterochromaffin cells
What are the glands that are found in the neck of the gastric pit of the fundus/body?
1) Mucus secreting neck cells
2) Stem cells
What are the glands that are found in the base of the gastric pit of the fundus and body?
1) Parietal cells (they have tooth-like intercellular spaces which secretes the HCl)
2) Chief cells (secretes pepsinogen)
3) Enterochromaffin cells (which secretes histamine)
What does the fundus contain?
1) Undifferentiated cells/stem cells: Neck of the gland, oval, basal nuclei, distinct nucleolus, mitotic stem cells, and the surface cells survives 3-7 days
2) Mucous neck cell (low columnar, basal nuclei irregular, mucus granules, secretes mucous
3) Parietal/oxyntic cells (large pyramidal cells with acidophilic cytoplasm, it secretes HCl and gastric intrinsic facto “necessary for VitaminB12 absorption”, in the upper half of the gland there are many mitochondria, and intracellular canaliculi
4) Enterochromaffin cells
5) Chief cells: Deeper in glands, basophilic, ribosomes, secrete pepsinogen
What is the difference between the microscopic photo of the parietal and chief cells?
Chief cells have a basophilic cytoplasm (darker) while parietal cells have eosinophilic cytoplasm (reddish/pinkish looking) and look like a cracked egg
Describe the histology of the pylorus
- It has deep and open gastric pits (2/3:1 pit to gland ratio)
- It has short coiled glands
- It mucus cells protects the entrance of the duodenum from the acid-pepsin attack and lubricates the passage for chyme
- It has few parietal cells and no chief cells
- Scatted among the pyloric mucus cells are neuroendocrine cells (G-cells) which secretes the hormone gastrin “which increases the gastric motility and juice”
What are the layers of the stomach muscle?
1) Internal oblique
2) Middle circular
3) Outer longitudinal
What is the final layer of covering of the stomach?
It is covered by serosa except for the bare areas where it is covered by adventitia
Describe the histology of the pyloric sphincter
- There is a sharp transition between the mucosa from the GLANDULAR arrangement in the stomach to the VILLOUS arrangement in the small intestine
- The duodenum is distinguished by a mucus-secreting gland (named Burnner’s glands), which are found in the submucosa
1) The pyloric sphincter consists of a marked thickening of the circular layer of the muscularis at the gastroduodenal junction
