Download Lab 7 Digestive and accessory organs

HISTOLOGY LABORATORY GUIDE
7
Digestive System and Accessory Organs
Digestive System
The digestive system propels food stuffs through a continuous lumen beginning in the mouth, and
proceeding along the alimentary canal (esophagus to the anal canal). Throughout the alimentary canal,
the tube wall is composed to 4 distinctive layers:
Mucosa: the epithelium and underlying connective-tissue rich lamina propria and the muscularis
mucosa.
Submucosa: a layer of dense irregular connective tissue
Muscularis externa: usually two layers of smooth muscle
Serosa: a simple squamous epithelial membrane, and some connective tissue. A connective tissue
wrapping called the adventitia is found is some portions where the structure is fixed in place.
For a concise description of the functions of these layers throughout the alimentary canal consult your
textbook: p. 519-522
1. Tongue:
Slides 8–10
Ross et. al. Plates 45, 46
The primary component of the tongue is skeletal muscle arranged in bundles that extend in three planes
giving the tongue exquisite flexibility for speech, eating, and swallowing. As a result, a section of the
tongue usually shows skeletal muscle in several planes of section. The dorsal epithelial surface of the
tongue contains elevations or irregularities called papillae. The taste buds are found at the four different
types of papillae. The tongue is coverd by a stratified squamous epithelium that may be partially
keratinized and rests on a lamina propria.
2. Esophagus:
Slides 11-13
Ross et. al. Plates 50
The esophagus has the 4 characteristic layers of the alimentary canal. The lumen of the esophagus is
lined by a nonkeratinized stratified squamous epithelium that sits on a connective tissue submucosa.
Lymphoid tissue and nodules may be observed in the lamina propria. The muscularis mucosae, a
smooth muscle layer, is present in some portions of the esophagus. In the submucosa of the esophagus,
one can observe the hallmark irregular dense connective tissue as well as blood and lymphatic vessels,
nerves, mucous-secreting glands and lymphoid tissue.
The muscularis externa of the esophagus consists of 2 muscle layers: an inner circular and an outer
longitudinal layer. In humans, the upper third of the muscularis externa consists of striated muscle
fibers (demonstrating the voluntary component to swallowing), the middle third has both striated and
smooth muscle, and the lower third contains primarily smooth muscle. Thus, one can make a pretty
good guess where the section was taken based on the composition of this layer. Clusters of neurons, the
myenteric plexis, can be found between the two muscle layers. The connective tissue surrounding the
esophagus is usually an adventitia except for the short stretch of the esophagus in the abdominal cavity,
which is covered by a serosa. The gastro-esophageal junction is marked by an abrupt transition from a
stratified squamous epithelium to a simple columnar epithelium.
3. Stomach:
Slides 33, 34
Ross et. al. Plates 52, 53, 54
The stomach is divided into three regions based on the types of glands observed: cardiac, fundic,
pyloric (Ross et al, p. 524, Figure 17.5). While you should be aware of these classifications, you will
not be responsible for identifying the various regions of the stomach on a practical exam. The luminal
surface of the stomach is thrown into folds called ruggae, that allow for the expansion of the stomach
when full.
The gastric (stomach) mucosa has numerous openings in its surface called gastric pits. See Figure
17.8 on page 527 of your text to appreciate the relationship between gastric pits and gastric glands. The
gastric lumen is lined with a simple columnar epithelium. Numerous light-staining mucous cells are
present and the surface and they continue into the gastric pits. Other specialized epithelial cells are
distributed among the mucous cells in the lower gastric pits and in the gastric glands. The apical regions
of gastric glands contain large eosinophilic parietal (oxyntic) cells that secrete hydrochloric acid and
intrinsic factor. The basal region of gastric glands contains the basophilic chief (zymogenic) cells, that
secrete pepsinogen that is converted to the proteolytic enzyme, pepsin, by stomach acid.
Enteroendocrine cells are found in the basal region of gastric glands. These cells secrete hormones
into the capillaries in the lamina propria; the secretory granules of these cells are in the basal region of
the enteroendocrine cells. Beneath the gastric glands is a scant lamina propria and a thin 2-layered
muscularis mucosae.
The submucosae contains the nerve plexus of Meissner, that is responsible for segmental contraction.
The stomach contains 2-3 smooth muscle layers in the muscularis externa that appears less layered and
more random in orientation in section. Parasympathetic ganglion cells of the myenteric plexus of
Auerbach are evident between the smooth muscle layers. The stomach is covered by a serosa.
4. Small Intestine:
Slides 19-22
Ross et. al. Plates 55, 56, 57
The small intestine is the longest component of the alimentary canal and is divided into three sections:
the duodenum, the jejunum, and the ileum. Three features of the small intestine amplify its absorptive
surface area:
plicae circularis: transverse folds that include submucosa (Figure 17.17, p. 535)
villi: finger-like projections of mucosa (Figures 17.18, 17.19, p. 536-537)
microvilli: form the striated border on the surface of intestinal absorptive cells
The intestinal mucosa is made up of villi and intestinal glands both lined with a simple columnar
epithelium. The lamina propria extends into the core of the villus and contains blood capillaries as well
as a lacteal, a blind-ended lymphatic capillary. Lacteals can be distinguished from blood vessels due to
their lack of blood cells in the lumen. At the base of villi are intestinal glands called crypts of
Lieberkuhn. The lamina propria also contains lymphocytes and nodules of lymphatic tissue (part of
GALT: gut-associated lymphatic tissue). The predominant aggregation of lymphatic nodules, Peyer’s
patches, are found in the ileum. The muscularis mucosa is 2 thin layers of smooth muscle, some cells of
which extend into the lamina propria of villi.
The intestinal epithelium contains several specialized cells listed below:
Enterocyte: tall columnar cells; basal nucleus; striated border of microvilli; glycoclayx, secretion of
digestive enzymes and absorption of nutrients.
Goblet cell: goblet shaped; basal nucleus; produce mucous; mucin granules in cell apex; increase in
number form duodenum to ileum.
Paneth cell: at base of intestinal glands or crypts; large distinctive intensely acidophilic granules;
secrete lysozyme to digestive bacterial cell walls.
Enteroendocrine cells: most common at base of glands; release hormones from basal membrane of
cell.
M cells: sit atop Peyer’s patches and other lymphatic nodules; lack microvilli; extend processes to
sample contents of intestinal lumen and provide antigens to nodules.
The intestinal submucosa is distinguished by the presence of Brunner’s glands that secrete an alkaline
substance that helps to neutralize the acidic chyme arriving from the stomach.
The two layers of the muscularis externa include an inner circular layer of smooth muscle and an outer
longitudinal layer. Clusters of neurons (myenteric or Auerbach’s plexus) can be found between these
muscle layers and are responsible for the movements of the small intestine. The small intestine is
covered by a serosa.
5. Large Intestine:
Slides 23-26
Ross et. al. Plates 58
The large intestine, whose purpose is reabsorption of electrolytes and water as well as elimination of
waste materials, contains all layers characteristic of the alimentary canal. Distinguishing features of each
layer when compared to the small intestine are described here:
Mucosa lacks villi and contains straight and deep intestinal glands with absorptive cells and goblet cells.
The number of goblet cells increases throughout the length of the large intestine. Mucus from the
goblet cells serves to lubricate the lumen for the passage of fecal material.
Lamina propria contains a well developed GALT (lymphatic nodules).
Teniae coli: the outer layer of the muscularis externa is thickened into three equally-spaced bands
Digestive Accessory Organs
6. Pancreas:
Slides 33, 34
Ross et. al. Plates 64; Figures 18.22, 18.23, p. 598-599
The pancreas has both exocrine and endocrine functions served by distinct populations of cells.
The endocrine cells are localized together in islets of Langerhans and secrete the hormones insulin and
glucagons involved in regulating blood sugar levels. An islet of Langerhans appears as a cluster of pale
staining cells surrounded by a sea of strongly stained exocrine acini. The islets are composed of a
network of fenestrated capillaries and three major cell types although they cannot be distinguished
without special stains:
B (beta) cells: in center of islet, about 70% of islet cells, release insulin
A (alpha) cells: at periphery of islet, about 15-20% of islet cells, release glucagon
D (delta) cells: at periphery of islet, about 5-10% of islet cells, release somatostatin
The exocrine component of the pancreas is serous and composed of acini and ducts. The acinar cells
are a simple epithelium of pyramidal-shaped cells with a broader basal than apical surface. The acinar
cells are basophilic in the basal end and acidophilic in the secretory-vesicle rich apical end. The serous
acinar cells secrete digestive enzymes into an intercalated duct that begins within the acinus with pale
staining centroacinar cells with flattened nuclei and limited cytoplasm (see p. 596, Figure 18.20). The
centroacinar cells and associated intercalated duct appear as a pushed into a balloon (the acinus). The
intercalated ducts feed into interlobular ducts with a low columnar simple epithelium and these in turn
contribute to larger ducts that leave the pancreas. Note: the exocrine pancreas closely resembles the
parotid gland; the presence of Islets of Langerhans can be used to distinguish the pancreas from the
parotid gland.
7. Liver:
Slides 37, 38
Ross et. al. Plates 61, 62
The histology of the liver is closely tied to an understanding of its function. The liver receives its blood
supply from both the hepatic artery and the portal vein (the portal vein delivers nutrient-rich blood
directly from the capillary beds of the small intestine). This blood enters sinusoidal capillaries that
bathe plates of hepatocytes and eventually drains into a central vein.
Central vein
http://www.niaaa.nih.gov/Resources/GraphicsGallery/Liver/lobulep295.htm
Hepatocytes of the liver secrete bile which is transported via bile canaliculi between neighboring
hepatocytes; the canaliculi fuse to form bile ducts and then the hepatic duct. The cystic duct then carries
the bile to the gallbladder. The cuboidal epithelium lining the bile duct distinguishes this structure from
the blood vessels. The combination of the hepatic artery, portal vein and bile duct is called the portal
triad. The liver can be represented as lobules based on the function described (see Figs. 18.3, 18.4,
18.5,18.6, 18.8; p. 581-2).
See http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Liver/Images/LiverAni.gif for an animation of
the liver lobules.
Hepatocytes are large polygonal cells with large central nuclei and generally acidophilic cytoplasm.
The surface of hepatocytes not in contact with their neighbors face hepatic sinusoids specialized
capillaries) and are separated from the sinusoids by a perisinusoidal space or the space of Disse. These
surfaces of the hepatocytes have small, irregular microvilli. The perisinusoidal space is the site for
exchange between the blood and the heptaocyte. Phagocytotic Kupffer cells can be found in the
sinusoid and can extend processes into the perisinusoidal space; they are darkly stained. Also found in
the persinusoidal space are hepatic stellate cells (Ito cells) that store vitamin A but can be activated to
produce collagen in disease states. See Figure 18.12 on page 587.
8. Gall Bladder:
Slide 36
Ross et. al. Plates 63
The gall bladder stores bile produced by the liver. Lining the lumen of the gallbladder is a folded
mucosa with a simple columnar epithelium and underlying lamina propria. The simple columnar
cells have short apical microvilli. The wall of the gall bladder does not contain either a
muscularis mucosa nor a submucosa. Instead the lamina propria is directly adjacent to the
randomly oriented smooth muscle fibers of the muscularis externa.