DIGESTIVE SYSTEM

Objectives

You should be able to recognize the following structures of the GI tract:

                     

General                                                

Small intestine

mucosa

plica circularis 

lamina propria

villus

muscularis mucosa

enterocytes

submucosa

Brunner's glands

muscularis externa

crypts of Lieberkühn

myenteric plexus

Paneth cells

adventitia/serosa enterocytes

 

Large intestine

Esophagus

glands

submucosal glands

tenia coli

skeletal and smooth muscle

appendix

 

Stomach   

Ano-rectal junction

rugae

rectum

gastric pits

anus

surface mucous cells

hemorrhoidal veins

mucous neck cells

gastric glands

 

chief cells

parietal cells

pyloric sphincter

 

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Blue Histology


Slide descriptions
D-101Esophagus (H&E)
D-103Fundic stomach, monkey (H&E)
D-105Pyloric-duodenal junction, human (H&E)
D-106Upper duodenum and pylorus, monkey (H&E)
D-107Lower duodenum (H&E)
D-111Appendix (H&E)
D-114Colon. (H&E)
D-115Ano-rectal junction (H&E)

Optional slides
D-104Stomach, monkey (PAS stain)
D-108Duodenum, monkey (Masson stain)
D-109Duodenum, rabbit (FeH)
D-110Ileum (H+E)
D-113Colon ,monkey (Muc. Carm.)
D-135Thyroid, trachea, esophagus (PASH)


Prologue

Although at first it might seem most sensible to start histological studies of the gut with the esophagus and simply progress along the digestive tract to the rectum, it is, in fact, the small intestine which demonstrates the fundamental features of this system most fully. If one understands this organ, it is easy to understand other parts of the alimentary canal as variant specializations. For this reason we are asking you to begin your study with the duodenum, proceed with the stomach and esophagus and finally, study the colon and rectum.

The diagram on the facing page of the printed syllabus labels the basic organizational features of the duodenum. A very important distinction for you to make is between mucosal and submucosal structures. The diagram clearly shows the difference between mucosal and submucosal glands. Make sure that you also understand why villi are mucosal projections and plicae are submucosal.


Slide descriptions

D-107 Lower duodenum (H&E)

This is an unusually well preserved example of human lower duodenum, sectioned longitudinally. The wall is thick, but much of its thickness is accounted for by a succession of transverse ridges called the plicae circularis. Under low power it becomes evident that plicae are folds in the submucosa covered with mucosa (epithelium, lamina propria and muscularis mucosa) (illustration). They are covered on their surface by a multitude of villi. These are smaller protrusions of mucosa made up of only epithelium and lamina propria. The lamina propria is sharply demarcated from the underlying submucosa and the characteristics of these two connective tissues are totally different (illustration). While the submucosa is rather ordinary-looking areolar tissue, the lamina propria is highly cellular with many lymphocytes. A thin layer of muscularis mucosa marks the base of the lamina propria. Try to imagine what this tissue would look like if it were sectioned transversely instead of longitudinally.

Most, but not all, of the interesting specializations of different parts of the alimentary tract are in the mucosa, so turn now to higher magnifications and study its details. Here, in the lower duodenum, stubby and relatively inconspicuous glands (crypts of Lieberkühn) invade the mucosa between the villi down close to the muscularis mucosa. The cells at the bottom of the glands (Paneth cells) contain red secretory granules in their apical cytoplasm. These may be visible under high power, but in some slides the granules have been leached out, leaving a foamy-looking apical cytoplasm instead. The epithelium of the villi is high columnar. At the lumenal surfaces. Its predominate cells are enterocytes (also called absorptive cells in view of their main function). They are noted for their short brush-border, which is visible with high magnification. Distinctively shaped goblet cells also are sprinkled through this epithelium. Their upper parts are swollen and their cytoplasm appears more textured than that of the enterocytes. Blebs of mucus may have oozed from their apexes. The nuclei of goblet cells are distinctively heteropyknotic (compact and darkly staining) and triangular in shape, in contrast with the paler, oval nuclei of enterocytes. The population of cells in the lamina propria of the villi also should be examined, at least briefly. Lymphocytes and plasma cells dominate, but some fibroblasts and smooth muscle cells can be seen (illustration). Both capillary and lymphatic channels richly supply this region, so that many of the elongate nuclei are endothelial. Eosinophils are often seen in the lamina propria, although perhaps not in your slide. Lymphocytes commonly invade the overlying epithelium. Some eventually to work their way across the epithelium to be discharged into the gut lumen. In places with proper orientation, a well-defined basement membrane can be seen underlying the epithelium.

Turning to the submucosal region, follow this layer from the core of the plicae to the tunic beneath the plicae where a rich plexus of large blood vessels and lymphatic channels is visible under low power (illustration). These reveal the main function of the submucosa. Outside of this layer lie the two principal muscle layers of the wall structure. The inner circular layer of the muscularis externa is seen here in cross section and the outer longitudinal layer is sectioned longitudinally. This smooth muscle is regulated by a well developed plexus of autonomic ganglia which you studied earlier as a component of the peripheral nervous system.  Review it.  Even under relatively low power, random sections through the myenteric nerve plexus can be seen as pale islands of cells, sandwiched between the two muscle layers. View them at higher power (illustration).  What types of cells do they consist of?  While harder to identify, nerve ganglia of the smaller submucosal plexus (example 2) are present in the submucosa. Some of you will enjoy the challenge of looking for them under the microscope.

The two plexuses also have been named after the persons who described them, Auerbach and Meissner. Increasingly histologists are trying to get away from personal names for structures. Thus, you are forward looking if you call Meissner's plexus the myenteric plexus and Auerbach's the submucosal plexus.

D-108 Duodenum, monkey (Masson stain)

The lower duodenum of the monkey was stained with a trichrome procedure for this slide. It demonstrates most of the features that you saw in the last slide. The tissue has been cut between the plicae and shows only villi. How do you know that this is a cross section? Paneth cells are few and far between but goblet cells are highly visible.

This slide reveals the main function of the crypts: the production of cells. The stem cells for the epithelium of the gut are sequestered in the glands. Here they are far more sheltered than they would be on the surface. All cell division in the epithelium takes place in the middle to lower part of the glands. The new cells push the old ones along out of the glands and up over the villi. Here they are shed. Cells last only several days on the villi so their progenitors must divide actively in the glands. The same stem cells give rise to enterocytes, goblet cells and Paneth cells.

You can see mitotic figures in the crypts even at fairly low power. When the chromosomes have condensed on the metaphase plate they show up as dark lumpy masses. These look entirely different from the pale staining, large nuclei of the surrounding interphase cells. The nuclei tend to be located at the basal end of the cells during interphase. When the cells go into mitosis, the nuclei migrate to the apical part of the cell, making it even easier to pick them out. Those of you who have had a genetics course may be interested looking for cells in various stages of mitosis; prophase, metaphase and anaphase. For others, just note that, indeed, cell division is confined to the lower parts of the glands and keep an eye out for mitotic figures in the crypts on other slides.


D-106 Upper duodenum and pylorus, monkey (H&E)

One end of this slide illustrates features of the upper duodenum (in an animal much smaller than the human). The other end is the pylorus of the stomach. In between, the inner circular layer of the muscularis externa is greatly expanded to form the pyloric sphincter. A mass of mucous glands, Brunner's glands, fills the submucosa. They open into the intestinal lumen and constitute the distinctive specialization of the upper duodenum. In the human, Brunner's glands extend down about 10 cm from the sphincter. Villi are not particularly well developed in the upper duodenum. All of this can be seen to advantage at low power.

The stomach side shows the appearance of the pylorus.  The pits are deep and the glands have only mucous secretory cells. Otherwise the wall has the same fundamental layers as the rest of the GI tract: mucosa with muscularis mucosa, submucosa, muscularis externa and serosa. 

D-105 Pyloric-duodenal junction, human (H&E)

This slide shows the enormously greater development of the pyloric-duodenal junction in the human than in a small monkey. The downside of the slide is that the preservation is poorer. In particular, the epithelium at the junction of the two types of mucosae over the sphincter has been lost. Fortunately, you saw that sharp transition on the previous slide.

Observe the much greater development of Brunner's glands than in the last slide. They fill up the lamina propria as well as the submucosa. This isolates the muscularis mucosa (which marks the boundary of the two layers) as streaks of smooth muscle coursing through the glands. This is characteristic in the human.

Brunner's glands are long, branched, and tubulo-alveolar in form . Ducts are sparse and those present are lined with mucous-secreting cells. Spend a moment looking at the glands and thinking about their shape. Can you find a place which shows their tubulo-alveolar morphology (illustration)?



D-103, Stomach, Fundus, monkey (H&E)

It is difficult to get well-preserved human fundic stomach because the tissue autolyses rapidly. The monkey illustrates the essential features but on a much reduced scale. Distinguish the main layers of the wall; mucosa, submucosa, muscularis externa and serosa (illustration).

Examine the mucosa of the fundic portion. The surface epithelium is fundamentally different from that of the intestine with its absorptive brush border. In the stomach the surface epithelium consists of "mucous cap cells," a unique cell type filled with tiny secretory granules of mucus.  These cells differ in appearance from typical mucous cells. They are narrower, their mucus-laden apical ends look clearer and their nuclei are plump and round instead of heterochromatic and pushed out of shape. Also, the mucus that they produce is biochemically distinctive in that it stays slippery at the low pH of the stomach. (Anyone who has had lemonade has felt ordinary mucus precipitate into ropey strands at low pH). Such mucus would not serve its function in the stomach. (What is that function?)

The gastric epithelium does not form a flat surface but, instead, is crisscrossed by a network of tiny creases about a mm deep. These are called "gastric pits" from their appearance in transverse section. The pits are typically several millimeters long. They increase the stomach surface area in a way different from the protruding villi of the intestine. From the bottom of the pits, simple, tubular branched glands (without ducts) extend down into the lamina propria. They run pretty much straight down except for the very distal ends, which twist around a bit.

Two cell types populate the upper parts of the glands. One is the brightly eosinophilic parietal cell which secretes HCl and intrinsic factor. The other, mucous neck cell, manufactures the special form of mucus that does not precipitate at low pH. They are hard to pick out among the jumble of cell nuclei in the glands with H&E.

The cells at the base of the glands close to the muscularis mucosa are of another type. These chief cells secrete the digestive enzymes of the stomach. Their cytoplasm is very basophilic (indicative of large quantities of rough endoplasmic reticulum, and therefore, protein synthesis). The apical portion is filled with secretory granules but on our slide these granules have been dissolved out leaving a foamy-looking cytoplasm. Several other cell types are found in the stomach (and intestinal) wall, but generally require special sorts of preparations for their demonstration. These include stem cells and scattered hormone-secreting cells. As in the intestine, the surface cells are born from cell divisions deep in the mucosal glands.

Before leaving the stomach note that at the pylorus (the lower end of the stomach) the mucosal glands are purely mucous, as they are at the cardiac (upper) end of the stomach. Thus, if a bit of gastric juice leaks into the duodenum or esophagus it will be mainly mucus instead of rich in HCl and peptic enzymes. It is worthwhile observing this mucosal specialization in the pylorus by returning to slide D-105. Here, the pits are deeper than elsewhere in the stomach, presumably to produce more mucus, while relatively stubby glands extend to the muscularis mucosa (illustration).  

The tissue on slide D-103 is mainly from the fundus of the stomach but one end is from near the cardia.  You can find this end by looking at the glands.  The cardiac glands also produce only mucus.  The pits are nicely seen, even over this portion.

D-101, Esophagus (H&E)

The esophagus is basically a pipe to convey food to the stomach. Its structure follows the same outline as the rest of the gut (illustration) with three obvious specializations. 1. The epithelium is stratified squamous and non-keratinized. This protects the mucosa from chewed food passing along it.

2. The muscularis externa is unusual in containing striated muscle fibers as well as smooth muscle. At the top of the esophagus the externa is made up of skeletal muscle. Proceeding down the esophagus, the amount of striated muscle dwindles and is replaced by increasing numbers of bundles of smooth muscle. The lower third of the esophagus has only smooth muscle. Fortunately our section is from the middle of the organ. You have already looked at this section as a special place where these two types of muscle can be compared side by side (low mag., high mag.) The muscularis mucosa (view #2) stays as smooth muscle all of the way up the esophagus. Towards the upper end, the layer dissolves into separated bands of smooth muscle and you will have to be careful to identify it.

 3. The lining epithelium invaginates as mucous-secreting glands (high mag) to in both mucosa and submucosa. They lubricate the passageway for food. Glands are most abundant at the upper and lower ends of the esophagus. In the middle region the glands are rather widely scattered and will not be included in some of your sections.

One further specialization, not particularly well shown by our section, is the presence of longitudinal submucosal folds. These essentially collapse the entire lumen when food is not being swallowed but can be stretched out when a large bolus of food passes down.

It is useful to make two comparisons with the esophagus. One is with the rest of the GI tract, as discussed above. The other is with other organ lined with stratified squamous epithelium, inner lip, vagina, etc. It might be interesting to turn to D-61 and compare the epithelium, mucosa, submucosa and muscle layers on the two slides.


D-114, Colon. (H&E)

The large intestine, the colon, conforms to the essential organization of the rest of the gastrointestinal tract (illustration). Villi are gone but the glands remain, and are still called crypts of Lieberkühn. In a section exactly perpendicular to the surface the glands might look like the boundaries between closely packed villi. However, if you look at an area where the knife has cut parallel to the surface you can see that the glands are well formed, small, circular glands in uniform pattern with a small sheet of connective tissue between them (illustration). Clearly the epithelium extends down into the lamina propria instead of protruding upwards over villi. Goblet cells dominate the epithelium of the crypts. Do you see mitotic figures? Where in the glands do they occur? Do you see any at the surface?

The connective tissue of the lamina propria is heavily infiltrated by lymphocytes and plasma cells. You can look around for eosinophils and the stromal cells. Lymph nodules are scattered in the submucosa near the junction with the mucosa (the black masses in the submucosa in the 0 magnification picture above).

This is a good slide for examining a muscularis mucosa carefully. It has two layers to it, inner circular and outer longitudinal. This is the case throughout the GI tract.

A principal specialization of the colon relates to its external longitudinal smooth muscle layer. This is reduced to a thin layer everywhere except for three heavy bands known as the tenia coli. These are easily found even without magnification as bulges in this full cross-section of the monkey colon. Between them and the inner circular layer of muscle are well developed ganglia of the myenteric plexus. Ask you partners what the function of the tenia coli are.

D-111 Appendix (H&E)

The appendix is an evagination off the large intestine. Not surprisingly, it continues the same basic organization of that organ. For example, the epithelium is arranged into simple glands with many goblet cells. For its size, the appendix has a relatively thick muscularis and a well-defined serosa. It is invariably specialized in having an enormous lymphatic infiltration into its lamina propria and submucosa. This slide shows a nice succession of lymphatic nodules, as well as a diffuse distribution of lymphocytes throughout the connective tissue (illustration). The lymphocytes accumulate here is in part because it has specialized tall postcapillary venules of the sort that you saw in lymph nodes and tonsil. Watch for them (illustration). The tissue was a surgical specimen and some of its surface epithelium was lost.

D-115 Ano-rectal junction (H&E)

Start at low power with the rectum on the right side of your slide (which will appear on the left through the microscope (illustration). Identify the simple columnar epithelium of the rectum. Notice the tubular infoldings of the epithelium as glands (many in cross section). Find the muscularis mucosa, the submucosa with rich venous plexus (some distended into hemorrhoids), the cross sections of the inner circular muscle layer, and the longitudinally sectioned fibers of the outer longitudinal muscle layer.  Follow the epithelium to the right, as seen under your microscope. Look for the abrupt transition into non-keratinized stratified squamous epithelium. Examine the transition at medium power and at high power. Further along, the epithelium becomes keratinized to blend with the skin at the distal portion of the anal canal (illustration). Find the point of transition and check the appearance of the unkeratinized and keratinized epithelia. The extreme anal end of your section may cut through skeletal muscle. The fibers are circularly arranged and, as in the upper end of the esophagus, give you some voluntary control. Note the loss of orderly layers typical of the GI tract at that anal end.

* * * * *

Optional

D-135 Esophagus, trachea, thyroid (PASH)

Observe the esophagus. Unfortunately, the mucosa has been treated roughly on some slides and only a detached fragment of epithelium survived. If this is the case, ask the course chairman if a replacement slide is available. PASH stains the inner and outer layers of the muscularis externa but only palely. From how far down towards the stomach was this section taken? Do you see any glands in this section? What does the muscularis mucosa look like (presuming that you are looking at a section with an intact mucosa)? How would you describe the submucosa?

D-104 Stomach, monkey (PAS stain)

The slide shows the rugae of the stomach. These are gross folds that the stomach wall collapses into when the stomach is empty. The stomach can extend greatly after a pig-out meal by stretching the mucosa flat. The rugae are composed of a ridge of submucosa overlain by mucosa. Therefore they correspond more to the structure of plicae of the stomach than to villi. However, they run longitudinally instead of transversely. Rugae are entirely different from the pits and are two orders of magnitude larger.

PAS stains the carbohydrate associated with mucus.  It shows that mucous-secreting cells line the surface and extend down into the glands. The ones on the surface are mucous cap cells. Those in glands are mucous neck cells. The glands, of course, are simple (branched) tubular structures that fill up most of the lamina propria. This specialized stain is poor for examining other cellular structures.

D-110 Ileum (H&E)

Our course does not go into the relatively minor differences between the divisions of the small intestine, other than the presence of Brunner's glands in the upper duodenum and Peyer's patches in the middle of the ileum. For the sake of completeness, however, D-110 is a nice slide of the ileum. Anyone who wishes may write up a description of this slide. I will add 2 points onto the written part of the final for the two best descriptions, assuming that they are really good.

 

D-109 Duodenum, rabbit (FeH)

D-109 is pretty much a repeat of D-107 and D-108 except for the iron hematoxylin stain for cellular detail. Some of you looked at this slide weeks ago for the smooth muscle highlighted by this stain. Plasma cells are abundant in the lamina propria cores of the villi and show up especially well. If you still worry about recognizing these crucial cells of the immune system do not miss the opportunity to examine them here.


D-113 colon, monkey (Muc. Carm.)

This slide has been specifically stained to illustrate the distribution of the goblet cells in the colon. They are abundant. The other non-mucous secreting cells of the epithelium show up as well. Their main function is to absorb water. This is another good slide for observing mitotic figures in the crypts.