GI tube histology week 1 Flashcards
alimentary canal
digestive system
GI tract
Alimentary Canal: includes the complete digestive tract or tube (from oral cavity to anus).
Digestive system = alimentary canal + digestive glands (salivary, liver, pancreas, gall bladder)
Gastrointestinal tract: includes stomach, small and large intestines, and usually esophagus. This unit will cover all organs/regions of the alimentary canal except the oral cavity. The digestive glands will be covered separately from the GI tract
List the 4 layers of the GI tract and generally what is in each layer.
- Mucosa - which consists of epithelium, lamina propria and muscularis mucosa
- Submucosa – dense irregular CT
- Muscularis Externa – which has an inner circular and an outer longitudinal layer
- Serosa (or Adventitia) – mesenteric (CT) covering
What are the 2 types of epithelia found in the GI tract? At what 2 places in the GI tract does the epithelium change from one type to another? Why are different types of epithelia preferred in different places?
Epithelia: Type is either stratified squamous or simple columnar.
There are two locations where it changes abruptly:
• esophageal-cardiac (stomach) junction
• rectal-anal junction
where in contact with external environment and have a lot of abrasion, such as in esophagus, tend to have this stratified squamous epithelium. is also in rectum
What are 4 epithelial layer fxns in the GI tract?
- barrier between lumen (environment) and body tissues –keeps out pathogens, antigens, toxins, encloses undigested food. Maintains barrier via tight junctions
- protection from abrasion via stratified type of epithelia
- absorption – of nutrients and water
- secretion – via glandular tissue. Provides lubrication, enzymes, HCl, buffer, hormones
What is the lamina propria? What is generally contained in the lamina propria?
What specific cell types are found in the lamina propria? What is the collective function of cells in the lamina propria?
Lamina propria: a loose CT layer that supports the epithelium
The LP houses:
1. Microvasculature – for nutrient absorption
2. Lacteals- lipid uptake
3. Glands (are embedded in and surrounded by LP)
- Immune cells
- CT cells
Cells in lamina propria:
- Fibroblasts
- Macrophages
- Lymphocytes
- Plasma cells
- Mast cells
- Neutrophils
- Eosinophils
Collectively, the LP cells are responsible for the body’s response to ingested antigens such as pathogens and allergens.
What 2 types of lymphatic tissue are found in the lamina propria? What is this the name given to this tissue in the GI tract?
- Diffuse lymphatic tissue
- Lymphatic Nodules with germinal centers
Both diffuse and nodular lymphatic tissue in the GI tract is called Gut-Associated Lymphatic Tissue (GALT)
What are the layers of the muscularis mucosa? Where is it found? What is its function?
- Inner circular and outer longitudinal bundles of smooth muscle
- Underlies the lamina propria
- Separates the mucosa from the sub-mucosa
Function:
contraction of muscularis mucosa produces localized movements of the mucosa which can aid in dislodging food particles that have adhered to the mucosa and in some regions may facilitate absorption/secretion. This movement is distinct from peristalsis!!!!!!!
What is the submucosa? What 4 things are contained in the submucosa?
Submucosa: a dense irregular CT layer (fibroblasts and collagen) which contains:
- Larger blood vessels (which send/receive branches to/from other three layers)
- Lymphatics
- Submucosal nerve plexus
- Submucosal glands (present only in selected regions of GI tract)
What is contained within the sumucosal nerve plexus? What is its function?
Submucosal nerve plexus comprised of parasympathetic ganglia (nerve cell bodies) and nerve fibers (both motor and sensory)- controls mucosal/submucosal motility and secretory activity of glands
What are the 3 main components of the muscularis externa? Explain the functions/components of each.
Muscularis Externa: has three main components
Two concentric and relatively thick layers of smooth muscle (in most parts of GI tract):
1. Inner circular layer spirals around the gut tube. Contraction constricts the lumen and compresses and mixes the contents
2. Outer longitudinal layer contracts to shorten the tube
Both layers contract in a slow, wave-like fashion to propel contents (bolus of food) through the gut tube—This is called Peristalsis
- Myenteric nerve plexus is located between the two muscle layers of the muscularis externa. It is comprised of parasympathetic ganglia (nerve cell bodies) and nerve fibers (both motor and sensory) and also has sympathetic input- controls mucosal/submucosal motility and secretory activity of glands.
What is a serosa? What is it characterized by? What is its function? Where in the GI tract is it found?
Remember that serosa is the outermost CT layer covering the gut tube.
Serosa: a serous membrane (smooth membrane) consisting of a thin layer of simple squamous epithelium—specifically, the mesothelium—with a limited amount of underlying CT.
- The mesothelium produces a serous fluid that lubricates the surface to reduce friction
- Serosa covers portions of the GI tract that are suspended in the peritoneal cavity, i.e. are intraperitoneal.
What is adventitia? What is its function? Where is it found?
Adventitia: a dense, irregular CT layer found where an organ is held in a fixed position.
x If the organ is retroperitoneal, then the adventitia adheres it to the body wall
x If the adventitia is between two organs it acts to adhere them to each other. In this case it may contain adipose CT as a shock absorber/filler, rather than dense irregular CT (such as btwn trachea and esophagus).
An organ may have both serosal and adventitial coverings. Why is this the case? List organs that have both types of coverings.
An organ may have both serosal and adventitial coverings.
- The gallbladder is covered by adventitia where the gallbladder bounds the liver, but by serosa for the rest of its surface.
- The ascending and descending colon and esophagus also have both coverings.
What is the enteric nervous system? What does it control? What is it made up of?
The functions of the gut are largely controlled by the enteric nervous system. This is a self-contained system which is comprised of many repeating ganglia that are organized into the two aforementioned plexuses: Submucosal and Myenteric.
What are the functions of the motor and sensory components of the submucosal (aka Meissner’s) plexus?
Answer the same questions for the myenteric (aka Auerbach’s) plexus.
Submucosal (aka.Meissner’s):
- has neuronal cell bodies located in the submucosa
- Motor component: controls mucosal and submucosal gland secretion and blood flow
- Sensory component: consists of mucosal mechanosensitive neurons (convey info regarding luminal contents, secretory status)
Myenteric (aka. Auerbach’s):
- has neuronal cell bodies between the inner circular and outer longitudinal layer of the muscularis externa
- Motor component: controls GI motility (contraction/relaxation of smooth muscle)
- Sensory component: consists of tension-sensitive neurons and chemosensitive neurons
The enteric nervous system (ENS) also has both parasympathetic and sympathetic innervation from the autonomic nervous system, which influences and complements the enteric functions. What are the general effects of parasympathetic and sympathetic innervation of the ENS? Via what nerves do these branches of the ANS influence the ENS?
Parasympathetic:
- innervation via the vagus nerve
- stimulates peristalsis
- inhibits sphincter muscles
- triggers secretory activity
Sympathetic:
- Innervation via the splanchnic nerves
- inhibits peristalsis
- activates sphincter muscles
At the esophageal-cardiac junction (cardiac refers to initial region of the stomach), there is an abrupt change from stratified squamous epithelium to simple columnar epithelium (see attached pic).
What 2 types of glands are found in this region?
What is the function of their secretions?
What layer are they found in?
What is significant about the muscularis externa at this junction?
- Cardiac gastric glands (mucosal glands) originating at the base of gastric pits
- Esophageal glands in the submucosa underlying the junctional region (distal portion of esophagus + initial part of stomach) – these glands secrete a neutral pH mucus which protects the junctional region from the acid produced in the fundic region of the stomach
- The muscularis externa layer becomes thickened to form the inferior esophageal sphincter/lower esophageal sphincter (LES)
What muscle does the stomach lie inferior to?
What are the functions of the stomach? What is the name of the mixture it produces?
What are the 3 histologically distinct regions?
The stomach is an expanded part of the gut tube and lies beneath the diaphragm.
Function: mixes and partially digests food to produce a mixture called chyme.
It has three histologically distinct regions:
1. Cardiac region
2. Fundic/Body region
3. Pyloric region
Where is the cardiac region of the stomach located? What 2 types of glands are in this region? What layers are they in? What is the muscularis mucosa like in this region?
Cardiac features:
- Located at and adjacent to the Esophagus-gastric junction
- Cardiac gastric glands are shallow simple mucus glands opening from gastric pits
- Esophageal glands in submucosa under transition zone (poorly distinguished muscularis mucosa in this area)
In the fundic region of the stomach, what is the arrangement of gastric glands and gastric pits?
Where are mucous surface cells located?
What is the thickness of the mucosa like in this region and why?
Fundic features:
- gastric pits with 2-4 deep gastric glands originating from each pit
- mucous surface cells – line gastric lumen and gastric pits
- relatively thick mucosa due to pits and fundic glands
There is a diverse cell population in glands of the fundic region of the stomach. List all of the cell types, their location within glands, and where/what they secrete.
a. Mucous neck cells– located in isthmus of pit and initial part (neck) of gland
b. Parietal cells – most numerous in neck and upper half of glands – produce HCl, intrinsic factor (vit B12 binding)
c. Chief cells – located in the bottom half of glands produce pepsinogen, lipase
d. Stem cells (undifferentiated cells)
e. Enteroendocrine cells – usually located in bottom half of glands – secrete across basal lamina (rather than into lumen of gland)
- G cells: produce gastrin which stimulates HCl secretion
- D cells: produce somatostatin which inhibits gastrin release and HCl secretion
- Enterochromaffin-like cells: produce histamine which stimulates HCl secretion
parietal cells (eosinophilic, look like fried egg)
see reverse side of card
chief cells (basophilic)
Compared to the fundic region of the stomach, how deep are glands in the pyloric region of the stomach?
What is the morphology of glands in this area?
What do these glands secrete? What is the function of their secretion?
What are 2 ways to tell the pyloric region of the stomach apart from the cardiac region?
Pyloric features:
-Shallow glands (compared to fundic region) with mucus-producing cells at base
-morphology: simple tubular glands that appear coiled due to repeated ‘s-curves’
-secrete highly basic mucus which helps to neutralize acidity of chime prior to its entrance into the duodenum
The pyloric region is distinguished from the cardiac region by:
1. has slightly deeper mucosal glands that appear coiled (are actually ‘S-curves’)
2. absence of esophageal glands (submucosal) and esophagus-cardiac junction (i.e. no epithelial transition)
In what 2 ways is surface area of the small intestine increased at the gross level?
How is surface area of the small intestine increased at the cellular level?
• Surface area is increased at the gross level by:
- submucosal folds called plicae circularis (called rugae in stomach)
- mucosal villi that protrude from the intestinal surface into the lumen
• Surface area is further increased at the cellular level by microvilli of columnar intestinal absorptive cells (enterocytes) that populate the epithelium
What are the 5 cell types in the epithelium of small intestine? Explain their locations and functions.
- absorptive cells (enterocytes): located both on villi and in crypts; have microvillus border, produce glycoproteins which reside in apical plasma membrane
a. crypt enterocytes secrete water, electrolytes
b. villus enterocytes absorb nutrients and water - Goblet cells: located on villi; produce mucus to protect and lubricate gut tube
- Enteroendocrine cells: more than 10 types
- M-cells: overlie lymphoid nodules of GALT and function to transport antigens from lumen to macrophages (Ag presenting cells) to eventually activate lymphocytes
- Paneth cells: eosinophilic cells located at base of crypts; produce lysozyme
- Stem cells (undifferentiated cells): originate in base of glands
What is the morphology of Brunner’s glands? In what layer of the duodenum do they lie?
What do these glands produce? What is the function of its secretion?
Where do its secretions go?
Brunner’s glands are:
- Simple alveolar glands located in submucosa underlying the crypts
- Produce alkaline mucus that helps to protect the mucosal surface and neutralize the acidic chyme entering from the stomach
- Have ducts which perforate the muscularis mucosa and empty into the base of crypts
How long is the jejunum?
What is its primary role?
What is a distinguishing feature of the jejunum?
Jejunum:
- Is approx. 2.5 meters long
- Principal site of absorption
- Villi are long and finger-like
How long is the ileum?
What is its primary function?
How do the villi in the ileum compare to the jejunum?
What is a distinguishing feature of the ileum? In what layer(s) does it lie?
- Is approx. 3.5 meters long
- Site of water and electrolyte reabsorption in small intestine
- Villi are broader compared to jejunum
- Contains collections of lymphoid nodules called Peyer’s patches (type of GALT) that are located in the submucosa but commonly extend into the mucosa beneath the epithelium.
What are 4 distinguishing features of the large intestine?
- ‘Smooth’ surface: Mucosa has no villi, and no plicae circularis
- Numerous straight tubular crypts that secrete abundant mucus
- Outer longitudinal layer of Muscularis externa is organized as three bands called teniae coli (see attached)
- Both serosal and adventitial coverings
What are 4 distinguishing features of the appendix?
• Large number of lymphatic nodules in lamina propria
that extend into submucosa
• No villi
• Sparse crypts (compared to colon)
• Relatively thin muscularis externa with 2 uniform layers (no teniae coli)
What are 3 distingushing features of the colon?
- Epithelium is mostly goblet cells (fewer enterocytes)
- Closely-space crypts
- Teniae coli (when visible in advantageous sections) are a distinguishing feature
What is the function of the rectum and anus?
What kind of glands are located in the rectum?
How does the muscosal surface of the rectum compare to the colon?
What epithelial transition occurs here?
In what layer are the anal sphincters located?
What 2 things are located in the perianal region that can be used to distinguish this junction from the cardiac-esophageal junction?
Rectal-anal junction : function is to eliminate feces
- Rectum has colonic glands
- Mucosal surface is infolded (rather than smooth as in colon)
- Has an abrupt epithelial transition from simple columnar to stratified squamous
- Anal sphincters in muscularis externa
- Perianal (skin) region has hair follicles and apocrine sweat glands
