Download 7.Development of mid..

By: Dr. Mujahid Khan
The derivatives of the midgut are:
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The small intestine including most of the
duodenum
The cecum, appendix, ascending colon and the
right half to two-thirds of the transverse colon
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Midgut derivatives are supplied by the
superior mesenteric artery
The midgut loop is suspended from the dorsal
abdominal wall by an elongated mesentery
As the midgut elongates, it forms a ventral Ushaped loop of gut, the midgut loop
It projects into the remains of the
extraembryonic coelom in the proximal part of
the umbilical cord
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At this stage the intraembryonic coelom
communicates with extraembryonic coelom at the
umbilicus
This movement of the intestine is a physiological
umbilical herniation
It occurs at the beginning of the 6th week
The midgut loop communicates with the yolk sac
through the narrow yolk stalk or vitelline duct
until the 10th week
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Umbilical herniation occurs because there is not
enough room in the abdomen for the rapidly
growing midgut
The shortage of space is caused mainly by the
relatively massive liver and two sets of kidneys
during this stage of development
The midgut loop has a cranial limb and a caudal
limb
Yolk stalk is attached to the apex of the midgut
loop where the two limbs join
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The cranial limb grows rapidly and forms
small intestinal loops
The caudal limb undergoes very little change
except for development of cecal diverticulum
which is a primordium of the cecum and
appendix
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While it is in the umbilical cord the midgut
loop rotates 90º counterclockwise around the
axis of superior mesenteric artery
This brings cranial limb to the right and the
caudal limb to the left
During rotation the midgut elongates and
forms intestinal loops e.g. Jejunum and ileum
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During the 10th week the intestines return to the
abdomen
What causes the return of the intestine is not
known
The decrease in the size of the liver and kidneys
and the enlargement of the abdominal cavity are
important factors
This process is called reduction of the
physiological midgut hernia
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The small intestine formed from cranial limb
returns first
It passes posterior to the superior mesenteric
artery and occupies the central part of the
abdomen
As the large intestine returns, it undergoes further
180º counterclockwise rotation
Later it comes to occupy the right side of the
abdomen
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The primordium of cecum and appendix, the
cecal diverticulum appears in the 6th week as a
swelling on the antimesenteric border of the
caudal limb of the midgut
The apex of the cecal diverticulum does not
grow as rapidly as the rest of it
The appendix is initially a small diverticulum
of cecum
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The appendix increases rapidly in length so
that at birth it is a relatively long tube arising
from the distal end of the cecum
After birth the wall of the cecum grows
unequally with the result that appendix comes
to enter the medial side
Appendix is considerably variant in position,
retrocecal, retrocolic and pelvic appendix
The derivatives of the hindgut are:
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The left one-half of the transverse colon
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Descending and sigmoid colons
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Rectum and the superior part of the anal canal
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The epithelium of the urinary bladder and
most of the urethra
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All hindgut derivatives are supplied by the
inferior mesenteric artery
The junction between the segment of transverse
colon derived from the midgut and that
originated from the hindgut is indicated by the
change in blood supply
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Superior mesenteric artery is the midgut artery
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Inferior mesenteric artery is the hindgut artery
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The descending colon becomes retroperitoneal
as its mesentery fuses with the peritoneum on
the left posterior abdominal wall and then
disappears
The mesentery of the sigmoid colon is retained
but it is shorter than in the embryo
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This terminal part of the hindgut is an
endoderm-lined chamber that is in contact with
the surface ectoderm at the cloacal membrane
This membrane is composed of endoderm of
the cloaca and ectoderm of the proctodeum or
anal pit
The cloaca, the expanded terminal part of the
hindgut receives the allantois
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The cloaca is divided into dorsal and ventral
parts by a wedge of mesenchyme, the urorectal
septum
It develops in the angle between the allantois
and hindgut
As the septum grows toward the cloacal
membrane , it develops forklike extensions that
produce infoldings of the lateral walls of the
cloaca
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These folds grow toward each other and fuse to
form a partition that divides the cloaca into two
parts
The rectum and cranial part of the anal canal
dorsally
The urogenital sinus ventrally
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By the seventh week, the urorectal septum has
fused with the cloacal membrane
Dividing it into a dorsal anal membrane and a
larger ventral urogenital membrane
The area of fusion of the urorectal septum with
the cloacal membrane is represented in the
adult by the perineal body
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The perineal body is a fibromuscular node and a
landmark of perineum where several muscles
converge and attach
The urorectal septum also divides the cloacal
sphincter into anterior and posterior parts
The posterior part becomes the external anal
sphincter
The anterior part develops into the superficial
transverse perineal, bulbospongiosus and
ischiocavernosus muscles
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This developmental fact explains why one
nerve, the pudendal nerve supplies all these
muscles
The anal membrane usually ruptures at the end
of the eighth week
This brings the distal part of the digestive tract
into communication with the amniotic cavity
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The superior two-thirds of the adult anal canal is
derived from the hindgut
The inferior one-third develops from the proctodeum
The junction between the epithelia of the two parts is
indicated by the irregular pectinate line
This line is located at the inferior limit of the anal
valves
It indicates the former site of anal membrane
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About 2cm superior to the anus is an anocutaneous
line or white line
This demarcates where the anal epithelium
changes from columnar to stratified squamous
At the anus, the epithelium is keratinized and
continuous with the skin around the anus
Because of its hindgut origin the superior twothirds of the anal canal are mainly supplied by the
superior rectal artery
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Because of its origin from the proctodeum, the
inferior one-third of the anal canal is supplied
mainly by the inferior rectal arteries
The differences in blood supply, nerve supply,
venous and lymphatic drainage of anal canal
are important clinically, when considering the
metastasis of cancer cells
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The characteristics of carcinomas in the two
parts are different
Tumors in the superior part are painless and
arise from columnar epithelium
Tumors in the inferior part are painful and
arise from stratified squamous epithelium
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This anomaly is a persistence of the herniation
of abdominal contents into the proximal part of
the umbilical cord
Herniation of intestines into the cord occurs in
about 1 in 5000 births
Herniation of liver and intestines in 1 of 10,000
births
Size of the hernia depends on its contents
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The abdominal cavity is proportionately small
when there is an omphalocele
Immediate surgical repair is required
Omphalocele results from failure of the
intestines to return to the abdominal cavity
The covering of the hernial sac is the
epithelium of the umbilical cord which is a
derivative of the amnion
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When the intestines return back to the abdominal
cavity during the 10th week and then herniate
through an imperfectly closed umbilicus, an
umbilical hernia forms
This is different from the omphalocele
In umbilical hernia the protruding mass is covered
by subcutaneous tissue and skin
Hernia reaches its maximum size at the end of the
first month after birth
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It usually ranges from 1 to 5 cm
The defect through which the hernia occurs is the
linea alba
Hernia protrudes during crying, straining, or
coughing
It can easily be reduced through the fibrous ring at
the umbilicus
Surgery is not usually performed until it persists to
the age of 3 to 5 years
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This outpouching is one of the most common
anomalies of the digestive tract
This congenital ileal diverticulum occurs in 2 to
4% of people
3 to 5 times more prevalent in males than
females
It sometimes becomes inflamed and causes
symptoms that mimic appendicitis
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The wall of the diverticulum contains all layers of
the ileum and may contain small patches of gastric
and pancreatic tissues
The gastric mucosa often secretes acid, producing
ulceration and bleeding
It is the remnant of the proximal part of the yolk
stalk
It typically appears as a fingerlike pouch about 3 to
6 cm long
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It arises from the antimesenteric border of the
ileum 40 to 50 cm from the ileocecal junction
It may be connected to the umbilicus by a
fibrous cord or an omphaloenteric fistula
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A part of the colon is dilated because of the
absence of autonomic ganglion cells in the
myenteric plexus distal to the dilated segment of
colon
The enlarged colon has the normal number of
ganglion cells
The dilation results from failure of peristalsis in
the aganglionic segment
In most cases only rectum and sigmoid colon are
involved
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It is the most common cause of neonatal
obstruction of the colon
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Accounts for 33% of all neonatal obstruction
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Males are affected more often than females
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It results from failure of neural crest cells to
migrate into the wall of the colon during the 5th
and 6th weeks
This results in failure of parasympathetic ganglion
cells to develop in Auerbach plexuses
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It occurs about once in every 5000 newborn
infants
More common in males
Most anorectal anomalies result from abnormal
development of the urorectal septum
Results due to incomplete separation of the
cloaca into urogenital and anorectal portions
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There is normally a temporary communication
between rectum and anal canal dorsally, from
the bladder and urethra ventrally
It closes when the urorectal septum fuses with
the cloacal membrane
Lesions are classified as low or high depending
on whether the rectum ends superior or
inferior to the puborectalis muscle
Following are the low anomalies :
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Anal agenesis with or without a fistula
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Anal stenosis
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Membranous atresia of anus
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Anorectal agenesis with or without fistula