Cell and Tissue Biology

Liver, Pancreas, Gallbladder & Gastrointestinal Tract

In this and all second semester labs, it may be necessary to examine more than one slide per organ. Not all organs are fixed the same and some structures are best illustrated in one species over another; therefore the best way to prepare for pathology is to observe tissues under various fixations and staining. Most of the slides used in the Atlas are plastic sections stained with Toluidine blue. That technique provides very high resolution sections of cell and tissue structures. However, the blue stain does not always give a contrast that is optimal for comparison with the most common stains used in pathology—H&E. Therefore, the best of both worlds will be achieved this semester by starting with the Atlas high resolution slides and then moving to the H&E stained virtual slides for identification of all listed structures.

In this lab, you will be looking at the organs associated with digestion and filtration of blood from the gastrointestinal tract.

Terms List

The key objects to concentrate on during this lab are listed below. You need to learn 1) how to recognize each object, 2) understand it's primary functions, and 3) understand how structure is related to function.

The "terms list" formatted as a worksheet can be downloaded here: Lab 6 Grid.

Exocrine Glands (Liver, Gallbladder, Pancreas):

  • hepatic cords
  • central vein (venule)
  • stem cells
  • liver sinusoids
  • sinusoidal endothelium
  • classical hepatic lobule
  • hepatocyte
  • polyploid hepatocyte
  • binucleate hepatocyte
  • Kupffer cell
  • portal triad
  • periportal limiting plate
  • hepatic portal vein (venules)
  • hepatic artery (arteriole)
  • bile canaliculus
  • bile duct
  • lymphatic vessel
  • gallbladder epithelium
  • pancreatic lobule
  • septa
  • Islets of Langerhans
  • pancreatic acinus
  • zymogen granules
  • centroacinar cell
  • intercalated duct
  • intralobular and interlobular ducts
  • pancreatic duct

Oral Cavity and Associated (Tongue, Salivary Glands, Teeth):

  • tongue filiform and circumvallate papillae
  • taste bud
  • tooth enamel, dentin, cementum, dental pulp
  • Identify and describe the basic structure of taste bud
  • Identify and understand tooth development; enamel organ, ameloblasts, odontoblasts
  • serous, mucous and mixed salivary glands
  • zymogen granules
  • striated duct
  • intralobular and interlobular ducts
  • serous demilune

Gastrointestinal Tract (Esophagus, Stomach, Small and Large Intestine)

  • mucosa, epithelium, lamina propria/submucosa, muscularis mucosa, muscularis externa, adventitia (where appropriate in esophagus, stomach, duodenum, ileum, jejunum, colon)
  • esophageal submucosal glands
  • Cardiac, fundic and pyloric stomach
  • gastric pits
  • parietal, chief cells and enteroendocrine cells
  • parietal canaliculus
  • mucous surface and neck cells
  • villi and epithelium (Duodenum, ileum, jejunum, colon)
  • intraepithelial lymphocyte T-cells
  • plasma cells of lamina propria, lacteals
  • absorptive cell (enterocyte), microvilli
  • goblet cell
  • crypts of Lieberkuhn, stem cells, and Paneth cell
  • enteroendocrine cells
  • Brunner’s glands
  • Meissner’s submucosal and Myenteric nerve plexi
  • Peyer’s patch, lymphatic nodules, M-cells (Microfold-cells)

w37 Fundic stomach

Click link [1] and find the mucosa, submucosa and the muscularis externa. These are the three basic layers of the gut wall. Click link [1]. Find the labeled fundic glands and the pits. The glands of the fundus and body of the stomach are straight, tubular glands that secrete HCl, enzymes, and hormones. Note that the glands are so numerous and tightly packed that you can hardly see the lamina propria between them. The pits are invaginations of the gastric epithelium and several fundic glands open at their base. (The pits extend approximately one quarter of the depth of the mucosa.)

Click link [1]. The thick mucus that provides lubrication is labeled. It protects the stomach epithelium from digestion by acid and pepsin. Zoom out once and click link [2]. Highlight the surface mucous epithelium. It is columnar and lines the pits. For a higher magnification of the surface mucous cells click [1]. The intracellular mucus can be seen as a light staining area in the apical pole. Also note the mast cell. The presence of mast cells is important in the fundic stomach because they produce histamine, one stimulator of acid secretion.

Zoom out twice and click link [3]. Find the neck mucous cell nuclei. The cells containing those nuclei are the stem cells from which fundic gland cells and surface epithelial cells derive. Unlike other parts of the GI tract, stem cells of the stomach do not reside at the bottom of the gland, but are located in the neck or middle of the gland. Maturing cells then migrate both downward (oxyntic, enteroendocrine, and chief cells) and upward (surface mucous cells) from this area. The cells themselves are very hard to recognize since they are "squeezed" by the parietal cells. Highlight the lamina propria. It is obscured by the numerous glands. Note that the lumen of the gland is also narrow.

Click link [1]. Highlight the parietal (oxyntic, acid-producing) cell. They are large, roundish cells with pink-staining (acidophilic) cytoplasm and a large round nucleus. (The parietal cells contain more mitochondria than cardiac muscle cells. Why do you think this might be the case? Intramitochondrial proteins are responsible for the acidophilia. What does that suggest about their amino acid composition?) The parietal cells are distributed along the length of the gland but are more numerous in the middle portion. Highlight the canaliculus of the parietal cell. These provide the route of secretion for the HCl. The HCl is not stored inside the cell, but is produced by the H+-K+ ATPase located on the cell surface of microvilli lining the canaliculi. In this view there are a few helical microorganisms that may be Helicobacter pylori bacteria, now thought to be one causative agent of peptic ulcer. Zoom out once and click link [2] for more highlighted bacteria.

Click the thumbnail and then [1,2,2,1]. This is a high magnification view of the base of the fundic gland. You should be able to recognize the parietal cells because of their pink staining cytoplasm and large round nuclei. Two other cells types are highlighted. The chief cells (pepsinogen secreting cells) have large purple secretory granules and basophilic RER. The cell outlines are not clearly seen. The nucleus has condensed chromatin and is located more basally than in the parietal cells. The enteroendocrine (argentaffin) cells are much smaller than either the chief or parietal cells and have a clear-appearing cytoplasm and large nucleus. Some of the enteroendocrine cells secrete gastrin, another stimulator of HCl secretion.

Zoom out twice and click [1,1]. Using the drop-down menu, highlight the parietal cells, the pepsinogen-secreting cell (chief cell) and the secretory vesicles containing the pepsinogen. The clear cell is an enteroendocrine cell, possibly a somatostatin (an inhibitor for HCl secretion) secreting cell. In the stomach, enteroendocrine cells secrete a variety of hormones, e.g., gastrin, serotonin, histamine, and somatostatin. In addition to stimulating HCl secretion, gastrin also stimulates the growth of gastric mucosa. The third stimulator of acid secretion, acetylcholine, is released from vagal and enteric nerve endings.

Use this virtual slide to practice on an unlabeled image: 112 Stomach Fundic Glands. This slide is from a large piece of stomach tissue, which is a tubular organ. When it was cut and placed in fixative, the thick smooth muscle layer contracted and caused the tissue to fold back on itself. Thus, the stomach lumen is on the outside of the curved surface to the bottom and right side of the slide. Identify the following: mucosa, lamina propria, muscularis mucosa, submucosa, muscularis externa, smooth muscle, villus, epithelium, Chief cell, parietal cell, parietal cell canaliculus, enteroendocrine cells, surface mucous cells, mucous neck cell region (can be confused with Chief cells in the upper part of the gland), Isthmus region and region of stem cells.

w36 Cardiac stomach

The cardiac stomach is a narrow area just at the entrance of the esophagus. Click link [1]. Use the mouse-over to highlight the mucosa, submucosa, muscularis externa, and the peritoneal covering. (In the stomach the muscularis externa contains a third oblique inner layer.) Note that the pits of the cardiac stomach are about one half the depth of the mucosa.

Click [2,1] . Highlight the surface mucous cells. They are columnar, with a basal nucleus and light staining intracellular mucus at the apical pole.

Zoom out twice and click [3,1,1]. In this specimen there are numerous protein-secreting cells. They most likely produce lysozyme. Highlight a lysozyme secreting cell. They may be identified by the basophilic RER in the basal portion of the cell and numerous acidophilic secretory granules in the apical portion. Highlight these. The nuclei have prominent nucleoli, typical for protein secreting cells. Scattered throughout the cardiac glands are enteroendocrine cells. These cells have clear-appearing cytoplasm with numerous small dark secretory granules visible in this specimen. The nuclei are large.

Zoom out three times and click link [4]. The coiled glands of the cardiac stomach are not as tightly packed as in the fundic stomach. The lamina propria contains occasional lymphoid aggregations. Highlight the lymphoid aggregation. Click [1] and rerecognize the lymphocytes.

Zoom out twice and click link [5]. Find the coiled cardiac mucosal glands in cross-section.

Use this unlabeled virtual slide for practice: 111 Esophagus & Cardiac Stomach Dog. This slide shows all three regions. Going from right to left, the tissue transitions from esophagus to cardia to fundic stomach tissue. You must look at the mucosa epithelium, then the glands and their location. Can you find the following stuctures in the cardiac stomach: cardiac stomach junction, gastric pits, mucus cells, mucosa, muscularis mucosa, submucosa, muscularis externa? Can you identify when the fundic stomach beings? We will cover the esophagus later during this lab.

w38 Pyloric stomach

The pyloric glands occupy the distal portion of the stomach. Click link [1] and highlight the mucosa, submucosa and the muscularis externa. The rugae or folds seen in the gastric lining appear only in the empty stomach. They are composed of mucosa and submucosa. Note that there are many pits contained on one ruga. The pits in the pyloric stomach extend about one half of the depth of the mucosa. Highlight the muscularis mucosae. This forms the border between the mucosa and submucosa.

Click link [1]. Note the orientation. Click [2]. The glands in the pyloric stomach are tubular, coiled, and thus often seen in cross-section. The predominant cell type in the bottom of the glands is a mucus-secreting cell. Highlight these cells. They look honey-combed because the mucus has dissolved away. There are numerous enteroendocrine cells (most of the gastrin secreting cells are located in the pyloric stomach). Highlight these cells. Enteroendocrine cells have a homogenous, pale staining cytoplasm and may reach the lumen of the pyloric glands. They seem larger here than in the fundic stomach.

Zoom out once and click [4] to see more enteroendocrine cells and the surface mucus secreting cells. Click [1] to view a higher magnification of an enteroendrocrine cell and its secretory granules.

Now practice with this unlabeled virtual slide: 113 Stomach Pyloric & Duodenum. This slide shows the junction between pyloric stomach and duodenum (from right to left). Gastric pits with mucosal glands of mucous cells transition into intestinal villi comprising the mucosa and submucosal mucous Brunner's glands that open into the crypt area of the villi. Identify the following: pyloric stomach, mucosa, mucous cells, muscularis mucosa, submucosa, muscularis externa Duodenum junction and transition from stomach (follow the muscularis mucosa), villi, epithelium, absorptive cell with microvilli (near lumen), crypts of Lieberkuhn, Brunner's glands in submucosa, muscularis externa.

w41a Duodenum

Highlight the mucosa, submucosa and the muscularis externa. Click link [1]. A single villus tip is highlighted. You should be able to recognize many more. The villi line the entire small intestinal lumen to increase the surface area for absorption. They give it a velvety appearance in the freshly opened organ. The villi are most numerous in the duodenum and proximal jejunum and are diagnostic for the small intestine. Remember that the esophagus, stomach and large intestine lack them.

Click link [1]. Intestinal glands open to the base of the villi. There are more glands than villi, so several glands open at the base of one villus. The intestinal glands are called crypts of Lieberkuhn and they extend to the muscularis mucosa. The crypts are very tightly packed and seem to obscure the lamina propria. In the bases of the crypts new cells derive from stem cells, differentiate, and migrate to replace the cells continually lost at the tips of the villi. The cell renewal rate at the tip of the duodenal villi may be as short as two days. This is the area most vulnerable to the acid contents delivered from the stomach.

Zoom out once and click [2,1]. This is a higher magnification view of one villus. The intestinal surface epithelium is a simple columnar epithelium. The goblet cells are highlighted in this view. The predominant cell type is the simple columnar absorptive cell. Lamina propria is more evident here than around the crypts. Highlight the lymphatic vessel (lacteal). These are blind-ended vessels that begin in the tips of the villi. They are important in transporting the fat absorbed by the columnar cells. Small capillaries are distinguished by their small size(and the red blood cells within their lumen in this specimen).

Click the thumbnail and then click [2,1]. Highlight the Brunner's glands within the submucosa. These submucosal glands are diagnostic for the duodenum (as compared to other parts of the intestine). The secretion of the Brunner's glands is a viscous, alkaline fluid that contributes to the alkaline pH of the duodenum and helps protect it from the strongly acidic gastric contents. The Brunner's glands open into the crypts of Lieberkuhn and thus to the lumen of the duodenum.

Now practice with this unlabeled virtual slide: 113 Stomach Pyloric & Duodenum. Find the duodenum junction and transition from stomach (follow the muscularis mucosa). Within the duodenum, can you find: villi, epithelium, absorptive cell with microvilli (near lumen), crypts of Lieberkuhn, Brunner's glands in submucosa, muscularis externa?

Continue with 123 Duodenum Rabbit Plastic. This slide shows the duodenum of a newborn rabbit and the crypts of Lieberkuhn are somewhat collapsed and stain a bit dark, but the resolution is very good. Don't confuse the serous cells that are part of the rabbit Brunner's glands with the cells of the Crypt area. If unsure, skip it and go to another area because there are plenty of good Crypts. Identify the following: villi, mucosa, epithelium, absorptive (enterocyte) cell, microvillus border, terminal web of actin filaments, goblet cell, intraepithelial lymphocyte (T-cell), enteroendocrine cell (not many), crypts of Lieberkuhn, stem cells, mitotic cells, Paneth cells (in crypts; larger granules but not dark red in this section), lamina propria, smooth muscle cell, capillary versus lacteal lymphatic vessel, Plasma cell, macrophage, submucosa, Brunner's glands (mixed in rabbit but mucous in human), muscularis externa, inner circular, outer longitudinal smooth muscle, Meissner's plexus (submucosal), Auerbach's plexus (Myenteric plexus).

And because practice makes perfect, here is another view of the duodenum: 122 Duodenum & Pancreas. This slide shows the pancreas attached by connective tissue to the side of the dog duodenum. It is somewhat a thick section, so the staining is dark. In the duodenum lumen, note the two cross sections through parasites (worms). Parasites a great inducers of an eosinophilic response, which you should be able to observe in the lamina propria of the mucosa. Brunner's glands have large lumens and are mostly in the submucosa, but because they open into the Crypts of Lieberkuhn, you can find some parts of the gland even in the mucosal area. Identify the following: duodenum, villi, mucosa, muscularis mucosa, crypts of Lieberkuhn, stem cells, (no Paneth cell in dog), goblet cells, absorptive cells, eosinophils (don't confuse with Enteroendocrine cells), submucosa, Brunner's glands, submucosal plexus (Meissner's plexus), muscularis externa, Myenteric plexus.

Everyone loves a nice duodenum, so here's a final unlabeled slide: 56d Duodenum Monkey. This slide is slightly understained but is similar to that of human tissue and the resolution is excellent. Unfortunately, the villi collapsed during the tissue collection and fixation, so the tips are folded over, but the bases of the villi are good. The demarcation between the crypts and the submucosa is easier to identify than in slide #122 and the Paneth cells are easier to identify, although the cytoplasm does not always show it reaching the lumen. Paneth cells are ALWAYS found at the very bottom of the crypt. Identify the following: villi, mucosa, crypts of Lieberkuhn, Paneth cells, some goblet cells start appearing close to the crypt (but their nuclei appear more dark and constricted as the handle of the goblet), muscularis mucosa, submucosa, Brunner's glands, muscularis externa, myenteric plexus.

w41b Jejunum

This slide has been seen in previous labs. The three basic layers are highlighted. We will briefly review the pertinent structures for this lab. Click [3]. Notice the presence of the villi. Click link [1]. Highlight a villus. Click link [1]. Highlight the smooth muscle within a villus. This muscle is continuous with the muscularis mucosae and can contract and shorten the villi. This motion propels the fluid out of the vessels toward the submucosa. Zoom out once and click [2,2]. Using the mouse-over highlight the goblet cell, mast cell, intraepithelial lymphocyte, simple columnar cell and the brush border. The brush (microvillous) border enormously increases the surface area for absorption.

Click the thumbnail and then [6]. Notice that the crypts of Lieberkuhn are not as crowded as in the duodenum. Highlight the crypts. Notice that the crypts extend to the base of the villi. Click [4]. There are many (relative to the rest of the gland) mitotic figures at the base of the crypts. Highlight the mitotic figures (stem cells dividing frequently). Click link [3] to see mitotic figures in metaphase and telophase. Then zoom out and click [2] to see plasma cells and a mast cell. These cells are present in large numbers throughout the lamina propria in the whole GI tract.

The enteric nervous system can be divided into two nerve plexi within the intestinal wall. Click the thumbnail and then [6,5]. The myenteric or Auerbach's plexus is located between the circular and longitudinal muscle layers. It is partly responsible for the peristaltic activity of the small intestine. Click the thumbnail and then [4,2]. Highlight the ganglion (nerve) cells. The submucosal plexus of Meissner also participates in the intrinsic innervation of the muscularis mucosae and the glands. Both plexi consist of ganglion cells and interconnecting nerve fibers.

Click the thumbnail and then [2,3,1]. Highlight the enteroendocrine cells. Hormones (and neurotransmitters) released from the enteroendocrine cells also participate in regulating GI function. (Taken together, the enteroendocrine cells are probably our largest endocrine gland.)

w41c Ileum

Click link [2]. Notice that the villi are shorter and fewer than in the duodenum and jejunum. Click [1,1,2]. Using the mouse-over find the absorptive columnar epithelium with brush border, goblet cells (notice that there are more here than in the duodenum and jejunum), smooth muscle cells, and macrophages.

Click the thumbnail and then [2,2,2,1,1]. Stare at the picture for some time. Recognize the plasma cells and the mast cells. Remember the distinguishing features of the plasma cell are the lightly basophilic cytoplasm, heterochromatic nucleus, and the acidophilic Golgi complex near the nucleus. These cells are good examples.

Click the thumbnail and then [1]. Notice the lymphoid-tissue-filled submucosa. Click [2]. Highlight a Peyer's patch. This is a lymph nodule within the submucosa. Zoom out once and click [1]. Highlight the high endothelial venules, similar to the ones in the lymph nodes.

Zoom out once and click [4]. Using the mouse-over find the circular and longitudinal muscle layers and the Auerbach's plexus between them.

Now practice with this virtual slide: 125B Ileum. This slide is excellent for the study of Peyer's patches. Identify the following: villi, mucosa, muscularis mucosa, epithelium, microvilli, absorptive cell, goblet cell, Paneth cell, enteroendocrine cell, lamina propria, smooth muscle, crypts of Lieberkuhn, submucosa, Brunner's glands, muscularis externa, myenteric plexus, Peyer's patch, lymphatic nodules, M-cells, M-cell pockets, lymphocytes, B-cells of lymphatic nodule, germinal centers.

w43 Appendix

Click [2,1]. First notice that there are no villi, just crypts. The epithelium is simple columnar. Find these using the drop-down menu.

Zoom out twice and clink [1]. Highlight the lymph nodule located in the submucosa and invading the mucosa. Pay attention to the overall abundance of lymphoid tissue within the appendix. Click [2]. The lymphoid tissue extends to the surface epithelium in places and crowds out the crypts.

Practice with this virtual slide: 140 Cecum (like appendix). This slide is similar to the appendix but cecum has larger lymphatic nodules (Peyer's patches). The dome of the nodule contains the M-cells and shows the pockets of immune cells nicely. Over the top of the dome will be located the villus portions that contains absorptive and goblet cells. Identify the following: Peyer's patches, mucosa, goblet cells, submucosa, lymphatic germinal centers, M-cell regions, absorptive cells.

w42 Colon

One function of the colon is to absorb water and electrolytes from the chyme it receives from the small intestine. Click link [1]. As in the ileum and appendix there are large aggregations of lymphoid tissue. Highlight these. Click link [4]. Using the mouse-over highlight the layers of the colon wall. Using the drop-down menu highlight a crypt. Within the crypt notice the abundance of the pale-staining goblet cells. The crypts are straight and tubular in the colon and there are no villi.

Zoom out once and click [2,1]. Identify the structures using the drop-down menu. Zoom out twice and click [1,1]. Here is a higher magnification view of the epithelium with brush border and goblet cells.

Click the thumbnail and then [2]. The longitudinal layer of the muscularis externa is aggregated into three evenly spaced bands called taenia coli. Highlight the taenia coli. These are always partially contracted so that the intervening portions of the wall bulge outward forming haustrae.

Here's a virtual slide for practice with an unlabeled image: 42 Large Intestine (colon) Human. This slide is not as well fixed as is most animal tissue because it is either from a biopsy or necropsy and fixation can be delayed under both conditions. A delay in fixation can create tissue that does not embed well in paraffin and stains too dark at times, so the crypt areas do not show the cells very well. However, if there were excess proliferation of the mucosal cells producing a polyp, it would still be recognized. Nevertheless, this slide does emphasize the importance of physicians trying to help the pathologists obtain well fixed tissues when a biopsy is taken in the surgery room. It is best to get the tissue sliced into thin pieces and into a generous amount of fixative quickly (no more than 10-15 minutes if possible). Thinner pieces allow the fixative to penetrate the tissue better. Identify the following: mucosa, absorptive cells in between the goblet or mucous cells, lamina propria, crypt of Lieberkuhn, submucosa, muscularis externa, myenteric nerve plexus.

w50 Esophagus and trachea

The esophagus is the first part of the gastrointestinal tract and has the same basic three layers. Click link [1]. Highlight the layers of the esophagus. Click link [1]. The epithelial lining of the esophagus is stratified squamous to resist the abrasive nature of swallowed food. The beginning of the esophagus contains mucus secreting submucosal glands. Highlight these. The upper part of the esophagus lacks muscularis mucosae. Click link [1]. The squamous cells are continually being sloughed off and replaced to overcome wear and tear, as is the case in all parts of the GI tract.

Zoom out twice and click link [2]. The muscularis externa of the upper third of the esophagus is composed of skeletal muscle, though under unconscious control.

To practice with the esophagus, start with: 107 Esophagus and identify the following structures: mucosa, epithelium, submucosa, submucosal mucous gland and duct, submucosal collagen bundles, muscularis externa, skeletal muscle (Why is there skeletal muscle here?) Next, find the esophagus in this slide we've already looked at above: 111 Esophagus & Cardiac Stomach Dog. Can you identify the following structures: esophagus mucosa, epithelium, submucosa, muscularis externa, submucosal gland.

w29 Tongue

The bulk of the tongue is composed of skeletal muscle and is under conscious control. The muscle fibers run in many different directions. Click link [1]. For this lab we will concentrate on the papillae or projections on the surface of the tongue. There are four types of lingual papillae located on the dorsal and lateral aspects of the anterior two thirds of the tongue; filiform, fungiform, foliate and circumvallate. This section of rat tongue shows two types. Zoom out once and click [2,1]. Highlight the fungiform and the filiform papillae in the keratinized epithelium.

Filiform papillae are the most numerous. Click link [1] for a higher magnification view. These papillae are slender and have a thick keratinized (in rat tongue) stratified squamous cap. They make the tongue surface rough. There are no taste buds associated with filiform papillae. Zoom out once and click link [2]. Fungiform papillae resemble mushrooms sitting on the tongue surface. They have a non-keratinized stratified squamous surface, a loose connective tissue-filled core and may have taste buds on the dorsal aspect of the cap.

b28 Tongue

Click link [1]. Highlight the foliate papillae. These are located on the posterolateral aspect of the tongue and are numerous in the rabbit. They appear in rows or furrows on the surface of the tongue. Foliate papillae have taste buds in the human neonate, but they probably degenerate with age. Minor salivary glands called glands of von Ebner empty into the base of the furrows. Highlight these. They secrete a watery fluid that helps to dissolve food constituents, helping to taste our food.

Click link [2,1]. Highlight the taste bud and the opening of the duct of the gland of von Ebner. Zoom out twice and click [1,1] to see more taste buds. The taste buds are groups of 30-80 elongate neuroepithelial cells, that extend from the basal lamina to the taste pore. Highlight the taste pores.

Now practice with this unlableled virtual slide: 102 Tongue Dog. Find the following: filiform papillae, skeletal muscle, fungiform papillae with taste bud (only one).

w31 Submandibular gland

Click link [1]. The submandibular gland is similar to other exocrine glands composed of lobes. Each lobe is divided into many lobules by connective tissue septa that are continuous with the outer capsule. Find a lobe and lobule.

Click link [1,2,1]. There are two types of secretory cells in the submandibular salivary glands; mucous cells and serous cells. Serous cells have a triangular shape and a round basally located nucleus. In the apical portion of the cell are deep purple secretory granules that contain enzymes. Highlight the granules. Serous cells form serous acini. Highlight an acinus. Zoom out once and highlight these again. Click [1]. Mucous cells are similar in shape to the serous cells but are very pale staining because the large mucous secretory granules have dissolved away during tissue preparation. The nucleus is also basally located, but is flattened instead of round. Mucous cells form mucous acini. Highlight these cells. The mucous acini are often surrounded by the serous acini which may form so-called demilunes.

The duct system of the submandibular gland is highly branched. The secretory acini empty into the intercalated duct. Zoom out once and highlight the intercalated duct. The wall of the ducts are composed of a single layer of low cuboidal epithelium. Zoom out once and click [1]. Highlight the intercalated duct in this view. Several intercalated ducts join together to form a striated duct. Highlight this. These ducts are lined by cuboidal to low columnar cells. Click link [1]. The striations result from the large number of infoldings of the basal plasma membrane where the mitochondria reside. The saliva that is produced by the acinar cells is modified by the cells of the striated ducts. The ducts remove sodium and chloride ions and replace them with potassium and bicarbonate ions.

Click the thumbnail and then [1,2]. Highlight the striated (intralobular) duct in this view. The striated ducts from several lobules join to form interlobular ducts. Highlight this.

For practice, start with this slide: 106 Salivary gland Submandibular. This slide has large enough tissue to show lobes and lobules and the associated intra and interlobular ducts. Identify the following: Lobe, lobule, mixed acinus, mucus cell, serous cells (serous demilunes), myoepithelial cell, intercalated duct, striated duct, intralobular duct (just starting to show stratified cuboidal epithelium), interlobular duct (more connective tissue and mostly stratified cuboidal epithelium), septa connective tissue. Continue with this slide: 136 Salivary Gland Parotid. This slide has large enough tissue to show lobes and lobules and the associated intra and interlobular ducts. Identify the following: lobe, lobule, serous acinus, zymogen granules, myoepithelial cell, short intercalated ducts, striated ducts (artifacts of fixation pull the epithelium away from basement membrane), intralobular duct, interlobular duct, septa connective tissue. and identify the following structures: lobe, lobule, serous acinus, zymogen granules, intercalated duct, striated duct, intralobular duct, interlobular duct, septa connective tissue.

w47 Gall bladder

Click link [1]. Highlight the epithelium, muscle and peritoneal layers. The epithelium is folded so that the gall bladder can expand when filled with bile. Click [1,1]. The epithelium is simple columnar and mostly absorptive. The smooth muscle propels bile into the cystic duct when needed. (Humans have a thicker muscle layer.) Zoom out once. Highlight the structure labeled epithelial pouch. This resembles a gland, but is really a cross-section of a fold in the epithelium.

Use this unlabeled image for practice: 133 Liver & Gallbladder. This slide is only good for gallbladder because the liver is not well fixed and overstained. Mucosa of gallbladder is folded and the epithelium is simple low columnar with microvilli (small and hard to see). Identify the following: gallbladder structure, epithelium, lamina propria, note the wall of smooth muscle, note the inflammatory cells in the lamina propria (a gland that is often inflammatory).

w48a Pancreas

The pancreas is a large, lobulated gland composed of two distinct portions. The bulk of the pancreas is exocrine and secretes a bicarconate-rich fluid containing digestive proenzymes (zymogens) into the duodenum via a duct system. The endocrine pancreas is formed of islets of Langerhans which secrete hormones in paracrine fashion or directly into the blood stream.

Click link [1]. Highlight the islets of Langerhans which are pale staining aggregates of small cells scattered throughout the exocrine portion of the pancreas. Click link [3] for a higher magnification of view of an islet and the surrounding exocrine pancreas.

Zoom out once and click [1,1,1]. showing a pancreatic acinus. Using the drop-down menu highlight each identified structure. The acinus is composed of an irregular cluster of secretory acinar cells that are roughly pyramidal in shape. They have intensely acidophilic secretory granules in the apical portion of the cell (that are staining a dark magenta in this section). These granules contain the precursors of digestive enzymes (zymogens). The RER is basophilic. In the lumen of each acinus is a centroacinar cell, which is the beginning of the duct system and is characteristic of the pancreas. Zoom out once and click link [2]. Highlight the zymogen granules and the basophilic RER.

Click the thumbnail and then [1,2,2] to see a high magnification view of a pancreatic duct. The duct carries zymogens exported from acinar cells and also secretes bicarbonate. The pancreas does not contain striated ducts.

Zoom out once and click link [1]. Highlight the islet of Langerhans. The islets are composed of alpha, beta, delta, PP and G cells, although they can only be distinguished by special staining. Notice the many capillaries surrounding and within the islet.

Use this unlabeled virtual slide to practice: 122 Duodenum & Pancreas. This slide shows the pancreas attached by connective tissue to the side of the dog duodenum. It is somewhat a thick section, so the staining is dark. Some areas of the pancreas did not section well and show chatter lines, so it is best to stay away from that area. Identify the following: pancreas, lobules, islets of Langerhans, acinar cells, zymogen granules, centroacinar cell, collapsed intercalated duct and epithelial cell nuclei, large pancreatic duct, very large common bile duct. Continue with this slide: 197 Pancreas Monkey. This slide shows two large lobes of the pancreas and in one lobe the large central pancreatic duct is seen surrounded by an abundance of connective tissue and wall of smooth muscle. Note the height of the epithelium compared to the inter and intralobular ducts and intercalated ducts. At higher magnification you should be able to identify the following: Islets of Langerhans, acinus, acinar cells, zymogen granules, hematoxylin stained RER, centroacinar cell, intercalated duct (sometimes collapsed, but some with small lumen), intralobular duct with cuboidal epithelium and small amount of connective tissue, and interlobular duct (with more connective tissue and smooth muscle and beginning to show stratified cuboidal epithelium), and one large pancreatic duct.

b26 Tooth

This is a ground specimen of an incisor and the cells have mostly fallen off. We will discuss the layers of the mature tooth from the crown to the root. Highlight the enamel. The enamel is the hardest substance in the body. It consists of 96% calcium hydroxyapatite and is translucent. Enamel is laid down by ameloblasts as the tooth is developing. Click link [1]. The enamel is formed in segments, and can vary in thickness if nutrition is compromised. The segmental formation of enamel results in striations called lines of Retzius. Highlight these. When the tooth erupts the ameloblasts die, and thus enamel cannot be repaired.

Dentin forms the main substance of the tooth. It is laid down by odontoblasts that survive for the life of the tooth. Their cell bodies are located at the periphery of the pulp, and their cytoplasmic extensions occupy tunnel-like spaces called dentinal tubules within the dentin. Highlight the dentinal tubules.

Zoom out once and highlight the cement which is harder than bone but resembles it. The cement covers the root of the tooth. Collagen fibers from the periodontal ligament are embedded in the cement and alveolar bone and suspend the tooth within the bony socket. Cement is laid down by cementoblasts that remain viable through the life of the tooth.

b27 Developing tooth

Highlight the lower jaw and the tongue. We will be looking at a tooth from the upper jaw. Click link [1,1]. As the structures are discussed highlight them using the drop-down menu. Tooth development begins between the 6th and 7th week of gestation. The migration of neural crest cells to the oral mesenchyme induces the oral epithelium (ectoderm) to proliferate and form the dental lamina. The inferior aspect of the dental lamina proliferates and forms a structure known as the cap. There is a convex simple squamous outer dental epithelium and a concave simple squamous inner dental epithelium. These layers form the enamel organ. The inner epithelium will form the future ameloblasts. The cluster of cells encased by the concave inner epithelium is the dental papilla and is derived from neural crest mesoderm. The dental papilla forms the dentin and pulp of the mature tooth.

b27a Developing tooth

This tooth is at a slightly later (so-called bell) stage of development than b27. Click link [1]. Identify the enamel organ and the dental papilla. Click link [1]. As the tooth develops the simple squamous cells of the inner dental epithelium differentiate into enamel-producing columnar cells called ameloblasts. The neural crest mesoderm differentiates into odontoblasts and starts producing dentin. Dentin production induces enamel production by the ameloblasts. Highlight these using the drop-down menu.

Now practice with this virtual slide: 104 Tooth. The growing tooth shows the Ameloblasts on top of the tooth cap (the part that will penetrate the oral epithelium). Identify the following: stellate reticulum, ameloblasts, pre-enamel, odontoblasts, dentin.

w44 Liver (human)

Click link [2]. Highlight the hepatic lobule. The "classic" hepatic lobule has as its center a branch of the hepatic vein, called the central vein. This is not clearly visible in this view. At the periphery of the lobule are the portal triads, consisting of branches of the portal vein, hepatic artery, and bile duct. (Other types of liver lobules have been defined; see your text for those descriptions). The notion of a lobule in the human liver is rather artificial because there are no connective tissue septa (as in pigs or in the human submandibular gland) that physically separate the lobules. Click link [1] and highlight the three structures of the portal triad. The portal vein is the largest vessel in the triad and has an irregular lumen. The bile duct is lined by cuboidal epithelium. The hepatic artery branches usually have a small amount of smooth muscle in their wall and much smaller lumen than the portal vein.

Zoom out twice and click link [1]. This is another view of a portal triad. Highlight the three structures in the triad. The hepatocytes directly surrounding the triad are called the limiting plate. Highlight the limiting plate. Some authors consider it to contain hepatic stem cells. When injury occurs to the liver, it will regenerate more fully if the limiting plate hepatocytes are intact and undamaged.

Zoom out once and click [2,2,1,1]. This demonstrates the general structure of the liver. Hepatocytes form the parenchyma of the liver. Highlight the parenchyma. Hepatocytes form anastomosing plates usually one, or at most two, layers thick, between which the blood passes within the sinusoids. There are also liver macrophages (Kuppfer cells) and endothelial cells lining the sinusoids. Hepatocytes are frequently binucleate. Highlight this cell. One function of the liver is to produce lipoproteins. In humans lipid droplets are frequently seen within the hepatocytes.

Sinusoids are a subtype of capillaries. Typical sinusoids have a fairly large, irregularly shaped lumen, an endothelium that has fenestrae, and lack basal lamina (the last two are not visible at the LM level). Between the endothelial cells and the hepatocytes there is a space filled with plasma, the space of Disse (also not visible at the LM level).

w45a Liver (dog)

With the lowest power notice the uneven staining of the liver. Unevenness of the staining reflects the blood supply (and thus the nutritional and oxygen supply) of the hepatocytes.

Click link [3]. Highlight the classic hepatic lobule. Notice that the points of the lobule are contained in the darker staining areas. Click link [2]. In the middle of a darker staining area there is usually a portal triad: hepatic artery, portal vein, and bile duct. Click link [1]. Highlight the elements of the portal triad. Often there are small lymphatic vessels in this area as well.

Click the thumbnail and click link [1,4]. The central vein is very prominent in this view. This is the first vein of the efferent venous system within the liver. This is a normal vein with a thin wall. Continue with [1] to view macrophages within the sinusoids and mast cells.

Zoom out twice and click link [1,1,1]. Study the sinusoids, which are a subtype of capillary and in this view have red blood cells in their lumen. There are macrophages that lie in the sinusoids or on the wall with the endothelial cells. Mast cells are also numerous and a few lymphocytes can be found. Some of the lymphocytes are Natural Killer cells. Many of the hepatocytes have small lipid droplets in the cytoplasm. Zoom out once and click link [2] to see phagocytosed red blood cells.

w45b Liver (guinea pig)

Click link [1,1,1,1]. This slide demonstrates the bile canaliculi well. Highlight the bile canaliculi and the sinusoids. The cross-sections of bile canaliculi appear as small, clear holes between hepatocytes. Longitudinal sections of bile canaliculi appear as narrow tubes between the hepatocytes. Highlight the binucleate hepatocytes, typical of a normal liver. Click link [1] and find the bile canaliculi again.

Time to practice with unlabeled images. Start here: 138 Liver Rat Plastic. This slide is of tissue that was fixed by perfusion of the blood vessels, so no blood is seen and this makes it more difficult to see some cells in the sinusoids (such as the Kupffer cells). However, resolution of the remaining cell types is excellent. Perfusion pressure made some of the central veins to appear quite large, so don't confuse with Portal veins. Identify the following: Hepatocytes, hematoxylin stained RER, bile canaliculus, binucleated hepatocytes, polyploidy in hepatocytes, sinusoids, endothelial cell, portal triad, portal vein, bile duct, hepatic artery, periportal limiting plate, central vein, stem cell area. Continue with 130 Liver Pig. This slide helps to see the classic hepatic lobule because the pig has more connective tissue following the branches of the portal veins between the portal triads. Find the following: hepatic cords, classical hepatic lobule, central vein (venule), hepatocytes, sinusoids, portal triad, portal vein, bile duct, hepatic artery (typically much smaller but with 1-2 layers of smooth muscle).