- Be able to distinguish the pancreas from salivary glands at the light microscope level.
- Be able to identify acinar cells, centroacinar cells, intralobular (intercalated) ducts and interlobular ducts in the pancreas, and name the major secretory products of the first two cells.
- Identify the islets of Langerhans and cells that produce major hormones (i.e. insulin, glucagon, etc.).
A. Exocrine Pancreas Slide 188B (pancreas, H&E) WebScope ImageScope WU Slide 98 (pancreas, thin section, H&E) WebScope ImageScope --note, since this is a thin section (from Western University's digital slide collection), 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 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 (W pg 299, 15.14). 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 acini (W pg 300, 15.15a), 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 (W pg 300, 15.15b).
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 (W pg 300, 15.15b). 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.
Observe the islets of Langerhans (W pg 299, 15.14) in slide 188, 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.
Before histological preparation, the arterial supply of this pancreas was injected with a red material. The main point of the slide is to show you how much richer the vascular supply is to the endocrine tissue, the islets of Langerhans, than to the surrounding exocrine pancreas. Find islets in the parenchyma, and observe the denser concentration of capillaries. Many of these slides in our collection (including the virtual slide) contain some pieces of lymph nodes, so make sure that you are looking at the pancreatic tissue [ORIENTATION].
Scan the parenchyma of this slide to find islets of Langerhans [example]. The staining procedure used here allows you to differentiate the two principal cell types found in the islets in slide #190 and UCSF294. Although the nuclei in both a and b cells are reddish, the insulin secretory granules in the beta (or B) cells cause the cytoplasm to stain a pale blue-green with the chrome-alum hematoxylin. The alpha (or A) cells, containing secretory granules of glucagon, are stained reddish. Note that the beta cells are usually more numerous and occur in the interior of the islet, while the alpha cells are found more peripherally. You will not be able to distinguish delta (or D) cells, the source of somatostatin, but you should know that they are there. Incidentally, the secretory granules of the acinar cells stain quite well in this slide and can be seen clearly in the exocrine pancreas.
Use this section to study the pancreas [example] in the mouse.
Electron Micrograph Wall Charts
#124 HEPATOCYTE WebScope ImageScope Most of the typical organelles are well developed in liver cells, reflecting the many functions of these cells. Note the nucleus, rough (granular) endoplasmic reticulum, smooth endoplasmic reticulum (not labeled here), mitochondria, Golgi complex, lysosomes, peroxisomes and occasional lipid droplets. The liver cell stores glycogen and lipid. The cytoplasm contains clusters of glycogen particles (black), which can be metabolized to glucose for release into the blood when needed by the body. The glycogen occurs primarily in areas rich in smooth endoplasmic reticulum. The diverse secretory and absorptive functions of the hepatocyte take place primarily across two surfaces, shown clearly here: (1) The cell surface facing the blood in the space of Disse and adjacent sinusoid. (2) The cell surface involved in the bile canaliculus. Note the junctional complexes that seal the two sides of the bile canaliculus, and keep the bile products isolated from the blood.
#125 EXOCRINE PANCREAS WebScope ImageScope In this low power electron micrograph, observe the organization of the acini, composed of acinar cells. Within the acinar cells you will see the basal rough endoplasmic reticulum, and the numerous secretory granules in the apical region of the cells, facing the small lumen of the acinus. Note the centroacinar cell in one of the acini. The intercalated duct shown here is of intermediate size.
DG6: What are the three classifications of liver lobules and what structures define each? Answer
- The cell indicated: [medium magnification] [high magnification]
- produces bile.
- is in the space of Disse.
- produces pancreatic pro-enzymes (such as trypsinogen).
- adds bicarbonate and water to the pancreatic exocrine secretion.
- removes sodium from the pancreatic exocrine secretion.
- secretes insulin.
- secretes glucagon.
- The asterisk is in:
- liver sinusoid
- space of Disse
- central vein
- branch of hepatic artery
- bile duct
- pancreatic intercalated duct
- pancreatic interlobular duct
- parotid gland intercalated duct
- parotid gland interlobular duct