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Boxster
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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 |