Plates 14-1 through 14-6, pp. 300-311

Ch 15, Gastrointestinal System

Overview:

Upper GI tract: Esophagus and Stomach

Lower GI tract: Small and Large Intestine

The basic goal of these two labs is to learn the microscopic anatomy of the alimentary tract.  The labs involve examining a number of slides from different regions of the tract, so there is a lot of information to assimilate. Here are a few tips that should be helpful in processing this potentially confusing material:

1. First, always scan each section at low power.  Look for obvious junctions, for example between the esophagus and stomach.


2. Next, identify the four layers characteristic of the G.I. tract: mucosa, submucosa, muscularis externa, adventitia/serosa.   If these 4 layers are present you must be viewing part of the alimentary tube.


3. Once you have identified the 4 layers of the G.I. tract, look for the key structural features within each layer for each specific region of the tract.  For example, stratified squamous epithelium lines the esophagus and anal canal, whereas simple columnar epithelium lines the stomach, small intestine, and large intestine.  However, these GI organs with simple columnar epithelium can be distinguished by several criteria, most importantly by differences in mucosal folding.  There are pits leading into coiled glands in the stomach, villi and straight glands (crypts) in the small intestine, and straight glands (crypts) in the large intestine.   Thus villi are unique to the small intestine. TO HELP YOU KEEP TRACK OF CHANGES IN THE VARIOUS LAYERS, CLICK HERE FOR AN EXCEL SPREADSHEET THAT YOU CAN FILL IN AS YOU WORK THROUGH THESE SLIDES (click here to see a filled in version to check your work).


4. Cross-sectioned villi and glands will both appear as circular structures.  However, in villi the surface epithelium will be seen enclosing cellular connective tissue (lamina propria), whereas in glands the epithelium will ring open spaces--the lumens of the glands.


5. Use the orientation of the muscle layers of the muscularis externa to determine the plane of section for each slide. A cross-section of the tube will show longitudinally-sectioned muscle in the inner layer and cross-sectioned muscle fibers in the outer layer, whereas these relationships are reversed in a longitudinal section.  (An oblique section of the tube cuts both muscle layers obliquely).


6. The textbook, lecture Powerpoints, and lecture notes should be helpful during slide examinations.

Slide Descriptions

I. Esophagus

Slide UCSF 226 (esophagus, H&E) WebScope ImageScope
Slide UMich 126 40x (trachea & esophagus, H&E) WebScope ImageScope
Slide UMich 153 20x (esophagus, H&E) WebScope ImageScope
Slide UMich 155 40x (gastro-esophageal junct, H&E) WebScope ImageScope

For the purpose of histological descriptions, the esophagus is subdivided into upper (entirely skeletal muscle in the muscularis externa), middle (mixed smooth and skeletal muscle) and lower (entirely smooth muscle) portions. Slide UCSF 226 is from the upper 1/3; slides 126 and 153 are from the middle 1/3; and slide 155 is from the lower 1/3 (at the esophageal-cardiac junction). The esophageal epithelium [example] is the non-keratinized stratified squamous type and is supported by a connective tissue lamina propria. Note the presence of isolated lymphoid nodules [example] and scattered leukocytes in the lamina propria. Small mucous glands may be found scattered in the lamina propria of the upper and lower esophagus in some of our glass slides. A rather thick layer of longitudinally arranged smooth muscle fibers form the muscularis mucosae [example]. The connective tissue of the submucosa consists of mostly collagenous fibers with some elastic fibers and varying amounts of fat as well as submucosal sero-mucous glands which can be readily observed in both slide 126 [example] and slide 153 [example] (those in slide 155 are not very well preserved).

In the upper esophagus, as shown in Slide UCSF 226, the muscularis externa consists of both inner and outer layers of skeletal muscle only. In the middle esophagus, the muscularis externa contains a mixture of skeletal and smooth muscle as seen in slide 126 [example], whereas in the lower esophagus only smooth muscle is found as seen in slide 155 [example]. Present in all regions of the esophagus (upper, mid, and lower) is the myenteric (Auerbach’s) plexus [example] between the two layers of the muscularis externa (W pg 267, 14.3). For most of its extent, the esophagus is retroperitoneal, so its outermost layer consists of a connective tissue adventitia which merges with the adjacent connective tissue associated with nearby structures (such as the trachea as shown in slide 126). Below the diaphragm, however, the esophagus is suspended within abdominal cavity and is therefore covered by a connective tissue serosa as shown in slide 155.

II. Stomach

A. Cardiac glands (cardiac region of stomach)
Slide UMich 155 20x (gastro-esophageal junct, H&E) WebScope ImageScope
Slide UMich 155 40x (gastro-esophageal junct, H&E) WebScope ImageScope

These mucosal glands of the stomach are composed mainly of mucous cells with pale staining cytoplasm and basally located nuclei. Present, but not seen, are stem cells and endocrine cells. In slide 155, locate the cardio-esophageal junction (W pg 268, 14.6) [example]. Note the abrupt transition from the mucosa of the esophagus with its stratified squamous epithelium to the glandular mucosa of the stomach. The cardiac gastric glands [example] are present only in a very small segment of the stomach mucosa adjacent to this junction. They are mucous glands comprised of a HOMOGENEOUS population of pale-staining columnar cells with a “bubbly” supranuclear cytoplasm. As you move further into the stomach, the cardiac glands are very quickly replaced by gastric proper (or fundic) glands which, as described below, consist of a notably HETEROGENEOUS mixture of basophilic chief cells and eosinophilic parietal cells.

B. Gastric glands (fundus and body of stomach)
Slide UMich 155 40x (gastro-esophageal junct, H&E) WebScope ImageScope
Slide UMich 156 40x (stomach, H&E) WebScope ImageScope
Slide UMich 157 40x (stomach, H&E) WebScope ImageScope

“Gastric” or “fundic” glands are characteristic of the mucosa throughout the fundus and corpus regions of the stomach. At low magnification, notice that the gastric pits are relatively more shallow here [example] and the tubular gastric or fundic glands are relatively much longer than those in the cardia or pylorus. Observe that the tall columnar cells lining the luminal surface and pits have basally located nuclei and lightly staining cytoplasm (W pg 270, 14.10 a, b). Most of these cells secrete mucus, that is stored in the apical cytoplasm. You can see these cells in the stomach preparation stained with PAS (which will stain mucus and other glycoproteins) in slide 160 and in Wheater’s pg. 270, 14-10c). In this slide, the mucous cells of the gastic pits stain quite intensely. Also identifiable are lighter-staining “mucous neck cells” present in the neck region of the gastric glands and mucus-secreting cells of the cardiac glands found near the gastro-espophageal junction.

Moving into the body of a fundic gland, identify the parietal and chief cells in slides 155 [example] [ORIENTATION], slide 156 [example] [ORIENTATION], and slide 157 [example] [ORIENTATION]. Parietal cells are large, ovoid to pyramidal shaped cells with their broad side adjacent to the basement membrane. Each cell contains a round, centrally located nucleus and reveals a lightly eosinophilic cytoplasm that appears granular due to the presence of many mitochondria. The chief cells are present in the lower one-third to one-half of the gastric glands. The apical cytoplasm of these cells may appear granular due to the presence of zymogen granules, that may stain bright red in H&E preparations (as in slide 157). However, in other preparations (slides 155 and 156), the pepsinogen has been extracted and the empty secretory granules resemble many glass beads in the supranuclear cytoplasm. The base of the cytoplasm, on the other hand, is distinctly basophilic. 

Other cells of the gastric glands, such as undifferentiated (stem) cells and various endocrine cells, will NOT be studied in this laboratory session as they are not readily identifiable in the stomach, but you should be aware of their general characteristics. Notice the presence of a large number of lymphocytes and plasma cells in the lamina propria between the gastric glands, and, in some areas, aggregated as lymphoid nodules [example] (which will be studied in greater detail along with other lymphatic tissues but you should at least be able to identify them here). Also, notice that the strands of smooth muscle fibers from the inner layer of the muscularis mucosae extend between the glands toward the surface. The contraction of these strands may help the emptying of the glands.

C. Pyloric glands (pylorus or antrum of stomach)
Slide 162 40x (gastro-duodenal junct, H&E) WebScope ImageScope
Slide UCSF 242 40x (pyloro-duodenal junction, H&E) WebScope ImageScope

In slides 162 and UCSF 242, you can see the transition from pylorus of the stomach to duodenum of the small intestine. The pyloric region of the stomach is characterized by a thick wall due to the presence of the pyloric sphincter muscle [example], which is comprised primarily of the inner circular layer of the muscularis externa. Compare its wall thickness with that of the adjacent duodenum (W pg 273, 14.15). The pyloric glands [example] at the base of each gastric pit [ORIENTATION] are also composed again of a mostly HOMOGENEOUS population of mucous cells that are similar in appearance to those in cardiac glands although the pits are much deeper compared to cardiac glands. Present, but not seen, are stem cells and endocrine cells. An occasional parietal cell may be also found. Note that the bases of the pyloric glands abut the muscularis mucosae whereas in the duodenum, you will see abundant glands (Brunner’s glands) DEEP to the muscularis mucosae (i.e. in the SUBMUCOSA).

Just in case you are worried about identifying cardiac glands versus pyloric glands, it is admittedly very difficult to tell these two apart based only on high-mag views, but you can always use contextual information to help you out: cardiac glands will be right near the gastro-esophageal junction whereas pyloric glands are at the gastro-duodenal junction.

III. Small Intestine

A. Duodenum
Slide UCSF 242 40x (pyloro-duodenal junction, H&E) WebScope ImageScope
Slide UMich 162 40x (pyloro-duodenal junct, H&E) WebScope ImageScope
Slide UMich 161 40x (pylorus, duodenum, pancreas, H&E) WebScope ImageScope

Look at slide UCSF 242 first. Locate the duodenal portion in this slide and notice the presence of submucosal mucous glands (Brunner’s glands).  Observe that the ducts of these glands (and, occasionally, some acini) penetrate the muscularis mucosae and open into a crypt of Lieberkühn. After viewing slide 242, move to slides 162 and 161 and try to find the duodenal region in these tissue sections.

2. Jejunum and ileum
Slide UMich 29 40x (jejunum, monkey, H&E) WebScope ImageScope
Slide UMich 168 40x (ileum, H&E) WebScope ImageScope
Slide UCSF 246 40x (jejunum, H&E) WebScope ImageScope
Slide UCSF 247 40x (jejunum, H&E and Ammonium-Silver Nitrate) WebScope ImageScope

View these sections with the low power objective and identify the mucosa, submucosa and the muscularis externa.  Note that the mucosa consists of three sub-layers:

1. epithelium
2. lamina propria (or lamina propria mucosa –”propria” means “belonging to”
3. muscularis mucosae (or lamina muscularis mucosae –”mucosae” here is not plural, but genitive, so this literally means “muscular layer of the mucosa”)

The mucosa, which is clearly demarcated from the submucosa by the prominent muscularis mucosae layer, frequently shows heavy lymphocytic infiltration in the lamina propria.

The appearance of the submucosa layer is a bit variable, but, in general, it’s best considered as irregular connective tissue: in slide 29 the submucosa appears more “loose” whereas in slides 168 and 170 it is more dense, and, in slide 169, here the submucosa is edematous and exhibits unusually dilated blood vessels.  You can see the intestinal villi and intestinal glands (crypts of Lieberkühn).  Examine the villi at a higher magnification and note that the lining epithelium consists of simple columnar cells (aka enterocytes) with a brush border and interspersed goblet cells, particularly well-demonstrated in slide 168.  You can observe the distribution of goblet cells in the intestinal epithelium stained with PAS (slide 165).  

The epithelium lining the villi continues into the intestinal glands.  Examine several of these glands in slide 169 and note that goblet cells and enterocytes similar to those lining the villi, cover the upper portions of the gland.  Also, notice that there are many mitotic figures [example] .  The cells which line the lower portions of the crypts are less well differentiated.  You may be able to see the enteroendocrine cells [example] in this region.  These are the cells with spherical nuclei and clear cytoplasm –the secretory granules of these cells are not always stained very well, but, if they are, you should note that the granules are oriented basally.

Slides UCSF 246 and UCSF 247 have some excellent examples of enteroendocrine cells [example]. Again, the enteroendocrine cells have a clear cytoplasm and, if visible, basally-oriented granules. Slide 247 in particular has been stained with ammonium silver nitrate to demonstrate so-called “argentaffin” cells [example] (which, incidentally, are now known to be “S” or serotonin-secreting enteroendocrine cells –the serotonin in these cells reacts with the silver causing a black precipitate to form). Note that there are about 20 different types of enteroendocrine cell, and you are NOT expected to be able to identify a specific type of enteroendocrine cell (e.g. the “S” cells described above), but you should know the general histological characteristics and functions of enteroendocrine cells as a whole.

Paneth cells [example] occupy the base of the intestinal cypts/crypts of Lieberkuhn.  They are fairly well preserved in slides 168 and 29.  These cells are pyramidal shaped with round nuclei located near their base.  They contain brightly eosinophilic (almost orange) secretory granules in the apical cytoplasm.  In slide 168, the secretory granules in the Paneth cells stain a refractory brown or green. 

Just under the mucosal epithelium is the lamina propria (or lamina propria mucosa), which consists of loose connective tissuethat fills the spaces between the intestinal glands and forms the cores of the intestinal villi.  Within the core of each villus is a central lacteal, capillaries, and delicate wisps of smooth muscle that extend from the muscularis mucosae below.  However, in some regions, the lamina propria may be so packed with a heavy infiltration of lymphoid cells that these finer structures may not be visible.  You may hear the term “Peyer’s patches” used to describe such regions in the GI tract.  However, technically, Peyer’s patches are found ONLY in the ileum and they are big enough to be visible with the naked eye.

The muscularis mucosae (or laminae muscularis mucosae) consists of smooth muscle fibers.  Observe that strands of smooth muscle fibers from the muscularis mucosae extend into the cores of the intestinal villi along the central axis.  Contractions of this muscle layer are controlled by ganglion cells and nerve fibers of the submucosal (Meissner’s) plexus [example] located in the submucosa, (W pg 266, 14.4a). The muscularis externa consists of two layers of smooth muscle: inner circular and outer longitudinal.  Observe the ganglion cells and nerve fibers of the myenteric (Auerbach’s) plexus [example] located between the two muscle layers.

IV. Large Intestine

A. Colon
Slide UMich 176 40x (colon, H&E) WebScope ImageScope

The mucosa of the colon is lined by a simple columnar epithelium with a thin brush border and numerous goblet cells. Note that there are no plicae or villi.  The crypts of Lieberkühn are straight and unbranched and lined largely with goblet cells.  In many regions the mucus is partially preserved and stains with hematoxylin.  At  the base of the crypts, undifferentiated cells and endocrine cells are present; however,  Paneth cells are not usually present.  The appearance of the lamina propria is essentially the same as in the small intestine:  Leukocytes are abundant and the isolated lymphoid nodules present in this tissue extend into the submucosal layer.  The muscularis mucosae is a bit more prominent compared to the small intestine, and consists of distinct inner circular and outer longitudinal layers.  The submucosa of this specimen is particularly well fixed such that you may better appreciate the mixture of irregular connective and adipose tissue, numerous blood vessels, and several excellent examples of ganglion cells and nerves of the submucosal plexus.  The muscularis externa of the large intestine is different from that of the small intestine in that the outer longitudinal layer of smooth muscle varies in thichness and forms three thick longitudinal bands, the taeniae coli (taenia = worm). This section happened to be cut such that a piece of one of these longitudinal bands may be seen.  

B. Recto-anal Junction
Slide UMich 177 40x (recto-anal junct, monkey, H&E) WebScope ImageScope
Slide UMich 177-2 20x (recto-anal junct, human, trichrome) WebScope ImageScope
Slide UMIch 177-3 20x (recto-anal junct, human, H&E) WebScope ImageScope

Look at these slides (especially slide 177-3) at low magnification first to locate the recto-anal junction. Here you will observe a narrow zone of transition from the simple columnar epithelium of the intestine to the keratinized stratified squamous epithelium of skin.  Within the transition zone, you may find stratified columnar (or sometimes cuboidal) epithelium followed by nonkeratinized stratified squamous epithelium. While looking, move the image from the colon toward the direction of the recto-anal junction.  Observe that the crypts become shorter and shorter, eventually disappearing near the junction.  Also observe that the muscularis mucosae becomes tattered and disappears, allowing the lamina propria merge with the underlying submucosa in this area.  Note the presence of a large number of submucosal veins [example].  When these veins become dilated and varicose, they cause the mucosa to bulge and create the condition commonly known as hemorrhoids.  Examine the skin lining the anal region and observe sebaceous and sweat glands, hair follicles, etc., particularly evident in slide 177-2 [example].  Also, note the massive amount of smooth and skeletal muscle that form the internal [example] and external anal sphincters [example], respectively. The primate specimen (slide 177) shows these muscles quite well.