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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
I. SALIVARY GLANDS
A. Parotid Gland
Slide UMich 180-2: Parotid gland, H&E Webscope Imagescope
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.
B. Submandibular and Sublingual Glands
Slide UMich 183-1 Submandibular gland, H&E Webscope Imagescope
Slide UMich 184 Submandibular gland, H&E Webscope Imagescope
Slide UMich 185-2 Sublingual Gland H&E Webscope Imagescope
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.
II. LIVER and GALLBLADDER
A. Liver
UMich 001 liver H&E Webscope Imagescope
UMich 194 liver, gall bladder H&E Webscope Imagescope
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 not easily seen in conventional sections because it is only 200nm or so across and therefore at the limit of resolution for a light microscope. Remember that blood flows from the portal veins and hepatic arteries through the sinusoids to the central veins and the space of Disse is a site of both absorption (e.g. nutritents) and secretion (e.g. blood plasma proteins) for the liver cells. A classical liver lobule is based on the flow of blood has a central vein in its center and has several portal triads at its periphery.
A portal lobule, on the other hand, is based on the flow of bile and therefore places the bile ducts at its center. 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 canals 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.
B. Gallbladder
UMich 194 liver, gall bladder H&E Webscope Imagescope
UMich 195M liver, gall bladder Masson Webscope Imagescope
Upon gross examination of slides 194 and 195M (i.e. at the lowest power on the virtual microscope) you will see a portion of the gall bladder wall nestled in an indentation of the liver tissue. Examine the wall of the gall bladder with your microscope. Extensive folds of the mucosa extend into the lumen. The mucosa consists of a tall, simple columnar epithelium and its underlying connective tissue (constituting a lamina propria). Note that there is NO SUBMUCOSA. The muscularis consists of scattered bundles of smooth muscle. Deep to the muscularis is an adventitia consisting of rather dense connective tissue that binds the gall bladder to the liver. Where the surface of the gall bladder faces the abdominal cavity there is a serosa.
III. PANCREAS
A. Exocrine Pancreas
Slide 188B (pancreas, H&E) WebScope ImageScope
WU Slide 98 (pancreas, thin section, H&E) WebScope ImageScope --note, this is a thin section (from Western University's digital slide collection) so features of the endocrine and exocrine pancreas may be easier to find here, but you should definitely look at both slides.
Examine slides 188B and WU 98 at the lowest power and note that most of the section appears purple or bluish. This is the parenchyma (or functional tissue) of the exocrine pancreas. You will note that the parenchyma is rather indistinctly divided into smaller areas by slits of open space or by pink connective tissue (stroma). The smallest of these areas constitute the lobules of this gland. You may see a few circular structures of various size between the lobules. These are cross sections either through branches of the pancreatic duct or through blood vessels. If you observe the parenchyma carefully you will note scattered small spots that are a lighter blue-gray. These are the islets of Langerhans, which comprise the endocrine pancreas.
First observe the parenchyma, noting that it is made up of large numbers of secretory acini, although you may also see occasional fat cells in the parenchyma. Each acinus is a cluster of secretory cells arranged around a small lumen (which is generally collapsed and therefore not visible in your sections). The acini may vary considerably in shape, since they are cut randomly in the section. Note that the peripheral region of each acinus, which represents the basal portions of the individual acinar cells, stains more blue or purple. The hematoxylin component of the H&E stain is staining the ribosomal RNA in the abundant rough (or granular) endoplasmic reticulum found in this portion of the secretory cells.
This “cytoplasmic basophilia” is the reason why the whole section appears purple or blue. The central region of the acinus, representing the apical portions of the acinar cells, is pink (acidophilic) because of the presence of the Golgi complex and numerous secretory granules in this part of the cell (you will probably not be able to make out the individual granules). Here and there you may see a smaller cell, or cluster of cells, with pale cytoplasm in the central region of an acinus. These are centroacinar cells [example] and represents the initial portion of the excurrent duct that extends up into the acinus. These slender ducts extending from the acini to larger excretory ducts located outside the lobule are called intercalated ducts [example] and may be found by looking for small clusters of 3-5 slightly elongated nuclei lying between the acini; the cytoplasm of the duct cells is very pale, and you may or not be able to make out the lumen. As in salivary glands, intercalated ductal cells in the pancreas contribute bicarbonate ions (sodium and water follow passively) to the exocrine secretory product. However, unlike salivary glands, there are no striated ducts in the pancreas to recover sodium, so the final product is rich in both sodium and bicarbonate (as opposed to saliva in which the sodium content is about one tenth that of plasma).
Using intermediate or low power, observe the larger ducts that are located in the connective tissue septa between the lobules. These interlobular ducts can be distinguished from blood vessels by their lining epithelium, which is either simple cuboidal or, in the larger ducts, simple columnar.
B. Endocrine Pancreas
Slide 188B (pancreas, H&E) WebScope ImageScope
WU Slide 98 (pancreas, thin section, H&E) WebScope ImageScope
Observe the islets of Langerhans in slide 188 and WU 98, occurring as pale areas of cells here and there in the parenchyma (you can find them most easily under low power). Note the scattered distribution of the islets and their variation in size. You will not be able to distinguish the various cell types in the islets in this routine H&E preparation.
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