Download File - Wk 1-2

Learning Objective: Histology of the GIT
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Know the general structure and components of the GIT
Be able to label and describe the key components and cells of each GIT layer
Demonstrate an understanding of the different types of epithelia found in the GIT
Correlate the histological features of GIT tissues (including the liver and pancreas)
with the physiological function.
The GIT includes the following organs;
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Oesophagus
Stomach
Small Intestine
o Duodenum
o Jejunum
o Ileum
Colon
Each of these structures are made up of the following layers;
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Mucosa (epithelia types) and glands
o This layer is generally made up of columnar epithelium. The exception of this is
the oesophagus, which is comprised of stratified squamous epithelium
o There is also a variety of specialised cells in this layer, including mucous
producing, enzyme secreting and hormone producing
o This layer is supported by the lamina propria
Lamina Propria
o Can contain glands (oesophagus and duodenum)
o Helps to support the mucosa and also attach it to the muscularis mucosae
o Contains collagen, blood, vessels and lymphatic nodules (this layer is important
for immune surveillance)
Lymphoid Nodules
o Present in varying amounts throughout the GIT, but there is a large proportion
in the ileum. The ileum also contains Peyer’s Patches (diffuse lymphoid tissue)
o GALT: Gut Associated Lymphoid Tissue
o Site of surveillance
(Please refer to picture 1 on the DOC 2 for Peyer’s patches)
Muscularis Mucosae
o 2 fine bands of smooth muscle, located at the junction between the mucosa and
submucosa
o Continuous layer down whole of the GIT
Submucosa
o Anchors the mucosa to the muscularis externa
o This is the site where neurovascular bundles are found (A Neurovascular bundle
is a term applied to the body nerves, arteries, veins and lymphatics that tend to
travel together in the body)
o Contains type I Collagen
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This is also the place of the peripheral nerve ganglia (Meissner plexus location of
ganglion cells) (– think it may be important to remember this point)
Muscularis externa
o Usually 2 layers of smooth muscle (with the exception of the stomach). These
include an inner circular layer and an outer longitudinal layer
o Also contains the Auerbach plexus (Myenteric plexus) between the two muscle
layers
Serosa
o This is also called the adventitia or fibrosa
o Covered by a mesothelium layer
o This is also the layer where mesenteric blood and nerve supply enters the GIT
o Its function is to gather and anchor the GIT and is also the location for mast cells
(therefore is important in immunity)
Distinct features within the GIT
Oesophagus
 The mucosa consists of stratified squamous epithelium (instead ofcolumnar epithelium
like the rest of the GIT)
 The muscularis externa changes as it goes down: the first 1/3 is skeletal muscle, the
second 1/3 is made up of a combination of smooth and skeletal muscle, whilst the final
1/3 is made up entirely of smooth muscle
 The submuscosa is the oesophagus has tubule/alveolar musous glands (to make sure
bolus is well lubricated to move down to the stomach)
 Instead of a serosa, the esophagus has a fibrous adventitia composed entirely of
connective tissue, which blends with surrounding structures along its route
Stomach
 The surface of the stomach is folded into rugae
 The mucosal surface is much deeper than in the other structures (to ensure that the
acidic environment of the stomach does not start to digest the stomach walls)
 There are many mucous secreting cells and specialized cells (parietal cells, chief cells
and enteroendocine cells) in the stomach
(N.B Parietal cells secrete gastric acid (HCl), chief calls secrete pepsinogen and
enteroendocrine cells include all cells of the GIT which secrete hormone, therefore G
cells (which secrete histamine) are an example.)
 Muscularis externa is composed of 3 layers - The stomach is composed of an inner
circular layer, middle oblique layer and an outer longitudinal (this is because it have
more ‘motor’ functions in terms of massaging the food and also needs to be able to
distend and contract a lot. Food also spends a longer period of time in the stomach).
 Small intestine had villi and microvilli and the epithelium contains Paneth cells which
have a role in immunity as they secrete defensins. Crypts of Lieberkuhn are at the base
of the villi. These are digestive and absorptive cells
(please refer to picture 2 DOC 2 for a picture of the location of the different cells of the stomach)
Small Intestine
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Duodenum has Brunner Glands (Brunner Glands produce an alkaline secretion that acts
to protect the duodenum from the acidity of the chime, change the pH of the chime so
other digestional enzymes can act on it and also to lubricate the chime for it’s travels
down the GIT). These are above and below the muscularis mucosae
Jejunum has fewer and shorter villi, but has folds known as pilicae or values of
Kerckring. The Jejunum is the main site of absorption
The Ileum has Peyer’s patches (large lymphoid aggregates)
Large Intestine
 The Veriform Appendix has more lymphoid nodules and crypts of Lieberkuhn
 The large intestine has a folded surface, mucosa without villi, many goblet cells (which
produce mucous), Crypts of Lierberkuhn and a thick muscularis externa
Colon
Goblet Cell
Mucosa
Crypts of Lieberkuhn
Liver
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The liver is made up of hepatocytes, which are arranged in cords forming lobules
around a central vein
There are radiating cords of hepatocytes and peripheral portal tracts composed of a
vein, artery and one or more bile ducts
The sinusoids percolate blood between cords of hepatocytes. These are supported by
collagen
The purpose is to carry oxygenated blood from the hepatic arteries and to deliver the
nutrient rich blood that has come out of the central vein
Lymphatic vessels are also present
This is also the location of Kupffer cells (macrophages)
The bile is secreted from the central vein into the canaliculi between hepatocytes,
shunted by bile ducts to the gallbladder.
Duct epithelium also secretes a bicarbonate rich fluid which is added to the bile (which
will later act to neutralise the gastric acids)
(Please refer to picture 3 on DOC 2 for a diagram of the lobules of the liver)
Pancreas
Exocrine Component (glands):
 Intracellular zymogen granules synthesise enzymes.
Enzymes include:
 Amylase, lipase, elastase
 Ribonuclease, DNase
 Proenzymes: trysinogen, porcarboxypeptidase, chymotrypsinogen
 These glands are by far the most prominent component of the Pancrease
Endocrine Component:
 The endocrine component is primarily the Islets of Langerhans. These secrete the
following;
 Insulin
 Glucagon
 Gastrin
 Somatostatin
 Pancreatic peptide
(Please refer to DOC 2 for a histological picture of the pancreas)
5. Demonstrate an understanding of the histological arrangement of the muscle layers
within the GIT and how this relates to function
The arrangement of the muscle layers in the GIT allows for effective peristalsis to occur. The
circular and longitudinal muscles coordinate their action with the net effect of moving food
forward in the GIT.
This occurs by the following mechanism:
 Impulses sent proximally by cholinergic effector neurons cause contraction and
shortening of the circular muscle layer.
 Impulses sent distally to certain interneurons cause shortening of the longitudinal
muscle layer and distension of the intestine, in response to ACh-releasing effector
neurons.