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DIGESTIVE SYSTEM part 2
Joanna Sekita-Krzak MD PhDDepartment of Histology and Embryology
PARIETAL (oxyntic) CELLS
FUNCTION
The main functions of oxyntic cells
include:
the secretion of hydrochloric acid
(hydrogen ions, protons) and
the secretion of intrinsic factor.
PARIETAL (oxyntic) CELLS
FUNCTION
Secretion of intrinsic factor
Intrinsic factor plays very important function. It is
necessary for the normal absorption of vitamin B12
in digestive system.
Vitamin B12 is necessary for the normal maturation
of red blood cells.
The lack of intrinsic factor (for example in atrophic
gastritis that destroys the stomach mucosa) leads
to pernicious anemia that is the result of vitamin
B12 deficiency.
GASTROENDOCRINE (APUD, DNES)
CELLSGastroendocrine cells are also reffered to as APUD cells (amine precursor uptake and decarboxylation), or
DNES cells (they are members of the diffuse neuroendocrine system cells).
They are scattered among chief cells.
They possess cytoplasmic granules that contain hormones .
Hormones are secreted into the blood and regulate the activity of the gastrointestinal tract.
So, these cells have an endocrine function in the digestive system.
Endocrine cells are found not only in the stomach but also in the mucosa of duodenum, small and large intestine.
Lack of intrinsic factor
Normal peripheral
blood smear pernicious anemia
vitamin B12 deficiency
Peripheral blood smear in
iron deficiency anemia
showing microcytic,
hypochromic red blood
cells
GASTROENDOCRINE (APUD, DNES)
CELLSGastroendocrine cells are also reffered to as APUD cells (amine precursor uptake and decarboxylation), or
DNES cells (they are members of the diffuse neuroendocrine system cells).
They are scattered among chief cells.
They possess cytoplasmic granules that contain hormones .
Hormones are secreted into the blood and regulate the activity of the gastrointestinal tract.
So, these cells have an endocrine function in the digestive system.
Endocrine cells are found not only in the stomach but also in the mucosa of duodenum, small and large intestine.
GASTROENDOCRINE (APUD) CELLS
Gastroendocrine cells are divided into several types depending on the kind of hormone they secrete.
For example
⚫ A-cells located in the stomach produce glucagon,
⚫ G-cells located in the pylorus and duodenum secrete gastrin that strongly stimulates the production of hydrochloric acid.
⚫ D-cells located in the pylorus and duodenum secrete somatostatin. This hormone inhibits other endocrine cells of the digestive tract and is used in clinic for the inhibition of secretion in digestive system for example in patients with gastrointestinal haemorrhage.
PYLORUS
The mucous membrane of the
pylorus has very deep gastric
pits.
The lamina propria comprises pyloric glands which open into the bottom of gastic pits.
They are simple or branched tubular glands They secretemucus but
some oxyntic and gastroendocrine cells are found.
PYLORUS
PYLORUS - G cellsGastroendocrine cells located in thepylorus (G-cells) secrete gastrin- thehormone that
⚫ stimulates the parietal cells to producehydrochloric acid,
⚫ to a smaller degree, it also stimulates thechief cells to produce pepsinogen.
G-cells are intercalated among the mucouscells of pyloric glands.
Gastrin secretion is stimulated by:
⚫ vagus nerve (under the influence of sight, smell, taste or tought of food),
⚫ by the presence of protein foods in thestomach and in duodenum.
Gastrin is released from G-cells into theblood and with the blood is carried to thegastric glands where it stimulates theparietal and chief cells.
STOMACHMUCOUS MEMBRANE
Under the lamina
propria of the mucous
membrane there is
muscularis mucosae
that separates the
mucosa from the
underlying submucosa.
STOMACHSUBMUCOUS MEMBRANE
The submucosa is
composed of dense
connective tissue that
is infiltrated by
lymphocytes,
macrophages and
mast cells.
It contains nerves,
blood and lympatic
vessels.
STOMACHMUSCULARIS EXTERNA
The muscularis externa
is composed of smooth
muscle cells arranged in
three layers:
⚫ the external layer is
longitudinal,
⚫ the middle layer is
circular and
⚫ the internal layer is
oblique.
At the pylorus the middle
layer is thick and forms
the pyloric sphincter.
STOMACHSEROSA
The muscularis externa is
covered by serosa that
consists of a thin layer of
connective tissue covered
by mesothelium (simple
squamous epithelium) and
constitutes the most
external layer of the
stomach wall.
SMALL INTESTINE
The small intestine is the site of
terminal food digestion,
nutrient absorption and
endocrine secretion.
The small intestine consists of
three parts:
⚫ the duodenum about 25 cm long
⚫ the jejunum about 4 m long
⚫ the ileum about 2 m long.
SMALL INTESTINESTRUCTURE
The wall of the small intestine consists of the usual four layers:
the mucous membrane,
the submucous membrane,
the muscularis externa and
the serosa.
Small intestine
SMALL INTESTINEMUCOUS MEMBRANE
The mucous membrane is thrown into the numerous circular, permanent folds called the plicae circulares.
These permanent folds extend one-half or two-thirds of the way around the lumen. They effectively increase the surface area.
INTESTINAL VILLI
The free surface of the mucous membrane has small, fingerlike projections called intestinal villi.
Each villus measures about 0,5 to 1,5 mm long.
In the duodenum the villi are long and broad, they become shorter and thinner in the distal part of the small intestine.
INTESTINA VILLI
The villi are covered by simple columnar epithelium.Each villus has a core of connective tissue containing an artery, a capillary network, a vein and lymphatic vessel.
Narrow strands of smooth muscle derived from the muscularis mucosae extend into each villus and by their contraction aid the circulation of lymph through lymphatic vessel.
INTESTINAL VILLI
epithelium
The epithelium covering
the villi is a simple
columnar epithelium and it
contains three types of
cells:
absorptive cells
mucus-secreting goblet
cells and
enteroendocrine cells
(DNES cells).
ABSORPTIVE CELLSAre tall columnar cells.
They have oval nuclei located in
the basal half of the cell.
At the apex of each cell is a layer
called the brush border. This
border is a layer of densely
packed microvilli.
Each microvillus is a cylindrical
protrusion of the apical
cytoplasm covered with cell
membrane and a core of actin
filaments.
Microvilli greatly increase the
surface area of contact between
the intestinal mucosa and food
(absorptive surface) about 20
folds.
ABSORPTIVE CELLS
The outer surface of microvilli is
bounded with several enzymes which
are very important in the terminal
digestion of food. These are:
⚫ Disaccharidases: sucrase, maltase and
lactase – enzymes which hydrolyze
disaccharides into monosaccharides;
⚫ Lipase – an enzyme that hydrolyzes fats
into glycerol and fatty acids and
⚫ Dipeptidases which hydrolyze dipeptides
into amino acids.
Deficiences of these enzymes lead to
digestive disturbances
ABSORPTIVE CELLS
functionThe most important function of the columnar absorptive cells is the absorption of nutrient molecules.
Amino acids and monosaccharides cross the apical plasma membrane by facilitated diffusion, whereas the products of lipid hydrolisis (fatty acids and monoglycerides) cross passively and they are collected in the SER where they are resynthesized into triglycerides and then assembled into chylomicrons
GOBLET CELLS
The goblet cells are scattered in the epithelium between the absorptive cells.
They sectrete mucus.
The main function of this mucus is to protect and lubricate the lining of the intestine.
ENTEROENDOCRINE CELLS
The enteroendocrine cells of small intestine belong
to DNES cells (APUD cells).
The enteroendocrine cells of small intestine secrete
hormones such as:
⚫ GIP
⚫ serotonin,
⚫ secretin,
⚫ cholecystokinin,
⚫ motylin and
⚫ urogastrone.
These hormones are secreted into the bloodstream through
the basal surface of enteroendocrine cells.
These cells are small and are scattered between the bases
of columnar absorptive cells.
GIP Glucose-dependent insulinotropic peptide
Gastric inhibitory peptide
K-cells located in the small intestine
secrete GIP glucose-dependent
insulinotropic peptide that stimulates
insulin secretion.
ENTEROENDOCRINE CELLS
EC cells secrete Serotonin increases
gut motility.
S cells secrete Secretin stimulates
bicarbonate secretion by pancreas and
biliary tract.
I cells secrete Cholecystokinin
stimulates release of pancreatic
enzymes and contraction of gall
bladder (with release of bile)
Mo cells secrete Motylin increases gut
motility.
Urogastrone produced by Brunner
glands of the duodenum inhibits
gastric HCl secretion and enhances
epithelial cell division.
INTESTINAL GLANDSThe lamina propria of the mucous membrane consists of loose connective tissue and contains intestinal glands which are known as the crypts of Lieberkühn.
These glands are tubular in shape and extend through the entire thickness of mucous membrane almost to the muscularis mucosae.
Intestinal glands open between the bases of the villi.
Intestinal glands are lined with the simple columnar epithelium.
INTESTINAL GLANDS
The epithelium covering the
upper half of the intestinal
glands is identical to that
covering the villi.
In the lower half of the glands
two new types of cells are
found:
⚫ Paneth’s cells and
⚫ stem cells.
STEM CELLS
Stem cells (regenerative cells)
are continuously undergoing
mitosis to replace the cells
that are exfoliated from the
tips of the villi.
They can differentiate into
absorptive cells, Goblet cells
and enteroendocrine cells.
They are located in the basal
half of the crypts of
Lieberkühn.
PANETH’S CELLS
Paneth’s cells are found in
small groups at the bottom
of the intestinal glands.
They stain deep pink in
H+E stained slides.
They contain lysozyme –
an enzyme that digests the
bacterial wall and playes
an important role in
controlling the intestinal
flora.
Peyer’s patches and M-cells
The lamina propria of small intestine has numerous lymphatic nodules which form Peyer’s patches.
The epithelium covering Peyer’s patches has special type of cells which are called M-cells (Microfold cells).
Microphold cells can endocytose antigens and transport them to the underlying lymphoid cells (macrophages and lymphocytes) in underlying nodules. So, they help to initiate immune response.
Peyer’s patches and M-cells
The lamina propria of small intestine has numerous lymphatic nodules which form Peyer’s patches.
The epithelium covering Peyer’s patches has special type of cells which are called M-cells (Microfold cells).
Microphold cells can endocytose antigens and transport them to the underlying lymphoid cells (macrophages and lymphocytes) in underlying nodules. So, they help to initiate immune response.
SMALL INTESTINE MUSCULARIS MUCOSAE
The muscularis
mucosae consists of
a thin layer of
smooth muscle cells
and is the deepest
layer of mucous
membrane
separating it from the
underlying
submucosa.
SMALL INTESTINE SUBMUCOUS MEMBRANE
The submucosa consists
of dense connective
tissue and contains
Meissner’s plexus.
DUODENUMSUBMUCOUS MEMBRANE
In the duodenum submucosa
contains compound tubular
glands called Brunner glands (or
doudenal glands).
⚫ These glands are mucous glands.
⚫ They empty into the intestinal
glands.
⚫ They secrete mucus that is
alkaline (pH 8-9) and
urogastrone.
⚫ The former helps to increase the
pH of the acidic chyme comming
in from the stomach.
⚫ This increase of pH also helps
pancreatic enzymes in becoming
active to allow further digestion.
⚫ These glands help in protecting
the epithelium by neutralizing the
acidic chyme.
SMALL INTESTINE MUSCULARIS EXTERNA
The muscularis externa of the small intestine consists of an inner circular layer and outer longitudinal layer of smooth muscle.
Between the muscle layers the Auerbach’s plexus of autonomic nerves is located.
SMALL INTESTINE SEROSA
The serosa is the
outermost coat of the
small intestine and
consists of a thin
layer of loose
connective tissue
covered by a simple
squamous
epithelium.
LARGE INTESTINE
The large intestine is
divided into the:
⚫ cecum,
⚫ appendix,
⚫ colon
⚫ ascending,
⚫ transverse and
⚫ descending,
⚫ rectum and
⚫ anal canal.
LARGE INTESTINE
STRUCTUREThe structure of the large intestine shows the usual four layers that are characteristic of the alimentary canal:
the mucous membrane,
the submucous membrane,
the muscularis externa and
the serosa.
LARGE INTESTINE
STRUCTUREThe structure of the large intestine differs from
that of the small intestine in many respects:
There are no permanent folds of mucous membrane (no
plicae circulares)
There are no villi on the surface of the mucous
membrane.
The mucous membrane has long intestinal glands (crypts
of Lieberkühn) that have no Paneth’s cells but more
Goblet cells. Goblet cells constitute 60-70% of all
epithelial cells.
Although numerous solitary lymphatic nodules are
present there are no Peyer’s patches.
The muscularis externa differs from it counterpart in the
small intestine by the arrangement of its outer,
longitudinal layer into three thick bands called teniae
coli.
The serosa is arranged differently in the different parts of
the large intestine. It forms a complete covering of
peritoneum for the cecum.
It covers the anterior surface and sides of the ascending
and descending colon.
LARGE INTESTINEepithelium
The cecum and colon are lined with
simple columnar epithelium with
numerous
⚫ goblet cells,
⚫ surface absorptive cells, and
⚫ occasional endocrine cells
The rectum is lined with simple
columnar epithelium similar to that
of the colon.
The anal canal is lined with
⚫ simple columnar epithelium
proximal to the anal valves,
⚫ stratified nonkeratinized squamous
epithelium distal to the anal valves
and
⚫ stratified squamous keratinized
epithelium at the anus.
LARGE INTESTINE
FUNCTIONSFunctions:⚫ the large intestine functions
primarily in the absorption of electrolytes, fluids and gases.
⚫ It houses bacteria that produce ⚫ vitamin B12 (necessary for
hemopoiesis )and
⚫ vitamin K necessary for coagulation.
⚫ The large intestine produces abundant mucus, which lubricates its lining and facilitates the passage and elimination of feces.
APPENDIXIs a wormlike diverticulum of the cecum.
Contains a large amount of lymphoid
tissue.
The appendix has a usual four-layered
structure of the alimentary canal.
⚫ The mucous membrane is identical to that
of the large intestine, possessing shallow
tubular crypts of Lieberkühn. It does not
form villi.
The lamina propria contains numerous
lymphoid nodules and lymphoid cells. In
many areas lymphoid tissue extends
through the muscularis mucosae into the
submucosa.
⚫ The submucosa of the appendix is
composed of connective tissue containing
lymphoid nodules
⚫ The muscularis externa of the appendix is
composed of an inner circular and an outer
longitudinal layers of smooth muscle.
⚫ The serosa completely surrounds the
appendix.
Fotos and diagrams in presentation from:
Color Textbook of Histology by Leslie P. Gartner, James L. Hiatt, fourth edition 2017; Basic
Histology by L. Carlos Junqueira, 11th edition, 2005; Wheater’s Functional Histology by B. Young,
J.W. Heath 4th ed.
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