Overview:

In this laboratory session you will examine slides illustrating the histology of the accessory organs of the digestive system. As you work, be sure to keep in mind the relationship(s) between the organ you are viewing and the GI tube proper. Try to understand the blood flow in these accessory organs as well as the path(s) taken by their various secretory products.

Slide Descriptions

Webslide 0083_J: Submandibular, monkey, Mallory-PA-hematoxylin   [DigitalScope]

This slide shows a fairly good degree of preservation. Note the following:

1. Thin strands of connective tissue running in and between lobules. Can you find traces of a capsule?


2. Running in the connective tissue septa:
   1. Blood vessels
   2. Ducts: can you distinguish excretory ducts, striated ducts, intercalated ducts? Note that in the salivary gland the epithelium of the large ducts is pseudostratified or stratified, whereas in the pancreas the large ducts are lined by a simple epithelium.
   
   
3. Identify serous and mucous acini. What are you looking for? Can you find the lumen in any of the acini? How would you expect comparable sections of parotid and sublingual glands to differ from this slide?

Webslide 0038_J: Submandibular gland, monkey, AF-TB   [DigitalScope]

Compare this slide with Webslide 83 above. Note how the connective tissue septa between lobules have frayed apart. Working at high power:

1. Find "typical" serous and mucous acini. (You'll have to search for mucous acini, there are few on this slide.)  Study nuclear morphology as well as disposition and appearance of secretory granules.


2. Try to understand the topological relationships of the finest branches of the duct system (intercalated ducts) to the acinar cells.


3. Identify a striated duct. Can you see the striations? Where? What is their functional significance?               

Webslide UTz_Sublingual (courtesy of Univ. Tasmania): Sublingual gland, human, H&E   [DigitalScope]

Webslide Michigan 180-1 (courtesy of UMich): Parotid gland, human, H&E   [DigitalScope]

Compare these glands to the submandibular gland above. In the sublingual gland, note that most of the acini are mucous. By contrast, note that the acini in the parotid gland are entirely serous (the numerous "empty" spaces are adipocytes indicative of fatty infiltation of the parotid gland that occurs with aging in humans). You may also find it easier to see intercalated ducts in the parotid gland as the intercalated segments tend to be longer in the parotid gland and therefore show up more frequently in section. You may also note several rather large nerves in the parotid gland, what function do they serve?

Webslide 0095_J: Pancreas, Monkey, H&E   [DigitalScope]

This is a well-preserved section, showing:

1. Acini with basally located round nuclei and apical zymogen granules.


2. Several pale islets of Langerhans, with clusters of pale-staining cells.


3. Pale staining ducts. The part of the duct that starts inside of the acinus is made up of centroacinar cells. The smallest ducts, called intercalated ducts, have low cuboidal epithelium. These ducts lead into intralobular ducts that have simple cuboidal epithelium with centered, round nuclei. Also notice the large excretory duct lined with a simple columnar epithelium with brush cells (these have a chemosensory function and serve to monitor the composition of the pancreatic secretions) and bundles of smooth muscle around the periphery. Note that there are no striated ducts in the pancreas.

Webslide 0039_J: Pancreas, monkey, AF-TB   [DigitalScope]

Examine the slide first at low power, comparing it with Webslide 95 above. Then go to high power and observe the following:

1. In the islets of Langerhans, endocrine secretory cells with granules. It is not possible to distinguish alpha, beta, and delta cells.


2. Relationship of endocrine cells to vasculature.


3. Find an ideal exocrine acinus and note:
1. Zymogen granules; the different staining may represent different states of maturity.
2. Nuclear morphology; pronounced nucleolus.
3. Slightly basophilic basal cytoplasm.
4. Find regions with centroacinar cells and note relationship of acinar cells to ducts.


4. Find a region showing a large excretory duct. What sort of epithelium is present?

 Webslide MA102900_PancInsulin: Pancreas, human, insulin immunostain   [DigitalScope]

This slide has been immunostained with an anti-insulin antibody to label beta cells (red) in the islets of Langerhans. It has been counter stained with toluidine blue to show nuclei and also the cytoplasm of acinar cells (which contains abundant mRNA necessary for protein production). 

 Webslide MA103905_PancGluc: Pancreas, human, glucagon immunostain   [DigitalScope]

This slide has been immunostained with an anti-glucagon antibody to label alpha cells (red) in the islets of Langerhans. It has been counter stained with toluidine blue to show nuclei and also the cytoplasm of acinar cells (which contains abundant mRNA necessary for protein production). 

Webslide 0084A_J: Liver and gall bladder, monkey, Masson  [DigitalScope]
UMich 194: Liver and gall bladder, H&E   [DigitalScope]

Examine the liver in these well-stained slides. The blue staining of the connective tissue in the Masson slide should help you locate the blood vessels and functional units.

1. Locate a central vein cut in cross-section. Note that the cords of hepatocytes radiate from the central vein.
2. Locate a portal tract containing branches of the portal vein, hepatic artery, lymph vessels, and bile duct ensheathed in connectie tissue.
3. Try to find various sized branches of the hepatic bile duct.
4. Be sure you can distinguish hepatocytes from sinusoidal lining cells. Do you find erythrocytes in the sinusoids? In the space of Disse?

In examining the gall bladder, pay close attention to the lining epithelium and the unique mucosal folds.

Webslide 0011_J: Liver, guinea pig, thin section, AF-TB   [DigitalScope]

Examine this section first at low power, using the same guide points as for slide UMich 194:

• Locate a central vein cut in cross-section. Note that the cords of hepatocytes radiate from the central vein.
• Locate a portal tract containing branches of the portal vein, hepatic artery, lymph vessels, and bile duct ensheathed in connective tissue.
• Be sure you can distinguish hepatocytes from sinusoidal lining cells. Do you find erythrocytes in the sinusoids? In the space of Disse?

Can you visualize how the hepatocytes are arranged, in 3–D, as muralia or walls? Do you understand what is meant by the term “cords” of liver cells. Now go to high power and further study this slide.

1. Find bile canaliculi in cross-section. Can you find them in longitudinal section?


2. Understand the relationship between hepatocytes, Kupffer cells, sinusoids, spaces of Disse. Where does blood flow? Plasma? What do you know about bile flow? Lymph?

Webslide 0005_J: Liver & Bile Duct, Monkey, PAS-H-FG   [DigitalScope]

This is an additional slide with the same features that you have examined before, except it is a thin section that allows resolution of very fine details such as bile canaliculi, into which the hepatocytes secrete bile in an EXOCRINE fashion.

The space of Disse associated with the sinusoids can also be appreciated: it is the pale region located between the sinusoidal endothelium and the apical domain of the hepatocytes.  This is a true extracellular space in which the hepatocytes absorb materials (such as nutrients from the gut, unconjugated bilirubin, etc.) from the blood.  Hepatocytes also have an important ENDOCRINE function by secreting materials (such as albumin, clotting factors, HDLs, LDLs, etc.) into the space of Disse which are then transported into blood.

You may note the hepatocytes closest to the portal tracts are darker staining with a fairly uniform cytoplasm, whereas the hepatocytes around the central veins have a "foamy" appearance. This is due to lipid accumulation, also known as "fatty change" that can occur with injury or cellular stress (e.g. high-fat diet, alcohol or other toxicity, hypoxia, etc).

After you have briefly reviewed the hepatic parenchymal tissue, examine the large bile duct. What sort of epithelium does it have?

Webslide 0084_J: Gall bladder, monkey, H&E   [DigitalScope]

Examine the wall of the gall bladder which might be confused with portions of the alimentary tube proper. How would you make the distinction?

1. What sort of epithelium is present? Can you find any goblet cells (do you expect to)? What about microvilli?


2. Examine the subepithelial muscle. Can you define a muscularis mucosae or externa?


3. Do you find much lymphoid tissue?

Extra Slides

Slide UMich 180-2: Parotid gland, H&E  [DigitalScope]

At low magnification, observe the abundance of fat in this gland: the amount of fat increases with age. Note that the parenchymal (secretory) tissue is divided into many lobules by the stromal (supportive) connective tissue. In this connective tissue stroma, notice the presence of blood vessels, nerves and large, interlobular ducts [example]. These interlobular ducts are lined by a pseudostratified columnar epithelium. Find a well defined lobule [example] and observe the secretory acinar cells and ducts within each lobule. You will note that these acinar cells are fairly uniform in size, structure and staining, because they all produce proteinaceous, or serous, secretion. The secretory granules are not always well preserved by routine methods of light microscopic fixation and are not recognizable in this slide.

The ducts of various sizes present within a lobule (intralobular ducts) are lined by a simple columnar epithelium of varying heights. Select a transverse section of a medium-sized duct and observe the fine striations at the basal portions of the lining cells. Ducts with these striations are the striated ducts (or salivary ducts) [example]. The slender, intercalated ducts [example] are lined by low-cuboidal epithelial cells and connect the secretory acini and striated ducts, but the intercalated ducts in this gland are much shorter than those in the pancreas (which we will cover in another session), so they are not as easy to spot.

Slide UMich 183-1 Submandibular gland, H&E  [DigitalScope]
Slide UMich 184 Submandibular gland, H&E  [DigitalScope]
Slide UMich 185-2 Sublingual Gland H&E  [DigitalScope]

Unlike the parotid gland, the submandibular and sublingual glands possess both mucous and serous secretory cells. Slides 183-2 (submandibular) and 185A (sublingual) are stained with mucicarmine which specifically stains mucus red. Survey the two alternate slides to compare the relative proportions of mucous acini in these two glands. After this, go back to the H&E-stained slides to study the histology of mucous and serous secretory acini.

In slide 183 and 184 of the submandibular gland, mucous secretory acini are those that stain lightly. Compare the appearance of mucous and serous secretory cells (W pg 261, 13.17). The serous cells possess granular cytoplasm and nuclei which are spherical and vesicular in appearance. The mucous cells have pale staining cytoplasm and nuclei which appear to be pushed against the basal cell membrane. Now, observe the blind end of a mucous acinus and note the serous demilune cells [example] that cap this region of the secretory acinus. These demilune cells also have brightly staining granules in their cytoplasm. As an aside, the serous and mucous cells are actually adjacent to each other in vivo, and the formation of these demilunes is actually a fixation artifact (mucous cells swell with traditional fixation techniques and “squeeze out” the serous cells). Even so, the characteristic appearance of demilunes is nonetheless a useful diagnostic for identifying sero-mucous glands. Switch to a lower power and observe the distribution of ducts. Note the presence of large ducts in the connective tissue septae separating the lobules. These are interlobular or excretory ducts [example] and their epithelium appears to be stratified. The intralobular ducts within the lobules are smaller and similar in appearance to those of the parotid gland (i.e. they are striated ducts). The intercalated ducts in the submandibular and sublingual glands are very short and therefore NOT encountered often in sections.
           
Now examine slide 185, the sublingual gland, and note the greater prominence of mucous acini and fewer numbers of intralobular ducts than in the submandibular gland. Some lobules in slide 185 reveal almost all ducts with a few remaining secretory cells. This represents a pathological transformation of secretory acini to duct-like structures.

UMich 001 liver H&E  [DigitalScope]
UMich 194 liver, gall bladder H&E   [DigitalScope]

Using the low power objective on slide UMich 001, observe numerous small, pale spots in the parenchyma, most of which are either central veins or small branches of portal veins (in portal tracts). There may be a few larger channels, which are larger veins either entering or leaving this region of the liver. Try to identify classic liver lobules vs. portal lobules vs. acini of Rappaport.

The central veins [example] (also referred to as terminal hepatic venules) are surrounded intimately by hepatocytes similar to those that make up the bulk of the liver tissue. Portal veins [example] at medium power appear in section as a circle of rather prominent nuclei. In small branches of the hepatic artery [example] you will see primarily the ring of smooth muscle that makes up their wall. The three components together (portal vein, hepatic artery, bile duct) constitute a portal triad. Look for good examples of portal canals where all three components are seen well. Keep in mind that these structures twist and turn so there may be more than one cross section of a bile duct, artery, or vein, so it’s not always a “triad” of structures that you’ll see in the portal canal. Now, see if you can define a classic liver lobule at low power.

In the hepatic parenchymal tissue, note the plates of hepatocytes (the arrangement of these cells in plates is not always clear, due to plane of section and the frequent interconnections of plates). Occasional hepatocytes are binucleate. Between the plates of hepatocytes are intervening sinusoids lined by a thin endothelium. Larger eosinophilic cells lining the sinusoids are mostly Kupffer cells [example] (a type of macrophage, part of the mononuclear phagocyte system). Look for Kupffer cells using slide 194 as these cells are not readily recognized in slide 1. You should be able to distinguish Kupffer cells from endothelial lining cells.

The space between the endothelial cells and hepatocytes is called the “space of Disse” and is somewhat artificially enlarged in conventional sections. Remember that blood flows from the portal veins and hepatic arteries through the sinusoids to the central veins. A classical liver lobule has a central vein in its center and has several portal triads at its periphery. Bile flows through the bile canaliculi (too small to see) to the canals of Hering to bile ducts in portal canals, to hepatic ducts of increasing sizes and to the common hepatic duct, eventually to be emptied into the duodenum via the common bile duct. If you really want to find a canal of Hering, look for a line of low cuboidal cells [example] immediately adjacent to a portal canal –the canal of Hering connects canaliculi to the bile duct.

This portal inflow system can be distinguished from the portal outflow system which lacks accompanying arteries and bile ducts. The hepatic outflow system starts with central veins which empty into sublobular veins and into collecting veins of various sizes and eventually into the hepatic veins. One characteristic of the hepatic outflow system is that it cuts through the liver parenchyma without respecting the organization of the liver lobules. The portal inflow system, on the other hand, is always located at the periphery of each liver lobule.

Practice Questions

1. What is the primary function of striated ducts in salivary glands?

1. Storage of enzymes
2. Production of enzymes
3. Synthesis of mucous secretions
4. Modification of ionic content of saliva
5. Release of hormones

[ANSWER]

2. The pancreatic ductal epithelium primarily secretes:

1. Enzymes
2. Bicarbonate
3. Insulin
4. Mucus
5. Glucagon

[ANSWER]

3. The portal tracts ("triads") in the liver includes all of the following structures EXCEPT:

1. Hepatic artery branches
2. Central veins
3. Bile ducts
4. Portal vein branches
5. Lymphatic vessels

[ANSWER]