 | -epithelial lining (1) |
| -lamina propria (2) |
| -muscularis mucosa (3) |
Basic Gut Tube
The gut tube is formed of several layers which are modified in the different parts of the gut to suit
the function of that part of the gut. The basic layers are:
| Mucosa |
| -epithelial lining (1) |
| -lamina propria (2) |
| -muscularis mucosa (3) |
| Submucosa |
| Muscularis externa (4,5) |
| Serosa (6) |
The epithelial lining of the mucosa varies from stratified squamous, able to resist abrasion, to tall columnar with a brush border, suitable for absorption. The lamina propria is a connective tissue layer that supports the epithelium. Blood vessels in the lamina propria nourish the epithelium. Glands may also be present opening onto the epithelium. In some parts of the gut tube the lamina propria is present only as a small amount of connective tissue between the glands. The muscularis mucosa is an incomplete layer of smooth muscle cells which defines the boundary between the mucosa and the submucosa.
The submucosa is formed of connective tissue and may contain extensive blood vessels, glands and nerves. Meissner's plexus is formed by nerve fibres and ganglion cells, and controls glandular secretion. It is part of the enteric nervous system.
The muscularis externa is the muscle coat of the gut tube. It is usually formed of an inner
circular and outer longitudinal layers of smooth muscle. In some parts of the tube striated muscle
replaces the smooth muscle, while in others three layers of smooth muscle may be present.
Auerbach's plexus of nerve fibres and ganglion cells lies between the muscle coats and controls
their function.
The esophagus extends from the pharynx to
the stomach. Most of the esophagus lies in the
thorax, and only the last two or three centimetres are within the abdominal cavity. The mucosa of
the esophagus has a thick stratified squamous epithelium. This type of epithelium is ideally suited
to situations where there is abrasion. The germinal layer at the base of the epithelium divides to
form new cells. The surface of the epithelium is formed of dead cells which are rubbed off with
the swallowing of food. In the submucosa there are extensive mucous glands which open onto
the surface of the epithelium. The muscularis externa in the upper one-third of the esophagus is
formed of striated muscle. In the middle one third there is a transition to smooth muscle. The
esophagus is continuous with the stomach at the cardio-esophageal junction. The epithelium
changes abruptly from stratified squamous to columnar. The submucosa at this point produces a
lot of mucus, protecting the epithelium from acid reflux from the stomach. Extensive blood
vessels are also present in the submucosa. Venous blood drains upwards into the azygos system
or down wards into the hepatic portal system. Increased pressure in the hepatic portal system
results in expansion of submucosal veins forming varices.
The stomach shows changes in all layers.
The epithelium is formed into deep glands. The
entrance into each gland is the gastric pit. The surface epithelium is columnar with numerous
goblet cells. The epithelium is heavily coated with mucus. The glands are formed of mucous
neck cells located at the top, parietal cells located in the middle and chief cells located at the
bottom. Mucous neck cells produce mucus which coats the surface epithelium. Parietal cells
produce hydrochloric acid. Chief cells produce pepsinogen. In addition to these cells there are a
few stem cells and endocrine cells present. The stem cells replace lost cells, and become obvious
when there is recurrent damage to the epithelium, as in ulceration. Endocrine cells secrete
serotonin, somatostatin, and gastrin. Gastrin or G cells are usually present in the neck region of
the gland. The gastric pits are short in the body of the stomach where the glands are long. In the
pylorus the pits become deeper and the glands shorter. The lamina propria of the stomach is
present as connective tissue between the glands in which run blood vessels and nerve fibres. The
muscularis externa is formed of three layers of smooth muscle.The stomach becomes continuous
with the duodenum at the pyloroduodenal junction. At the pylorus the circular muscle is
thickened as the pyloric sphincter.
The cells in the image at right have been stained using a primary
antibody to gastrin. The G cells lie at the top of the gastric glands,
and are particularly numerous towards the pylorus.
The small intestine is modified to aid
absorption in three ways. The tube is elongated and folded,
the lining is formed into crescent shaped ridges and the surface area of the individual cells in
increased by the presence of microvilli. In the duodenum the mucosa is formed into finger-like
projections, the villi. The epithelium covers each villous which has a central core of lamina
propria. The epithelium is formed of tall columnar cells with surface microvilli forming a brush
border. There is a layer of glycoprotein associated with the brush border forming a glycocalyx.
The glycocalyx contains a number of enzymes such as lactase, sucrase, peptidase and
lipase.Goblet cells secreting mucus are scattered between the columnar cells. Between adjacent
villi the epithelium projects towards the submucosa forming crypts. Cell division occurs at the
neck of the crypts, and daughter cells pass up the villi to be released at the villous tip, a process
which takes about two days. The crypts
contain cells producing enzymes. The core of each
villous contains a capillary plexus and lymphatic channels, the lacteals. The villi are usually in the
form of finger-like processes but in juveniles may be more ridge-like. In the duodenum there are
extensive submucosal mucous glands (Brunner's glands). In the jejunum the villi become
more complex and submucosal glands are absent. Lymphocytes are present throughout the epithelium
but are particulary concentrated in the submucosa and lamina propria as Peyer's Patches in the
ileum.
Left
- the surface of a villus showing tall columnar cells and goblet cells.
Right - Brunner's glands in the submucosa
Some of the cells at the bottom of the crypts
secrete enzymes. The cells stained red in this section are
Paneth cells.
Throughout the gut tube there are cells of the immune system - gut
associated lymphatic tissue
(GALT). In three areas these cells are formed into follicles with germinal centres. The palatine
tonsils are located below the stratified squamous epithelium of the oral mucosa. The aggregations
of lymphocytes are formed into follicles with germinal centres. The epithelium is thin over the
follicles, allowing antigens easier access. The Peyer's patches of the ileum have a similar structure
to that of the tonsil, and lie in the lamina propria and submucosa. The appendix is an appendage
of the cecum. Again follicles are present in the lamina propria and submucosa. The follicles are
extensive in juveniles but gradually reduce in extent with age. Isolated patches of lymphocytes
may also be found throughout the gut.
The liver is constructed of morphological units, the lobules. Each
lobule is constructed of a radial
arrangement of plates of cells arranged around a central vein. Around the periphery of the lobule
are arranged the triads, groups of branches of the hepatic artery, portal vein and bile duct.
The vasculature
The liver receives arterial blood from the celiac trunk and venous blood carried to it by the hepatic portal vein from the venules of the intra-abdominal gut. The arterial and venous blood mixes in the liver sinusoids, the equivalent of a capillary bed. The mixed blood passes through the liver sinusoids where macrophages and hepatocytes modify its composition. The blood is unlike any other venous blood in that it contains the products of digestion and absorption from the gut, and the products of antigen recognition and red cell breakdown from the spleen. The sinusoids are lined by a discontinuous epithelium which facilitates transfer of large complex molecules from the liver into the circulation. Blood leaves the sinusoids to flow into the central vein. Between the point where the blood has entered the periphery of the lobule and its exit into the central vein there are large gradients with respect to the composition of the blood, and this has significance with respect to ischemic or toxic damage to the hepatocytes. The central veins drain into hepatic veins then directly into the inferior vena cava. Although there are no valves in the hepatic portal vein or liver, blood flow is usually into the inferior vena cava. However if there is an obstruction in the portal vein or in the liver itself, blood flow can reverse and flow through porto-systemic anastomoses into the systemic venous system and by-pass the liver.
Hepatocytes
The plates of the liver lobules are formed by hepatocytes. These large
cells have surfaces exposed
to the sinusoids and other surfaces in common with each other. At the sinusoidal surface
hepatocytes show extensive microvilli projecting into the Space of Disse. At this surface the cells
absorb and secrete. From absorbed amino acids, lipids and sugars the hepatocytes synthesize and
secrete glucose, plasma proteins, fibrinogen and other substances involved in blood coagulation.
Toxic surfaces such as alcohol and barbiturates are also absorbed at this surface. At the common
surface between adjacent hepatocytes, there is cell to cell communication. Some of these surfaces
adjacent hepatocytes form canaliculi, narrow channels between the cells. At this surface the
hepatocytes excrete bile into the canaliculi. The canaliculi carry bile in the opposite direction to
blood flow, i.e. to the periphery of the lobule where it flows into small branches of the bile duct
system. Bilirubin is absorbed at the sinusoidal surface and conjugated before being released into
the bile canaliculi. The intracellular organelles of hepatocytes reflect the wide range of activities
carried out by liver cells. There are extensive aggregations of both rough and smooth
endoplasmic reticulum, lysosomes, peroxisomes, and mitochondria. Free ribosomes and glycogen
granules are also prominent.
Kupffer cells
Among the discontinuous endothelial cells lining the sinusoids are Kupffer cells, macrophages
derived from monocytes. The cells in this section have taken up carbon
particles perfused through the circulation. The Kupffer cells around the
periphery of the lobule are most heavily labelled since they are exposed
to the blood first. This illustrates what happens when toxic substances
arrive at the periphery of the liver lobule, causing periopheral damage
before central changes occur. The central part of the lobule is however
more susceptible to ischemic damage.
The gallbladder stores and concentrates bile. A tall columnar
epithelium modified
for absorption. The cells have an apical brush border through which water and ions are absorbed.
Exocrine pancreas
The units of the exocrine pancreas are the acini. Acinar cells secrete precursors of proteolytic and lipolytic enzymes together with amylase. These products, such as trypsinogen, are activated in the lumen of the duodenum. The secretion from the exocrine pancreas is alkaline due to the secretion of bicarbonate ions. The acinar cells store the proenzymes in zymogen granules at the apex of each cell. The release of the proenzymes appears to be under hormonal control. Secretin and cholecystokinin are released by duodenal endocrine cells and promote proenzyme release. The duct system of the pancreas begins in the acini with centroacinar cells. The secretion from acinar cells is modified by the cells forming the ducts, particularly through the addition of bicarbonate.
Endocrine pancreas
The endocrine pancreas is represented by the Islets of Langerhans. The islets are clumps of endocrine cells clustered around a capillary plexus. The islets develop as buds from the exocrine ducts, eventually losing their ductal connections. There are many cell types present. Alpha cells producing glucagon are usually distributed around the periphery of the islet. Beta cells producing insulin are usually centrally located. About 10% of the cells present are of other types, producing somatostatin, pancreatic polypeptide, and VIP. As for other endocrine tissue there is an extensive fenestrated capillary plexus. The islets receive both parasympathetic and sympathetic innervation.
Note the extensive capillary plexus in this
section of an Islet, typical of all endocrine tissues.
The adrenal or pararenal glands lie
superiorly on each kidney. A section through the gland
illustrates the presence of a cortex and medulla. These two components have different origins.
The cortex is derived from mesoderm and the medulla differentiates from neural crest. A fetal
cortex is developed from the mesothelium lining the posterior abdominal wall. The permanent
cortex develops later to enclose the fetal cortex. During development the fetal cortex atrophies
and disappears. The medulla is formed by migration of cells from the neural crest. These cells lie
on the medial side of the developing cortex and are eventually enclosed as the medulla. The gland
is highly vascularized and innervated by the autonomic nervous system.
Adrenal cortex
The adrenal capsule surrounds the gland. Below the capsule the cortex is formed into three
structurally and functionally different layers, synthesizing steroid hormones produced from
cholesterol. The outer zona glomerulosa is formed of clumps, or glomeruli, of cells which
produce mineralocoticoids such as aldosterone. The zona fasciculata lies deeper and is formed of
radial cords of cells which produce glucocorticoids such as cortisol. This is the thickest layer of
the cortex. The inner layer of the cortex, the zona reticularis, produces androgenic steroids.
Capillaries run throughout the three layers.
Adrenal medulla
The cells of the adrenal medulla are equivalent to post-ganglionic sympathetic neurons. They secrete catecholamines. The cells of the medulla can be identified using an antibody to tyrosine hydroxylase, the rate limiting enzyme in the synthesis of the catecholamines.
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