Download The Digestive System

• Throughout the remainder of
the digestive system, the
histological composition of the
alimentary canal can be
described by the following four
layers.
• Mucosa
• The lumen is lined by an
epithelium, which rests on a
vascular c.t. the lamina
propria,which contains bl. vessels,
lymph nodules and glands. The
lamina propria is surrounded by a
band of smooth muscle(muscularis
mucosae). These three tissues are
collectively referred to as the
mucosa of the alimentary canal .
• Submucosa :Is a connective
tissue containing blood vessels,
lymph nodules,the submucosal
(Meissener's) nerve plexus and
submucosal glands
(oesophagus and duodenum).
• Muscularis Externa:
Inner circular layer and outer
longitudinal.
Between these two layers is the
myenteric (Auerbach) nerve
plexus
• Adventitia/Serosa:
The alimentary canal is
surrounded by a layer of loose
connective tissue, the
adventitia. In the case of the
intraperitoneal parts of the
alimentary canal, a simple
squamous epithelium to a layer
of loose of connective tissue,
forms the serosa.
• The Oesophagus
• In the oesophagus the mucosa
is formed by a stratified
squamous epithelium (nonkeratinised) and a well-defined
lamina propria and muscularis
mucosae.
• Oesophageal glands are located
in the submucosa.
• The muscularis externa
contains striated muscle in its
upper one third, a mixture of
striated muscle and smooth
muscle in its middle one-third
and smooth muscle in its lower
one-third.
• The adventitia surrounds the
intrathoracic part which
consists only of a layer of loose
connective tissue. A serosa
forms the outermost part of the
short intraperitoneal segment of
the oesophagus.
• GASTROINTESTINAL TRACT
• The gastrointestinal tract (GIT)
comprises the stomach,
duodenum, jejunum, ileum,
colon, rectum and anal canal.
The GIT and oesophagus form
the alimentary canal.
• The Stomach
• Anatomically, the stomach is
divided into :a cardiac part, fundus,
body or corpus, and a pyloric part
(pyloric antrum and pyloric canal)
The Mucosa (epithelium, lamina
propria, muscularis mucosae).The
mucosa is thrown into longitudinal
folds (gastric folds or rugae), which
disappear when the stomach is
fully distended.
• On the mucosal surface we see
small, depressions (gastric pits).
Simple, branched tubular gastric
glands open into the bottom of the
gastric pits.
The structure and cellular
composition of the surface
epithelium which is simple, tall
columnar, does not change
throughout the stomach. These
cells secrete mucus.
• Cardiac glands:
Cardiac glands are branched
tubular glands which contain
mainly mucus-producing cells.
• Principal (or corpus-fundic) glands
Each glandular tubule consists of :
a deep body, and neck .In principal
glands we find four cell types: chief
cells, parietal cells, mucous neck
cells and endocrine cells.
• Chief cells (or zymogenic cells) are
the most numerous of the four
types. They occur primarily in the
body(base) of the glands. They
produce pepsinogen.
Parietal cells (or oxyntic cells)
occur in the neck of the glands.
Parietal cell secrete HCL and
intrinsic factor, which is necessary
for the resorption of vitamin B12.
• Mucous neck cells are found
between the parietal cells in the
neck of the gland.
Endocrine cells are part of the
gastro-entero-pancreatic (GEP)
endocrine system. The endocrine
cells in the gastric mucosa are
gastrin-producing cells (G cells)
and somatostatin-producing cells
(D cells).
• Pyloric glands
Pyloric glands are more coiled than
principal glands.Gastrin-producing
cells, are more frequent than in
principal glands. A few parietal
cells may be present.
• The lamina propria is formed by a
very cell-rich loose c.t. (fibroblasts,
lymphocytes, plasma cells,
macrophages, eosinophilic
leukocytes and mast cells).
• The muscularis mucosae of the
stomach contains both circular
and longitudinal layers of
muscle cells.
• Large blood vessels, lymph
vessels and nerves and the
submucosal (Meissener's) nerve
plexus are located in the
submucosa which consists of
loose connective tissue.
• The muscularis externa
consists of three layers of
muscles: an inner oblique layer,
a middle circular layer and an
outer longitudinal layer.
• Serosa: is connective tissue
and mesothelium.
• Small Intestine
• The small intestine is divided
into duodenum , jejunum and
ileum. The three segments have
the same basic histological
organization.
• The mucosa of the small intestine
has various structural features
which increase the luminal surface
area .
• Plicae circulares (of Kerkering) are
macroscopically visible, crescentshaped folds of the mucosa and
submucosa. Plicae circulares are
permanent structures
• The entire intestinal mucosa
forms intestinal villi(elevations
of epithelium and lamina
propria).The surface of the villi
is formed by a simple columnar
epithelium. Each absorptive
cell, or enterocyte, of the
epithelium forms numerous
microvilli.
• Between the intestinal villi we
see the openings of simple
tubular glands, the crypts of
Lieberkühn. They extend
through the lamina propria
down to the muscularis
mucosae. Undifferentiated cells
close to the bottom of the
crypts regenerate the
epithelium.
• Other epithelial cells in the
crypts correspond largely to
those in the epithelium of the
intestinal villi.Paneth cell
release a number of
antibacterial substances and
goblet cells secrete mucus.
• The lamina propria is, similar to
the lamina propria of the
stomach.
• Lymph nodules may form
aggregations of 30-50
nodules in the lamina propria
of the ileum.
These large aggregations are
called Peyer's patches.
• The muscularis mucosae has
two layers and extends into the
intestinal villi, where the
smooth muscle cells form a
longitudinal bundle in the
centre of the villi .Blood vessels
and lymphatic
capillaries(lacteals) are also
present
• The Submucosa
The submucosa contains
glands only in the duodenum.
Submucosal glands of the
duodenum are also called
Brunner's glands. Their
secretion is mucous and
slightly alkaline and protects
the duodenal mucosa.
The large intestine constitutes
the terminal part of the
digestive system. It is divided
into three main sections: cecum
including the appendix, colon,
and rectum with the anal canal.
• The surface of the mucosa is
smooth as there are no plicae
circulares or intestinal villi.
• Crypts of Lieberkühn are
present. Goblet cells are more
than in the small intestine.
• There is lamina propria between
the glands. The muscularis
mucosae forms two layers. The
appearance of the muscularis
externa is different from that of
the small intestine.
• The inner circular layer of
muscle forms the usual sheath
around the large intestine, but
the outer longitudinal muscle
layer forms three flattened
strands, the taeniae coli.
• The adventitia forms small
pouches (appendices
epiploicae) filled with fatty
tissue along the large intestine.
• Specialized Sections of the
Large Intestine
• The vermiform appendix:
The most important of its
features is the thickening of its
walls primarily due to large
accumulations of lymphoid
nodules in the lamina propria.
• Intestinal villi are absent, and
crypts are rare. There is often fatty
tissue in the submucosa. The
muscularis externa is thinner than
in the remainder of the large
intestine.
• The anal canal
The mucosa has a characteristic
surface elevations of 5-10
longitudinal folds, the anal
columns.
Small folds between the anal
columns (anal valves) form the
pectinate line. Crypts disappear
below the pectinate line and the
epithelium changes from the tall,
columnar type seen in other parts
of the large intestine to a stratified
squamous epithelium. The
muscularis externa gradually
becomes thicker and forms the
involuntary internal anal sphincter.
• The Liver
• The Liver is surrounded by a
thin capsule of connective
tissue. The connective tissue
extends into the liver
parenchyma and divides it into
the basic structural units of the
liver, the "classical" liver
lobules.
• There are other ways of dividing
the parenchyma of the liver into
units. Two common ways are
divisions into portal lobules and
liver acini. The portal vein,
hepatic artery and bile duct
enter the liver through the porta
hepatis. These three vessels
travel together through the liver
parenchyma.
• Portal Triads:
A branch of the portal vein, a
branch of the hepatic artery and
a branch of the bile duct - are
called portal triads. Portal triads
are a key feature of the
organization of the liver. Portal
triads are embedded in
interlobular connective tissue.
• The Liver Lobule
The "classical" liver lobule is
hexagonal(six-sided). It is
delimited by interlobular
connective tissue (only little, if
any, visible in humans; plentiful
in e.g. pigs). In its corners we
find the portal triads.
• In cross sections, the lobule is
filled by cords of hepatic
parenchymal cells, hepatocytes,
which radiate from the central vein
and are separated by vascular
sinusoids.
Hepatocytes are separated from
the bloodstream by a thin
fenestrated simple squamous
epithelium, which lines the
sinusoids.
• Between the hepatocytes and the
epithelial cells is a narrow
perisinusoidal space (of Disse).
Contents of the blood plasma can
freely enter the perisinusoidal
space through the fenestrations of
the epithelium lining the sinusoids.
Fixed macrophages, Kupffer cells,
are attached to the epithelium.
• The liver lobule is drained by
the central vein, which open
into the sublobular veins of the
liver. These in turn coalesce to
form the hepatic veins. They run
alone through the tissue, are
usually covered by connective
tissue and eventually empty
into the inferior vena cava.
• Biliary System:
Intrahepatic
Adjoining liver cells form the walls
of the bile canaliculi. Bile canaliculi
connect via very short canals of
Hering(formed by both hepatocytes
and simple cuboidal epithelium) to
terminal bile ducts (ductules)
which empty into the interlobular
bile ducts found in the portal triad.
• Extrahepatic
Interlobular bile ducts merge to
form the left and right hepatic
ducts and then the common
hepatic duct, which connects to the
cystic duct to form the bile
duct.The bile duct carries the bile
to the duodenum. The cystic duct
leads to the gall bladder, where the
bile is concentrated and stored.
• The terminal and interlobular
bile ducts are lined by a
cuboidal epithelium. All other
parts of the bilary system are
lined by a tall columnar
epithelium.
• Gall Bladder:
• Mucosa (epithelium and lamina
propria):
• In the gall bladder the epith. is
highly folded and often caved
(diverticuli). The epith. is simple
column. doesn’t contain goblet
cells and a muscularis mucosae
is absent.
• Fibromuscular layer: Bundles of
smooth muscles separated by
C.T.
• Perimuscular: C.T layer(large
blood vessels and nerves).
• Serosa: mesothelium+
connective tissue.