Anatomy and Human Biology 2214
12 October, 2009
Prof. John Campbell
DIGESTIVE SYSTEM
Objectives: You should be able to recognize the following structures:
For all organs of the GI tract
mucosa
lamina propria
muscularis mucosa
submucosa
muscularis externa
myenteric plexus
adventitia/serosa
Esophagus
skeletal and smooth muscle
stratified squamous epithelium
submucosal glands (if present)
Stomach
rugae
gastric pits
surface mucous cells
gastric glands
chief cells
parietal cells
Small intestine
plicae circularis
villi
enterocytes
brush border
goblet cells
Brunner's glands
crypts of Lieberkühn
Paneth cells
Large intestine
glands
tenia coli
Ano-rectal junction
transitions from simple columnar to
stratified squamous to keratinized
epithelium
hemorrhoidal veins
Slide descriptions
D-101, Esophagus (H&E): The esophagus is basically just a muscular tube to convey food to the
stomach. Its structure follows the same outline as the rest of the gut with a mucosa composed of
epithelium, lamina propria and muscularis mucosa, a submucosa, a double-layered muscularis
externa and an adventitia. Be warned that the lamina propria is more abbreviated here than in the
rest of the alimentary canal and that the muscularis mucosa is made up of bands of smooth
muscle instead of a nice continuous sheet. The first thing you should do with this slide is pick
out these layers of the mucosa and distinguish them from the submucosa.
The esophagus has four distinctive specializations: 1. The epithelium is stratified squamous and
non-keratinized instead of simple columnar. 2. Glands are found in the submucosa as well as the
lamina propria. They are compound mucous glands and do not appear on all of your sections. 3.
Large projections occlude the lumen except when a bolus of food is being swallowed. Look to
see if these projections are mucosal or submucosal. 4. The muscularis externa is composed of
striated muscle fibers at the top of the esophagus. Proceeding down the esophagus, the striated
muscle is replaced increasingly by bundles of smooth muscle. You can compare these two types
of muscle side by side since this section is from the middle of the organ.
The muscularis mucosa stays as smooth muscle all of the way up the esophagus. Towards the top
of the esophagus this muscle layer become disorganized and you will have to look carefully to
locate it close to the epithelium.
1
D-103, Stomach, Fundus, monkey (H&E): Again, distinguish the main layers of the wall;
mucosa, submucosa, muscularis externa and serosa. These layers in the monkey, seen here, are
substantially thinner than they would be in a human.
Examine the mucosa. 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 (e.g. goblet 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.
The gastric epithelium does not form a flat surface but, instead, dips down as a series of gastric
pits. Looking down on the surface these pits would look like tiny creases, crisscrossing the
surface. They are typically several millimeters long and about a millimeter deep. They increase
the stomach surface area. Also, from the bottom of the pits, simple, tubular branched glands
(without ducts) extend down into the lamina propria. The gastric glands run pretty much straight
down except for the very distal ends, which twist around a bit. Typically, the glands are so
jammed together that it is hard to recognize their orientation.
Two cell types can be distinguished in the upper parts of the glands. Brightly eosinophilic
parietal cells secrete HCl. Mucous neck cells are on their way to becoming mucous cap cells.
The cells at the base of the glands, close to the muscularis mucosa, are chief cells. Their basal
ends are strongly basophilic, indicative of large quantities of rough endoplasmic reticulum, and
therefore, protein synthesis. Stem cells and hormone-secreting cells are scattered along the
glands, but require special preparations for their recognition.
D-104 Stomach, monkey (PAS stain): This slide shows the rugae of the stomach. These are
gross wrinkles that form when the stomach is empty. They allow the stomach to extend greatly
after a pig-out meal by stretching the mucosa flat. The rugae are composed of a ridge of
submucosa overlain by mucosa: Thus, they are submucosal structures. Rugae are entirely
different from the pits, which are mucosal and two orders of magnitude smaller.
The slide has been stained with the periodic acid-Schiff reaction to demonstrate carbohydrates (a
major component of mucus). It shows that mucous-secreting cells extend from the surface down
into the neck of the glands. Mucous neck cells are produced by cell division in the middle of the
glands and migrate up the gland to become mucous cap cells. This specialized stain is poor for
examining other cellular structures. Don’t do it.
D-107 Lower duodenum (H&E). This well preserved example of human lower duodenum has
been sectioned longitudinally. The wall is thick, due to ridges called the plicae circulares. Under
low power it becomes evident that plicae are thickenings of the submucosa covered with mucosa
(epithelium, lamina propria and muscularis mucosa). They are covered on their surface by a
multitude of villi. These smaller protrusions of mucosa are composed of only epithelium and
lamina propria. The lamina propria is easily distinguished from the underlying submucosa. While
the submucosa is rather ordinary-looking areolar tissue, the lamina propria is highly cellular with
many lymphocytes. A thin layer of muscularis mucosa separates the two connective tissue
layers.
Examine the relatively inconspicuous glands (crypts of Lieberkühn) which invade the mucosa
between the villi and extend down close to the muscularis mucosa. The Paneth cells at the
bottom of the glands contain red secretory granules in their apical cytoplasm. On some slides
these granules have been leached out, leaving a foamy-looking apical cytoplasm instead.) The
2
surface epithelium of the villi is tall columnar and dominated by enterocytes with a brush border.
Goblet cells and lymphocytes are scattered in between.
The cells in the lamina propria include lymphocytes and plasma cells, fibroblasts and smooth
muscle cells. Both capillary and lymphatic channels richly supply this region, so that many of the
elongate nuclei are endothelial.
Turning to the submucosa, 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 are visible under low
magnification. These reflect the main function of the submucosa.
The muscularis externa has two muscle layers. The inner circular layer is seen in transverse
section and the outer longitudinal layer is sectioned longitudinally. This smooth muscle is
regulated by a well developed plexus of autonomic ganglia. Sections through this myenteric
plexus can be seen as pale islands of cells, sandwiched between the two muscle layers.
D-105 Pyloric-duodenal junction, human (H&E): Brunner’s glands fill up the lamina propria as
well as the submucosa of the initial 10 cm or so, of the duodenum. They are long, compound,
branched, and tubulo-alveolar in form; mucous in function. 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? The downside
of this slide is that the epithelium over the sphincter has been lost.
D-114, Colon. (H&E): The large intestine, the colon, conforms to the essential features of the
gastrointestinal tract. the glands. Mucosal glands, still called crypts of Lieberkühn, are present
although villi and pliqae are gone. 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 small, circular tubes
with narrow sheets of connective tissue between them. Goblet cells dominate the epithelium of
the crypts.
The connective tissue of the lamina propria is heavily infiltrated by lymphocytes and plasma
cells. Lymph nodules are scattered in the submucosa. The muscularis mucosa is worth examining
carefully on this slide. It has two layers, the inner circular and the outer longitudinal. This is the
case throughout the GI tract but especially well demonstrated here.
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 tenia coli.
These are easily found 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.
D-115 Anal-rectal junction (H&E): Start at low power with the rectum. On the right hand side
of your slide (which will appear on the left through the microscope). Identify the simple
columnar epithelium of the rectum. Notice the tubular glands (many in cross section). Find the
muscularis mucosa, the submucosa with rich venous plexus (some distended into hemorrhoids),
the transversely sectioned 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.
Farther along, the epithelium becomes keratinized to blend with the skin at the distal portion of
the anal canal. Finally, note the loss of orderly layers typical of the GI tract at the anal end.
3
D-101, Esophagus (H&E)
D-104 Stomach, monkey (PAS stain)
D-103, Stomach, Fundus, monkey (H&E)
D-107 Lower duodenum (H&E)
D-105 Pyloric-duodenal junction,
human (H&E)
D-114, Colon. (H&E)
D-115 Anal-rectal junction (H&E)
4