Download The Digestive System

The Digestive System
Dr. Haythem Ali Alsayigh
Department of Human Anatomy And
Histology
DIVISIONS OF THE GUT TUBE
 Formation :
– Folded endoderm layer
– Ventral (floor) of gut
from portion of yolk sac
– As a result of
cephalocaudal and
lateral folding of the
embryo, a portion of
the endoderm-lined
yolk sac cavity is
incorporated into the
embryo to form the
primitive gut. Two
other portions of the
endoderm-lined cavity,
the yolk sac and the
allantois, remain
outside the embryo
DIVISIONS OF THE GUT TUBE
• In the cephalic and caudal parts of the
embryo, the primitive gut forms a blindending tube, the foregut and hindgut,
respectively.
• The middle part, the midgut, remains
temporally connected to the yolk sac by
means of the vitelline duct, or yolk stalk
DIVISIONS OF THE GUT TUBE
Parts of gut :
1. Foregut
2. Midgut
3. Hindgut
• Buccophryngeal
membrane
• Cloacal
membrane
• Vitelline duct
Mesenteries:
Definition
• Intraperitoneal organ
• Retroperitoneal organ
• Peritoneal ligaments
Initially foregut, midgut,
and hindgut are in broad
contact with posterior
abdominal wall
Development
• Development of the primitive gut and its derivatives is usually
discussed in four sections:
• (a) The pharyngeal gut, or pharynx, extends from the oropharyngeal
membrane to the respiratory diverticulum and is part of the foregut;
this section is particularly important for development of the head and
neck and is
• (b) The remainder of the foregut lies caudal to the pharyngeal tube
and extends as far caudally as the liver outgrowth.
• (c) The midgut begins caudal to the liver bud and extends to the
junction of the right two-thirds and left third of the transverse colon
in the adult.
• (d) The hindgut extends from the left third of the transverse colon to
the cloacal membrane
Development
• Endoderm forms the epithelial lining of the
digestive tract and gives rise to the specific cells
(the parenchyma) of glands,
• such as hepatocytes and the exocrine and
endocrine cells of the pancreas.
• The stroma (connective tissue) for the glands is
derived from visceral mesoderm.
• Muscle, connective tissue, and peritoneal
components of the wall of the gut also are
derived from visceral mesoderm.
MOLECULAR REGULATION OF GUT
TUBE DEVELOPMENT
• Regional specification of the gut tube into different
components occurs during the time that the lateral body
folds are bringing the two sides of the tube together
• Specification is initiated by transcription factors expressed
in the different regions of the gut tube.
• Thus, SOX2 “specifies” the esophagus and stomach;
• PDX1, the duodenum;
• CDXC, the small intestine; and
• CDXA, the large intestine and rectum.
•
MOLECULAR REGULATION OF GUT
TUBE DEVELOPMENT
• This initial patterning is stabilized by reciprocal interactions between the
endoderm and visceral mesoderm adjacent to the gut tube).
• This epithelial-mesenchymal interaction is initiated by Sonic Hedge Hog
(SHH) expression throughout the gut tube.
• SHH expression upregulates factors in the mesoderm that then
determine the type of structure that forms from the gut tube, such as
• the stomach, duodenum, small intestine, etc. For example, in the region
of the caudal limit of the midgut and all of the hindgut, SHH expression
establishes anested expression of the HOX genes in the mesoderm.
• Once the mesoderm is specified by this code, then it instructs the
endoderm to form the various components of the mid and hindgut
regions, including part of the small intestine, cecum, colon, and cloaca
•
MESENTERIES
• Portions of the gut tube and its derivatives are
suspended from the dorsal and ventral body wall
by mesenteries, double layers of peritoneum that
enclose an organ and connect it to the body wall.
Such organs are called intraperitoneal
• whereas organs that lie against the posterior
body wall and are covered by peritoneum on
their anterior surface only (e.g., the kidneys) are
considered retroperitoneal
MESENTERIES
• Peritoneal ligaments are double layers of
peritoneum (mesenteries) that pass from one
organ to another or from an organ to the body
wall. Mesenteries and ligaments provide
pathways for vessels, nerves, and lymphatics
to and from abdominal viscera
Mesenteries:
 5th week … Portions of the
gut tube and its derivatives
are suspended from the
dorsal and ventral body wall
by mesenteries, double layers
of peritoneum that enclose
an organ and connect it to
the body wall Extension of
dorsal mesentery
• Mesogastrium
• Mesodoudenum
• Mesocolon
• Mesentery proper
• Ventral mesentery, which exists only in the region of
the terminal part of the esophagus, the stomach, and
the upper part of the duodenum is derived from the
septum transversum.
• Growth of the liver into the mesenchyme of the
septum transversum divides the ventral mesentery into
• (a) the lesser omentum, extending from the lower
portion of the esophagus, the stomach, and the upper
portion of the duodenum to the liver and
• (b) the falciform ligament, extending from the liver to
the ventral body wall
Mesenteries:
 Ventral mesentery:
• Extension
• Divided by liver into:
1. Lesser omentum:
– Gastrohepatic lig
– Hepatodoudenal lig
Roof of epiploic foramen contain portal
triad
2. Falciform ligament
Its free margin contain umblical vein
forming round lig (lig teres hepatis)
Parts of the gut tube
Foregut: consists of 2 parts
1. Pharyngeal gut ( pharynx)
2. Foregut caudal to the pharyngeal tube until
the liver outgrowth.
Parts of the gut tube
 Midgut: begins caudal
to liver bud and end at
the junction between Rt
2/3rd with Lt1/3rd of
transverse colon
 Hindgut : begins from
there to the cloacal
membrane
Derivatives of the Foregut:
1. The Esophagus;
tracheoesophageal septum
(4th w) divide foregut :
1-When the embryo is
approximately 4 weeks old, the
respiratory diverticulum (lung
bud) appears at the ventral wall
– ventral part .tracheobronchial
of the foregut at the border with
diverticulum dorsal part ..esophagus, extends to stomach. the pharyngeal gut
– At first, the esophagus is short, but
with descent of the heart and
lungs, it lengthens rapidly
• muscle coat derived from
surrounding splanchnic mesoderm;
 upper 2/3rd striated (vagus supply)
 lower 1/3rd smooth (splanchnic
supply)
Clinical Derivatives of the Foregut:
• Esophagus
anomalies:
1. Atresia with or
without fistula
2. Stenosis
3. Shortenning with hiatal
hernia
tracheoesophageal fistula in order of their frequency of
appearance: A, 90%; B, 4%; C, 4%; D, 1%; and E, 1%.
Derivatives of the Foregut:
2. Stomach :
1-At 4th week stomach appaere
as a fusiform dilatation of the
foregutDuring the following
weeks, its appearance and
position change greatly as a
result of the different rates of
growth in various regions of its
wall and the changes in position
of surrounding organs.
Positional changes of the
stomach are most easily
explained by assuming that it
rotates around a longitudinal
and an anteroposterior axis
2-Rotates 90⁰ clockwise about
longitudinal axis Dorsal border
grow faster than ventral……
2 curvatures rotates 90⁰
clockwise around its AP axis
Derivatives of the Foregut:
2. Stomach :
– Attach to body wall by
dorsal and ventral
mesogasterium
– Rotation about long axia
will pull dorsal
mesogasterium to left
and formation of lesser
sac
– Spleen (5th w) between
the 2 leaves of dorsal
mesogasterium
Derivatives of the Foregut:
2. Stomach :
– Lengthening, fusion and degeneration of posterior
leaf of dorsal mesogastrium and peritoneum will
place the tail of pancreas in a retroperitoneal
position (secondarily retroperitoneal).
Derivatives of the Foregut:
2. Stomach :
– Stomach Rotation about anteroposterior axis will make
double layered greater omentum;
– layers fuse to form a single sheet
– The posterior layer of the greater omentum fuses with
the mesentery of the transverse colon.
Derivatives of the Foregut:
2. Stomach :
– Stomach anomalies:
1. Pyloric stenosis: develope during fetal life and it
results from hypertrophy of the circular
musculature of the pyloric region of the
stomach. The fetus complains of sever projectile
vomiting.
2. Duplication of the stomach.
3. Prepyloric septation
Derivatives of the Foregut:
3. Doudenum :
– Developed of 2 parts:
1. Caudal part of foregut down to major doudenal
papilla (hepatopancreatic duct)…… upper1/3rd
2. cephalic part of midgut; after the papilla…..lower
2/3rd
– Attach by dorsal mesodoudenum
– Rotate with stomach to the right
– Head of pancreas within the mesodoudenum
– Doudenum and pancreas be retroperitoneal
Derivatives of the Foregut:
Derivatives of the Foregut:
4. Liver and GB:
– Hepatic diverticulum ( liver
bud) appears (middle 3rd
week) as endodermal
outgrowth from foregut
(ventral) (doudenum)
penetrate the septum
transversum.
– bile duct; connection
between foregut and liver
bud
– gallbladder and the cystic
duct; ventral outgrowth from
the bile duct
Derivatives of the Foregut:
4. Liver and GB:
– hepatic sinusoids; liver cords
fuse with vitelline and umbilical
veins
– parenchyma (liver cells);from
epith of liver cords
– Hematopoietic cells, Kupffer
cells, and connective tissue cells
derived from mesoderm of the
septum transversum.
– The 90⁰ rotation will shift bile
duct opening from anterior to
posterior (or posteromedial).
Derivatives of the Foregut:
4. Liver and GB:
–
1.
2.
3.
Anomalies :
Accessory hepatic duct
Double GB
Biliary duct atresia
Derivatives of the Foregut:
5. Pancreas :
–
develops from 2 buds (endodermal lining) of duodenum :
1. Ventral bud:
•
•
from ventral wall close to the bile duct
Form lower half of the head and the uncinate process.
2. Dorsal bud:
•
•
From dorsal wall
Form the upper part of head, neck, body and tail
Derivatives of the Foregut:
5. Pancreas :
• Because of 90⁰ rotation, ventral bud rotates dorsally and
finally lie below and behind dorsal bud and then fuse
together.
• major pancreatic duct ; distal part of dorsal pancreatic
duct and entire ventral pancreatic duct.
• accessory pancreatic duct, if present, will be formed by
proximal part of dorsal pancreatic duct.
Derivatives of the Foregut:
5. Pancreas :
– Anomalies :
1. annular pancreas;
may causes
duodenal
obstruction.
2. Accessory
pancreatic tissues;
may be found in the
stomach or in
meckels
diverticulum.
Development and Derivatives of the Midgut:
Midgut attached to posterior body wall by
dorsal mesentery
it gives rise to the following structures:
1. The lower 2/3 of the duodenum below the
opening of the bile duct.
2. The jejunum and the ileum.
3. The cecum, appendix, ascending colon, and
right 2/3 of the transverse colon.
Development and Derivatives of the Midgut:
4 stages of midgut developments:
1. the preherniation stage:
2. The herniation stage:
3. Reduction stage:
4. Fixation stage:
Development and Derivatives of the Midgut:
1. Preherniation stage:
a) rapid elongation of gut
and its mesentery,
with formation of Ushaped loop with a
cranial and caudal
limbs.
b) The tip of the loop is
connected to the yolk
through the vitelline
duct.
Development and Derivatives of the Midgut:
2. Herniation stage:
a) rapid growth and elongation of intestinal loop, parts of
midgut will enter the umbilical cord during the 6th
week of development (physiological umbilical
herniation)
b) During herniation midgut loops rotate 90⁰
counterclockwise direction around the axis formed by
superior mesenteric artery.
Development and Derivatives of the Midgut:
2. Herniation stage:
c) This 90⁰ rotation will bring the cranial
limb to the right and the caudal limb
to the left side
d) right (cranial) limb will elongate faster
and becomes coiled and it will
develops into distal part of duodenum,
the jejunum, and part of ileum.
e) left (caudal) limb becomes lower
portion of ileum, cecum, appendix,
ascending colon, and proximal 2/3rd of
transverse colon
Development and Derivatives of the Midgut:
3. Reduction stage:
a) In 10th week of
development
b) During the process of
reduction, the intestinal
loops rotate
180⁰counterclockwise (so
the ultimate rotation of
the midgut is 270⁰
counterclockwise)
Development and Derivatives of the Midgut:
3. Reduction stage:
c) the180⁰ rotation make
right limb on the left and
left limb on right
d) proximal jejunum is first
part to reenter abdominal
cavity, and be on left side.
e) cecum, ascending and
transverse colon deviates
upward and to the right
Development and Derivatives of the Midgut:
4. Fixation stage:
a) Most of dorsal mesentery persists but that of
ascending and descending colon will disappear
making them retroperitoneal.
b) The appendix, lower end of the cecum, and sigmoid
colon retain their free mesenteries.
Anomalies of Midgut
1. abnormalities of the mesenteries:
• volvolus of cecum or colon; due to persistance
of the mesocolon
• retrocolic hernia; entrapment of portions of
the small intestine behind the ascending
mesocolon.
2. Body wall defects:
• Omphalocele
• Gastroschisis
CLINICAL CORRELATES
• Omphalocele
-
-
herniation of abdominal viscera through an
enlarged umbilical ring.
The viscera, are covered by amnion.
Due to a failure of the bowel to return to the
body cavity from its physiological herniation
during the 6th to 10th weeks.
Omphalocele
CLINICAL CORRELATES
• Gastroschisis
- herniation of abdominal contents through the body
wall directly into the amniotic cavity.
- It occurs lateral to the umbilicus, usually on the
right, through a region weakened by regression of
the right umbilical vein, which normally disappears.
- Viscera are not covered by peritoneum or amnion,
and the bowel may be damaged by exposure to
amniotic fluid.
- Both omphalocele and gastroschisis result in
elevated levels of α-fetoprotein in the amniotic fluid,
which can be detected prenatally.
•Gastroschisis mainly occurs in young women with cocaine
use
•Unlike omphalocele, gastroschisis is not associated
with chromosome abnormalities or other severe
defects.
• Therefore the survival rate is excellent, although
volvulus (rotation o f the bowel) resulting in a
compromised blood supply may kill large regions of the
intestine and lead to fetal death.
Anomalies of Midgut
3. Vitelline duct abnormalities:
• Meckels diverticulum (ileal diverticulum); out pocketing of the
antimesenteric border of the ileum through a persistent
vitelline duct.
• Enterocystoma (vitelline cyst); that may be twisted causing
strangulation and gangrene.
• Patent vitelline duct forming umbilical fistula with fecal
discharge from the intestine out of the umbilicus.
Anomalies of Midgut
4. Gut rotation defect
5. Gut atresia and stenosis; occur as a result of
vascular accidents
Derivatives of the Hindgut:
1. The left 1/3 of the transverse colon.
2. Descending colon.
3. Sigmoid colon.
4. The rectum.
5. Upper part of the anal canal
• -The endoderm of the hindgut also forms the
epithelium of the bladder and urethra-
Derivatives of the Hindgut:
• Hindgut enters posterior region of the cloaca (future
anorectal canal),
• Allantois enters anterior region (future urogenital sinus).
• Boundary between endoderm and ectoderm forms cloacal
membrane.
• Breakdown of cloacal membrane provides communication to
the exterior for the anus and urogenital sinus.
Derivatives of the Hindgut:
The urorectal septum, separates the allantois and
hindgut. This septum comes to lie close to the
cloacal membrane and its tip will form the
perineal body.
Derivatives of the Hindgut:
• Lower 1/3rd of anal canal derived from ectoderm around the
proctodeum.
• Ectoderm proliferates and invaginates to create the anal pit.
• Later, degeneration of the cloacal membrane (now called the
anal membrane) establishes continuity between the upper
and lower parts of the anal canal, which is delineated by the
pectinate line,
Derivatives of the Hindgut:
• Anomalies :
1. Imperforated anus;
occurring due to failure of
cloacal membrane to
rupture.
2. Rectoanal atresia.
3. rectovaginal or
rectourethral fistula.
4. Congenital megacolon is
due to an absence of
parasympathetic ganglia in
the bowel wall ( aganglionic
megacolon or Hirschsprung
disease)
THE END
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