XV. ALIMENTARY
CANAL
Mucosa
The gastro-intestinal tract of mammals consists of four main
layers, the mucosa, submucosa, muscularis and serosa. The innermost of these
layers, the mucosa can be divided into the epithelium, lamina propria, and
muscularis mucosa. The epithelial lining may be protective, secretory, and/or
absorptive in function. The lamina propria is a type of areolar connective
tissue that contains both blood and lymphatic capillaries as well as some
smooth muscle.
The epithelium is infolded in many areas of the gut. These
infoldings may be located in the lamina propria (crypts of Lieberkühn and
gastric glands), in the submucosa (Brunner's glands) or outside the gut (liver
and pancreas). The muscularis mucosa consists of an inner circular and an outer
longitudinal layer of smooth muscle. This outermost layer of the mucosa is
responsible for local movements of the lining of the gut.
Submucosa
Beneath the muscularis mucosa is a layer of rather dense
connective tissue containing blood and lymphatic vessels, multicellular glands
and a plexus of unmyelinated nerve fibers called Meissner's or the submucosal
plexus. In addition lymph nodes may be found in this layer.
Muscularis Externa or Muscle Layer
This layer consists, in general, of two layers of smooth
muscle (skeletal muscle in upper 1/3 of esophagus). The outer layer is
longitudinal and the inner is circular with reference to the long axis of the
gut. The coordinated activities of these muscles results in the peristaltic
movements that transport material along the lumen of the gut. The control of
these movements is mediated through a plexus of nerve fibers associated with ganglia
situated between the muscle layers called myenteric or Auerbach's plexus.
Serosa
The serosa consists of areolar tissue and a mesothelium in
those areas of the digestive tract that are supported by mesenteries. In other
areas of the track like the esophagus, the outer layer is called the adventitia
and consists of connective tissue associated with the connective tissue
elements of other organs.
A. Tongue
Slide 30 (vallate
papillae)
Most of the tongue is composed of striated muscle cells
running in three planes and crossing at right angles. The muscle mass is
covered by mucosa which contain numerous small salivary glands. The surface and
sides of the tongue are covered with a number of different papillae including
the tapered filiform, the mushroom-shaped fungiform, and the large
circumvallate papillae. The taste buds
are located in the epithelium of these papillae. Study the epithelium, lamina
propria, striated muscle, nerves, blood vessels, and associated connective
tissue elements.
B. Esophagus
Slide 31, 32, 43
(esophagus)
Examine under low power first to get the general structure
of the organ. Identify each of the layers of the esophagus. Note the type of
epithelium, the structure of the lamina propria, the presence of mucous glands, and the location and
orientation of the muscles. The vagina or urethral orifice have little or no
muscularis mucosa or glands.
C. Stomach (Table #13)
Slides 33, 94, 95, 96 (stomach)
Study the slides of the stomach and note the thick walls and
presence of the folds or rugae. The slides contain the gastric or fundic
glands. Locate the large eosinophilic parietal cells (HCl) and the basophilic
chief cells (pepsinogen). The surface of the stomach next to the lumen is
composed of surface mucus cells. Mucus neck cells extend down into the glands.
Neither of these cells stain well with H & E but can be identified with
PAS. Study the arrangement of the lamina propria, muscularis mucosa, muscle
layer and serosa. Note the absence of parietal and chief cells in the cardiac
glands in slide 94.
D. Small Intestine
Slides 33, 34, 35, 36,
(duodenum, jejunum, ileum and Payer's patches)
Examine the slide of
the duodenum and identify the villi and crypts of Lieberkühn. The glands of
Brunner are diagnostic of the duodenum. These are prominent mucus glands in the
submucosa.They will not be found in the lower parts of the small intestine
(jejunum and ilium). On the villi, identify the absorptive cells with the prominent
striated borders composed of highly ordered arrays of microvilli.
Find the PAS positive mucus secreting goblet cells which are
regarded as unicellular glands. Try to
identify the Paneth cells of the bottom of the crypts. These cells
contain apical eosinophilic granules seen in routine H & E preparations. Do
not confuse these cells with the eosinophilic
enteroendocrine cells which have granules located near the basal lamina
and have cytoplasm that does not extend to the lumen. Note the large masses or
patches of lymphocytes (especially abundant in the ilium) that are called
Payer's patches.
Along the sides of the crypts, the most abundant cells are
the undifferentiated cells that have some
microvilli, but no striated border. These are the stem cells of the
epithelium that frequently divide and differentiate into other cell types. The
epithelial cells of the villi are derived from these undifferentiated cells in
the crypts. They gradually move upward toward the tips of the villi where they
are sloughed off. By this process the epithelium is being constantly renewed,
every 2-4 days in humans.
Slides 37, 70, (colon
and rectum)
The colon unlike the small intestine, lacks villi, but
crypts are present. Goblet cells are exceedingly abundant in the crypts. Thick
strands of smooth muscle called the teniae coli (3 thick bands of outer,
long smooth muscle) are present in the muscle layer. Note the structure of this
organ and the abundant lymphoid tissue.
F. Diagnostic Features (Table 14.31, page 273)
The following outline of diagnostic features is an example
of the kind of summary you should make for all of the organs and tissues we
study. Some students feel that the atlas or the instructor should provide a
list like this for each organ, but the process of making a list is an essential
learning experience. Just looking at a list is not as useful in the learning
process.
The diagnostic features you come up with are useful in
identifying unlabeled slides such as you will encounter on examinations. The
diagnostic features do not include all of the details of an organ
or tissue. They include those details which are likely to be important in
distinguishing between similar looking tissues in histology. The following are
some examples:
1. esophagus vs vagina
2. duodenum vs colon
3. salivary gland vs pancreas
4. bladder vs vagina
5. smooth muscle vs collagen
6. cardiac
vs striated muscle
Of course, it does not help much to memorize a list of
diagnostic features if you can't recognize the details and you must be
objective when you look at an unknown tissue. If you start out thinking that a
ganglion is an ovary because we just finished the lab session on the
reproductive system and the nervous system was studied in the first half of the
course, you may force yourself into seeing details that are not present.
You will find it fun and a great learning experience to test
your laboratory colleagues with slides and ask for their diagnostic
characteristics. Be sure to let them test you. There is something about this
testing process that aids the memory and develops self-confidence for real
exams and for professional work that may involve this body of knowledge. The
following are some diagnostic features of the primate gut:
Esophagus
folded mucosa
(low magnification)
striated
squamous epithelium with a thick muscularis mucosa
no serosa,
thick muscle layer
small mucus
glands in lamina propria
Stomach
rugae present
(low magnification) and thick wall obvious
gastric pits
present but no goblet cells present and no villi
gastric glands
and oblique layer in muscle layer
Duodenum
villi present
with columnar cells with striated border and goblet cells
crypts with
Paneth cells, undifferentiated cells and endocrine cells
contains folds
or plicae, may have Brunner's glands
Jejunum
similar to
duodenum but no Brunner's glands and larger plicae
Ileum
similar to
duodenum but fewer villi and plicae
Peyer's
patches common and no Brunner's glands
no villi
long tubular
glands with many goblet cells
taeniae coli
and large lumen