Gastrointestinal System

Learning Objectives:

No Slide:

Draw a quick schematic depicting the general layers of the “gut tube,” and annotate with details including tissue types, sublayers, and function.

The gut tube/alimentary canal has four main layers:

1. Mucusa with epithelial lining, lamina propria, and muscularis mucosae.

2. Submucosa made of connective tissue with blood vessels and an autonomic nerve plexus (submucosal plexus, aka Meissner’s plexus)

3. Muscularis externa: Smooth muscle arranged in inner circular layer and outer longitudinal layer; Myenteric/Auerbach’s plexus is found in between these two layers.

4. Adventitia (connective tissue that binds tube to surrounding structures) or Serosa (CT with overlying mesothelium; aka visceral layer of peritoneum).

At low magnification, identify the lumen and its epithelium. What does the structure of this epithelium tell you about the function of the esophagus? Do you see any glands in the lamina propria or submucosa?

The esophagus is lined by a simple squamous epithelium, and does not does not secrete or absorb anything (other than a small amount of mucus), but is a passage to the rest of the gut.

 

If you see glands in the lamina propria, these are esophageal cardiac glands and indicate that you’re either at the proximal or distal ends of the esophagus. Glands in the submucosa are esophageal (proper) glands and are found throughout the length of the esophagus.

At higher magnification, locate the lamina propria, comprised of loose areolar connective tissue. Just deep to this layer, note the muscularis mucosa. This layer is discontinuous in the esophagus, so it will appear as discrete patches of smooth muscle. Deep to this layer, identify the submucosa as a layer of dense irregular connective tissue. Next, examine the two layers of muscularis externa. Take note of the type of muscle present in your sample. Where in the esophagus was this section taken from?

Depending on which slide you look at, you may see different types of muscle. Skeletal muscle indicates it’s from the proximal 1/3, smooth from the distal 1/3, and a mix is from the middle 1/3.

At low magnification, again scan the slide and take note of the epithelium which lines the lumen of the esophagus. Note that the lamina propria is relatively thin. Identify the muscularis mucosa in this section and note that it appears more continuous in longitudinal section than in cross section. What does this tell you about the orientation of smooth muscle in this layer? 

That it is in discontinuous longitudinal bands.

Identify the submucosa and note the larger blood vessels. You may also note several mucous glands, which secrete mucus to lubricate the surface of the esophagus.

 

Examine the muscularis externa to determine what type of muscle it comprises, and from where in the esophagus this section was taken.

Depending on which slide you look at, you may see different types of muscle. Smooth muscle indicates that you’re at the distal end of the esophagus, while skeletal muscle indicates the proximal end. A combination of smooth and skeletal indicates that you’re in the middle.

 

If you forget how to differentiate between smooth and skeletal muscle, take a look at your notes. It may be easier to differentiate the two in muscle fibers that have been cut cross-sectionally.

Examine the external layer of the esophagus to determine whether it is adventitia or serosa. Do you see a mesothelium? 

Scan this slide at low magnification to find the abrupt transition between the epithelium of the esophagus and the epithelium of the stomach. Which type of epithelium characterizes each organ? What do those epithelia tell us about the function of each organ?

Esophagus: Stratified squamous- for passage and protection, not secretion or absorption.

Stomach- simple cuboidal for absorption and secretion.

Examine the lamina propria of the esophagus, where you will find a collection of mucus-secreting cardiac esophageal glands. Tie the presence of these glands at this location with the transition that occurs here.

Glands near the stomach protect the esophagus from regurgitated or refluxing stomach contents, which are highly caustic.

Now examine the mucosa of the stomach in order to see the gastric pits. In the lamina propria you see tightly coiled gastric glands that may appear circular in cross section. In this region of the stomach, these glands primarily comprise mucous cells and not chief or parietal cells. Why might that be?

The secretion of cardiac glands in the stomach is another way to protect the esophageal lining from gastric reflux.

Note that the muscularis externa is comprised of smooth muscle, but that the planes that it runs in may be difficult to discern.

Begin at low magnification and distinguish the luminal surface from the external surface. Describe the epithelium lining the lumen.

The lumen is lined by a simple columnar epithelium with many mucous cells.

Take note of the short gastric pits at this location. Distinguish them from the gastric glands deep to them. Chief cells are concentrated toward the bottom of the gastric glands, and stain basophilic with one or two euchromatic nuclei. Why do these cells stain basophilic?

Chief cells synthesize pepsinogen and require a large amount of rough ER, which stains basophilic.

Above these cells you will find wide bands of parietal cells. Parietal cells are round, have a centrally placed nucleus, and stain intensely eosinophilic. Why?

Parietal cells pump H+ ions out of the cell in order to create HCl in the lumen. This requires a great deal of energy, so parietal cells have a large number of eosinophilic mitochondria in their cytoplasm.

Mucous neck cells are located at the neck of the gland (where it meets the pit).

Begin at low magnification to distinguish the lumenal surface from the external surface.

What kind of epithelium do you see?

Simple columnar epithelium with many mucous cells 

Note the depth of gastric pits at this location in comparison to the height of the mucosa. How does this compare to other regions of the stomach?

Pits in the pylorus extend about halfway into the mucosa of the stomach. Compared to other regions of the stomach, these long pits.

Note the glands here are tightly coiled as in the cardiac region. How can you tell that they are tightly coiled? What type of secretion do these glands secrete?

The glands appear in cross section (i.e., they are round) indicating that they are bending at many locations. Based on their “washed out” appearance, these are mucous glands.

Before you move on, sketch the mucosa of each of the cardia, fundus, and pylorus of the stomach and compare their features. How could you identify these different regions if asked to on an exam?

Beginning at low magnification, identify villi: Finger-like projections of the epithelium. Draw or take a screenshot of these structures and annotate important features, including blood vessels, smooth muscle, and specialized lymphatic vessels called lacteals.

Villi have a core of lamina propria and an overlying intestinal epithelium. Blood vessels, etc. are found in the lamina propria. The muscularis mucosa forms a distinct band deep to the villi and crypts. 

Large, branched glands (Brunner’s glands) occupy the submucosa and mucosa of the duodenum. They secrete mucus. Why do you need such a large amount of mucus in the duodenum?

To protect the stomach from the acidic chyme leaving the stomach (They can also help to neutralize it, but that is primarily done by secretions from the pancreas).

Beginning at low magnification, appreciate the large number of goblet cells in this part of the intestine. Also take note of Paneth cells and columnar enterocytes. Where are Paneth cells located? How can you distinguish them from the surrounding cells?

They are located at the base of intestinal crypts, and will stain intensely eosinophilic.

Iowa Virtual Slidebox: Ileum (183)

Note that you will see an increase in lymphoid tissue in this part of the small intestine. What are these aggregations?

Peyer’s Patches. They are discernable as large round clumps of small darkly staining cells (primarily lymphocytes, but also macrophages and plasma cells). These nodules are an example of Mucosa-Associated Lympathic Tissue (MALT), more specifically called Gastrointestinal Associated Lymphatic Tissue (GALT). You may not see these in these sections.

Iowa Virtual Slidebox: Stomach Pylorus duodenum (137)

Before you move on, decide how to distinguish the small intestine from regions of the stomach. Use this slide to help you. Also decide if there are certain structures that are more common in certain regions of the GI tract.

SLU Slide #82: Colon

Iowa Virtual Slidebox: Colon (142)

At low magnification, observe: Does this region of the GI system have villi? Crypts?

Villi are present in the colon, but crypts are not.

Examine the epithelium at higher magnification to see the presence of columnar enterocytes and the abundance of goblet cells. Relate the distribution of these cell types to the function of the large bowel.

Goblet cells secrete mucus to facilitate movement of fecal mass, and enterocytes absorb water to aid in the formation of the fecal mass.

Return to low magnification in order to observe the teniae coli of the muscularis externa. What do these muscular bands represent? Describe their role in defecation.

Outer longitudinal layer of muscularis externa. Teniae coli push the fecal mass through the colon by shortening the length of the muscular tube.

At low magnification, observe: Does this region of the GI system have villi? Crypts?

The appendix does NOT have villi, but DOES have intestinal crypts. I.e., the epithelium does not have projections into the lumen of the appendix, but does have invaginations into the submucosa.

Examine the lamina propria and submucosa for GALT. Observe that the lymphoid nodules in both of these examples are “active” in that they have a germinal center of rapidly dividing lymphocytes and outer corona of older lymphocytes.

Examine the epithelium. You should see an area with a typical colonic epithelium, and then a sharp transition. Describe each of these types of epithelium. You should also see an area with a stratified squamous nonkeratinized epithelium, and an abrupt transition between the two. How does this resemble the junction between the esophagus and the stomach?

The epithelium of the colon is a simple columnar epithelium with abundant goblet cells. This transition resembles the gastro-esophageal junction in its abrupt transition, simple columnar for absorption of material vs. stratified squamous for passage of material, etc.

The muscularis externa exhibits inner circular and outer longitudinal layers. Which layer forms the internal anal sphincter? Is this layer under voluntary or involuntary control? What is the external anal sphincter formed from? Is it under voluntary or involuntary control? See if you can spot these two structures in your slides.

Internal anal sphincter forms from inner circular layer of the muscularis externa, which is not under voluntary control. External anal sphincter is formed from skeletal (voluntary) muscle from the pelvic floor.

No Slide:

What is an acinus? Draw a basic, general one, including any ducts.

At low magnification, scan the tissue and note the homogeneity of staining. Also note the abundance of adipocytes, which you should be able to easily distinguish from components of a mucous acinus. What type(s) of acini does the parotid gland display?

Serous acini.

Continue at low magnification and observe the many lobules divided by connective tissue. Within this connective tissue, note the presence of vascular and nervous structures, as well as large excretory ducts, lined with simple  or pseudostratified columnar epithelium.


Locate a well-defined lobule and observe it at higher magnification. Note the uniformity of cell size, staining, and structure. Within the lobule, locate an intralobar duct. What are the subdivisions of this duct? 

Intercalated and striated 

Note that within these interdigitations are abundant mitochondria, which are effective in the active transport of water and ions necessary to modify the primary secretion of the acini. You will not be able to observe these in LM.

 

Try to identify an intercalated duct. They are much shorter, and will be lined by either a low cuboidal or squamous epithelium. Screenshot or draw it and make annotations.

Scan this slide to appreciate the differential staining of different acini in this section. What does this tell you?

That there are different types of acini- serous and mucous. Serous acini will appear similar to those in the parotid gland. The cells will have a single basal euchromatic nucleus and granular cytoplasm.

Mucous acini are identified by their large, pyramidal cells with small basally-placed nuclei and pale cytoplasm.

At low power, observe the distribution of ducts. Observe a large duct in the interlobar connective tissue. What type of epithelium do you see?

Stratified or pseudostratified columnar

Observe an intralobar duct within the lobule. What type of ducts are most common?

Striated ducts. Note that because the intercalated ducts are very short, you will not encounter them often in the submandibular gland.

What type of acini are present in the sublingual gland?

Serous and mucous, but mucous are more numerous.

It should be easier to locate serous demilunes here. Why?

Because of the higher number of mixed glands in the sublingual gland compared to the submandibular (has both mucous and serous acini but not as many mixed) and parotid gland (has mostly serous acini).

Slide #40: Pancreas

Iowa Virtual Slidebox: Pancreas (146; 472, 1463)

Observe that most of this section can be characterized as acini. These acini comprise the functional tissue (parenchyma) of the exocrine pancreas, and are divided by small slivers of connective tissue. Observe carefully to find small spots of more lightly stained cells. These are Islets of Langerhans.  How can you differentiate the acinar cells from the Islet cells? Draw/screenshot and annotate an example.

Acinar cells are “polarized” and have a basophilic basal portion and an acidophilic apical portion. This is because the rough ER is concentrated at the basal surface, and the Golgi is located near the apical surface. These cells are also filed with zymogen granules. Islet cells stain more lightly, and are concentrated together in little islands (or islets). 

Try to find a centroacinar cell by looking in the center of an acinus for a cell, or cluster of cells, with pale cytoplasm (many describe them as looking like a “fried egg”). These mark the beginning of the duct system. These may also lead to an intralobar duct with a cuboidal epithelium. These ducts will coalesce to form which structure(s)? Take a picture of or draw a centroacinar cell and some ducts.

Main and accessory pancreatic ducts. 

How can you differentiate the pancreas from another digestive gland?

One way is that only the pancreas has Islets of Langerhans; another is that the pancreas has polarized cells. 

At low power, scan the parenchyma of the liver to observe numerous small, pale spots. These are most likely central veins or portal veins (in portal canals).

 

At higher power, differentiate between a central vein, and a portal vein within the portal triad. Central veins are associated with less connective tissue and generally have a wider lumen. The branches of the portal vein are found within connective tissue stroma, and appear primarily as a ring of nuclei. Branches of the hepatic artery will appear as rings of smooth muscle. Take a picture and label a portal triad.

 

Try to define a classic liver lobule, a portal lobule, and a liver acinus based on the structures you found above. Sketch and label a schematic of each. 

Note plates of hepatocytes, which will appear as cuboidal cells in one or two layers. Between the plates of hepatocytes are sinusoids. What type of epithelium do these sinusoids possess?

Simple squamous/endothelium. Extremely permeable!

For this preparation, a rat was injected with a preparation of ink shortly before sacrifice. Why does this preparation target Kupffer cells? (Hint: What type of cells are Kupffer cells?)

Kupffer cells are macrophages and will phagocytize the ink in the bloodstream.

At low magnification, appreciate both the number of these cells (Kupffer cells) , as well as their location. Where are they? 

Within sinusoids.

The space between the endothelial cells and hepatocytes is somewhat artificially enlarged in normal sections. Describe and/or sketch  the inflow and outflow of blood through the liver.

Blood flows in through portal veins and hepatic arteries, through sinusoids, and to central veins, and eventually to the hepatic vein.

Locate the lumen of the gallbladder. What type of epithelium do you find? There are many folds of mucosa found in the gallbladder. How do these relate to its function? Does the mucosa have a muscularis mucosae?

Simple columnar epithelium. They expand and “unfold” when the gallbladder is full. The gallbladder’s mucosa lacks a muscularis mucosae.

As you explore the wall of the gallbladder, you will note an additional layer is missing from the wall of the gallbladder. What is it?

Submucosa

Iowa Virtual Slidebox: Liver:Gallbladder (379)

Does the gallbladder have an adventitia or a serosa?

Both - adventitia where it is adjacent to the liver, and a serosa where it faces the abdominal cavity.