Download EMBRYOLOGY Mid-Gut

‫‪4/3/2014‬‬
‫الثالثاء‬
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It grows faster than the vertebral column,thus
it will become U-Shaped structure with an
inverted apex.This U-shaped structure has a
proximal(cranial) & distal (caudal)limbs with the
superior mesenteric artery running in between
the 2 limbs,while the apex is connected to the
umbilicus by Vitelline ligament.
 The cranial limb will give rise to the following
parts of the G.I.Tract:
1-The caudal half of the duodenum(The cranial
half
comes from the fore-gut derivatives.
2-The Jejunum.
3-Most of the Ileum.
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While the Caudal limb will give the
following derivatives:
1-The terminal part of the Ileum.
2-The Caecum & Appendix.
3-The Ascending colon and 2 / 3 of the
transverse colon.
 The caecum with the apex attached to its tip
arises a conical diverticulum from the
caudal limb close to the apex of the loop i.ei
near the point of attachment of
Vitellointestinal duct.
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The U –shaped loop projects inside the
umbilical cord to form the umbilical
hernia,which is a normal feature between 6th –
12th weeks of intra-uterine life.Such a hernia is
due to the inadequacy of the abdominal cavity
which is largly occupied by the following
structures.
1-The rapidly growing liver.
2-The large sized mesonephros on each side of
the vertebral column.
 By the end of the 12th weeks of development,the
loop of the intestine starts to return back in to
the abdominal cavity because of the following
reasons:
1-Regression of the mesonephros.
2-No more increases in the size of the liver.
3-An increases in the expansion of the abdominal
cavity with an increase in the abdominal wall.
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Just before the start of reduction,the loop will
rotates 90 degree Unti-clock wise,thus the
original proximal limb comes to the right ,while
the distal limb comes to the left.
Once reduction starts the loop will complete its
anti-clock wise rotation to An angle of 270,so
that the original cranial limb comes to the
left(deeper),while the original caudal Limb
comes to the right side(Superficial).
The original cranial limb (to the left &
deeper)reurns first and occupies the right side of
The abdominal cavity(to form the coils of small
intestine).The caudal limb (left & superficial)
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Re-inters superficial to the cranial limb and
occupies the left side of the abdominal cavity
and forms the colon.The last segment to inter
is caecum which comes to lie superficial to
the small intestine,the caecum then descends
from just below the liver to reach its adult
positrion in the right iliac fossa.
1-Omphalocele ,where the loop is trapped out side & covers
only by amnion due to incomplete reduction of the intestinal
loop.
2-Congenital umbilical hernia ,which is mainly due to defects
in central part of the anterior abdominal wall and the skin
around the region of the umbilicus,thus the loop fails to
completes its reduction and stay out side and covers by
peritoneum ,amnion &skin.
3-The loop may rotates in clock wise direction instead of
being roting in Anti-clock wise,thus the duodenum becomes
in a reversed position & superficial to the transverse colon.
4-Anti-clock wise rotation for only 90 degree leads to
reversed position of the abdominal contents and is usually
associated with congenital heart defect ( Situs inversus).
5-Sub hepatic position of caecum & appendix as it fails to
descent ro right iliac fossa.
6-Anomalies related to the Vetelline ligament
including the followings:
A-Meckl's diverticulum is present 2 % of the
population , 2 feet from the ilio- caecal valve
& is about 2 inches in length.
B-Vitello-intestinal fistula connects the
intestine with the umbilicus & discharge some
of the Intestinal contents in to the umbilicus.
C-The whole duct may persist as a fibrosed
band extending from the umbilicus to the
ileum.
D-Part of the duct may persist & forms a
vitelline cyst along the course of the duct.
The hind gut gives rise to the following parts
of the G.I.Tract:
1-The left one third of the transverse colon.
2-The descending colon.
3-The sigmoid colon.
4-The rectum.
5-The upper half of the Anal canal.
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The most caudal part of the hind gut(caudal
to the origin of the allantois) is known as
Endodermal cloaca with the cloacal
membrane lying in its ventral wall.The
Endodermal cloaca is divided by
mesodermal septum (Uro-rectal septum)in to
two parts;
A-Ventral part known as Urogenital sinus.
B-Dorsal part forms the rectum & upper half of
the anal canal.
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At the mean time the cloacal membrane is divided
in to two parts too :
A-Urogenital membrane ventrally.
B-Anal membrane dorsally.
 The mesoderm surrounding the cloacal
membrane thickened & elevated,so the whole
membrane comes to lie at the depression of
Ectodermal cloaca.The anal membrane becomes
surrounded by anal tubercle (mesodermal
proliferation)that will form the external anal
sphincter.Thus the anal membrane becomes at
the bottom of the proctodeum(anal pit) Which
gives rise to the lower half of the anal
canal.Rupture of the membrane leads to
continuity between the 2 halves of the anal
canal& the site of the anal membrane
represented in adult by anal valves.
1-Imperforated anus due to failure of rupture
of the anal membrane.
2-Congenital Recto-vaginal(in females) or
Recto-Vesical (in males) fistula due to
defective Uro-rectal septum.
3-Rectal atresia i.e obliteration of the lower
part of the rectum due to replacement of the
caudal part of the ano-rectal canal by
fibrous tissues.
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Starts as a ventral out growth (Diverticulum) from the
ventral aspect of the caudal part of the fore gut.It
grows in to the ventral mesogastrium & divides in to
2 Buds with a stem
(stalk),these buds are known as pars
hepatica(develops in to R & L lobes of the liver) and
pars cystica which forms the gall bladder,while the
stem of the diverticulum forms the Bile Duct.The
cells of the liver are arranged in strands,which are
separated from each other by blood sinusoids(from
mesoderm of septum transversum).The mesoderm is
also responsible for the formation of stroma &
capsule of the liver,while kupffer cells form from
mesenchym
It takes origin from 2 pancreatic Buds (from 
the endodermal lining of the
Duodenum),these buds are one dorsal & one
ventral buds.The dorsal pancreatic bud
grows in to the mesoduodenum & forms the
tail,body & the major part of the head of the
pancrease.It also give the major part of the
main pancic duct & the accessory pancreatic
duct.
The Ventral pancreatic bud accompanies the
hepatic diverticulum & forms the remaining
part of the head of the pancrease and the
uncinated process,it also share in the
formation Of the main pancreatic duct which
drains the rest of the head &uncinated
process.
 The endodermal cells of the pancreatic buds
forms:
1-The secretory acini which secretes the
pancreatic enzymes.
2-Islets of Langerhans that secretes insulin
from B –cells & Glucagon from Alpha cells.
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1-Pancreatic tissue may invade the pyloric part
of the stomach.
2-Annular pancrease ,where a ring of
pancreatic tissue surrounds the whole
circumference of the Duodenum due to a
bnormal migration of the pancreatic buds.
3-Cystic Fibrosis of the Pancrease results in
very viscous pancreatic secretions as a result of
obstruction of the pancreatic duct.
It develops from the mesoderm in the dorsal
mesogastrium which becomes divided in to
gastro-splenic and lieno-renal ligaments.It is at
first lobulated,but later on the lobes become
confluent together.The presence of notches in
the upper border of the adult spleen is an
indication of its earlier lobulation.
 The main Congenital Anomalies of Spleen are:
1-Lobulated spleen due to failure of the lobes to
become confluent.
2-Accessory splenules. 
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Its cranial half develops from the fore-gut,while
its caudal half develops from the mid-gut.
This explains its blood supply from both Coeliac
trunk & Superior mesenteric arteries. It is at
first has a dorsal mesenmtery known as
Mesogastrium attacheit to the posterior wall of
the abdomen.As aresult of rotation of the
stomach,the Duodenum comes to lie at the right
side & becomes C-shaped structure tjat loses
its mesentery & becomes as retroperitoneal
structure exceopts its first one inch
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The face develops from five processes theses
are the followings
1-The Fronto-nasal Process.
2-Two Maxillary Processess.
3-Two Mandibular Processess.
The mesoderm covering the fore brain
proliferates to form a single median process
termed the frontonasal process.On the
ventromedial part of the free end of the
frontonasal process an ectodermal thickening
area termed olfactory placode(this will become
depressed to form the olfactory pit).The olfactory
pit has media &lateral folds.
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Part of the frontonasal process is called the
intermaxillary (Premaxilla)segment.It lies
between the 2 maxillary processes & gives
rise to the followings:
1-Philtrum of the upper lip.
2-Median part of the upper jaw.
3-The primitive palate.
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On each side of the Stomodaeum the first
branchial arch gives rise to 2 processes.
A-Maxillary processes,which fuses with the
lateral nasal fold.
B-Mandibular processes:The 2 meet together
in the mid line.
The maxillary process extends medially to
fuse with the lower end of the frontonasal
process to form the lower lip & upper jaw.
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Each maxillary process extends medially from
below the olfactory pit to fuse with the tends
medial nasal fold.As a result the olfactory pit
becomes converted in to the primitive nasal
cavity with 2 openings,one leads to the surface
called the primitive anterior naris & another one
opens in to the stomodaeum ( posteriorly) called
the primitive posterior naris. Then the 2
primitive cavities are separated from each other
by the primitive nasal septum ( from the
Frontonasal septum).
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The lower end of the frontonasal process grows
back ward to form the primitive palate,which is
limited anteriorly by the 4 incisors.It is derived
from the intermaxillary segment. A horizontal
palatine process extends medially from the inner
aspect of the side of each maxillary process.In
the mid line this palatine process meets & fuses
with each other as with the lower border of the
nasal septum.
Anteriorly each palatine process fuses with the
primitive palate at an oblique line.Posterior to the
nasal septum ,the fused palatine processes are
not ossified & forms soft palate&uvula
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1-Hare lip,is due to failure of fusion between
the Maxillary process & Medial Nasal fold..It
leads to a fissure between the philtrum &
upper lip, it could be.
A-Lateral hare lip
BMedian hare lip.
C-Bilateral hare lip.
2-Oblique facial cleft,which is due to failure
of fussion between the maxillary process &
the side of the frontonasal process along the
side of the nose.
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3-Cleft palate:
A-Unilateral ,due to failure of one horizontal
palatine process to extnd medially,thus failing to
fuse with the primitive palate..
B-Bilateral cleft palate,,here the 2 palatine
processes fail to fuse with the primitive palate.
C-Isolated cleft palate,here the defect is in the
mid line of the palate behind incisive fossa.
D-Bifid soft palate ( Bifid Uvula ), here the 2
palatine processes fail to fuse together in the mid
line place in their most posterior part.