Download GI EMBRYOLOGY OVERVIEW Primordial gut is closed at 4th week

GI EMBRYOLOGY
OVERVIEW
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Primordial gut is closed at 4th week by oropharyngeal membrane at cranial end and at caudal end by
cloacal membrane
Endoderm gives rise to gut epithelium and glands
Epithelium at cranial and caudal end is from ectoderm of stomodeum and proctodeum
Fibroblast growth factors are involved early in anterior-posterior axial patterning
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Ex) FGF 4 signals endoderm induction by ectoderm and mesoderm
Muscular, connective tissue, and other layers of the wall of digestive tract are derived from splanchnic
mesenchyme surrounding primordial gut
Gut is divided into foregut, midgut, and hindgut
FOREGUT
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Derivatives are
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Primordial pharynx and its derivatives
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Lower respiratory system
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Esophagus and stomach
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Duodenum, distal to opening of bile duct
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Liver, biliary apparatus (hepatic ducts, gall bladder, and bile duct), and pancreas
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All of these except for pharynx and lower respiratory tract and most of esophagus are supplied by
the CELIAC TRUNK—the artery of the foregut
DEVELOPMENT OF ESOPHAGUS
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Develops from foregut immediately caudal to pharynx
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Is partitioned from trachea by tracheoesophageal septum
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Reaches final relative length by week 7
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Epithelium and glands from endoderm and they grow to completely obliterate the lumen, then
are recanalized
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Striated muscle forming superior third of esophageal muscular externa is derived from
mesenchyme in the caudal pharyngeal arches
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Smooth muscle develops from splanchnic mesenchyme
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Both types are innervated by X which serve the caudal pharyngeal arches
PG. 213 for esophageal atresia, stenosis, and short esophagus (hiatal hernia)
DEVELOPMENT OF STOMACH
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At week 4, slight dilation in the tube indicates the site of primordium of the stomach
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Is initially oriented in the median plane
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Soon enlarges and broadens ventrodorsally
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Dorsal border grows faster than ventral border, forming greater curvature
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ROTATION OF STOMACH
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Rotates 90 degrees in a clockwise direction (viewed from cranial end) around its
longitudinal axis
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This has several effects
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Lesser curvature (ventral border) moves towards the right and the greater
curvature (dorsal border) moves to the left
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Left side becomes ventral surface and right side becomes dorsal surface
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Cranial region of the stomach moves left and slightly inferiorly and its caudal
region moves right and superiorly
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Explains why the left vagus nerve supplies anterior wall and the right vagus
innervates the posterior wall
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MESENTERIES OF STOMACH
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Stomach is suspended from dorsal wall of abdominal cavity by the primordial dorsal
mesogastrium—a dorsal mesentery
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Is carried left during rotation of the stomach and formation of the omental
bursa/lesser sac
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Primordial ventral mesogastrium attaches to stomach as well as connecting the
duodenum to the liver and the ventral abdominal wall
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OMENTAL BURSA
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Isolated clefts develop in the mesenchyme forming the thick dorsal mesogastrium—they coalesce
to form a single cavity—omental bursa/lesser sac
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Rotation of stomach pulls the dorsal mesogastrium to the left, enlarging the bursa
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Bursa expands transversely and cranially to lie between stomach and the posterior abdominal
wall
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Superior part is cut off as diaphragm develops forming a closed space—infracardiac bursa
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If it persists, it lies medial to the base of the right lung
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Inferior part of omental bursa persists as superior recess of the omental bursa
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Inferior recess of omental bursa is formed between the layers of elongated dorsal
mesogastrium—which forms the greater omentum PG. 215 for figure
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The greater omentum overhangs the developing intestines
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Inferior recess disappears as the layers of the greater omentum fuse
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Omental bursa communicates with main part of the peritoneal cavity through the omental
foramen—located posterior to the free edge of lesser omentum in adult
DEVELOPMENT OF DUODENUM
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Duodenum develops from distal foregut, the proximal part of midgut, and splanchnic
mesenchyme
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Its blood supply is therefore from both CELIAC TRUNK and SUPERIOR MESENTERIC
ARTERY
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Junction of two parts of duodenum is just distal to origin of the bile duct
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As the stomach rotates, the duodenal loop rotates right and comes to lie retroperitoneally
(external to peritoneum)
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Lumen is obliterated and recanalized
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By this time, most of the ventral mesentery of duodenum has disappeared
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PG. 216 for congenital hypertrophic pyloric stenosis, duodenal stenosis, and duodenal atresia
DEVELOPMENT OF LIVER AND BILIARY APPARATUS
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Liver, gallbladder, and biliary duct system arise as ventral outgrowth—hepatic diverticulum—from
caudal or distal part of the foregut early in week 4
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Diverticulum extends to septum transversum—mass of splanchnic mesoderm between developing
heart and midgut
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Septum transversum forms ventral mesentery in this region
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Hepatic diverticulum enlarges and divides into two parts as it grows between layers of ventral
mesogastrium
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Larger cranial part is liver primordium
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Proliferating endoderm gives rise to hepatic cords and to epithelial lining of the
intrahepatic part of biliary apparatus
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Hepatic cords anastomose around endothelium-lined spaces—hepatic sinusoids
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Fibrous and hematopoietic tissue, as well as Kupffer cells are derived from mesenchyme
in the septum transversum
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Small caudal part of the hepatic diverticulum becomes the gallbladder, and the stalk of the
diverticulum forms the cystic duct
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Stalk connecting the hepatic and cystic ducts to the duodenum becomes the bile duct
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Initially, the stalk connects to the ventral aspect of the duodenal loop, but as it rotates,
the entrance to bile duct is carried posteriorly
VENTRAL MESENTERY
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Thin double layered membrane gives rise to
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Lesser omentum, passing from liver to the lesser curvature of the stomach
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Hepatogastric ligament
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Lesser omentum also passes from liver to duodenum
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Hepatoduodenal ligament
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Falciform ligaement, extending from liver to the ventral abdominal wall.
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Umbilical vein passes in the free border of the falciform ligament on its way from the umbilical
cord to the liver
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Ventra mesentery is derived from mesogastrum
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Also forms the visceral peritoneum of the liver
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Liver is covered with peritoneum except for bare area in contact with diaphragm
PG. 220 anomalies of liver
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DEVELOPMENT OF PANCREAS
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Develops between the layers of mesentery from dorsal and ventral pancreatic buds of
endodermal cells, arising from foregut
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Is derived from dorsal pancreatic bud and ventral pancreatic bud
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As duodenum rotates right, the ventral bud is carried dorsally with the bile duct
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Eventually it lies posterior to the dorsal bud and fuses with it
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Ventral pancreatic bud forms the uncinate process of the head of pancreas
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As the two buds fuse, their ducts anastomose
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Pancreatic duct forms from ventral bud and the distal part of the duct of the dorsal bud
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Proximal part of dorsal bud persists as accessory pancreatic duct in some people that
opens into the minor duodenal papilla
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Sometimes the ducts fail to fuse, and you get two ducts
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HISTOGENESIS OF PANCREAS
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Parenchyma is derived from endoderm
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Pancreatic acini develop from cell clusters around the ends of these tubules
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Pancreatic islets develop from cells that separate from the tubules and lie between the
acini
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PG. 222 pancreas anomalies
DEVELOPMENT OF SPLEEN
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Derived from mesenchymal cells located between the layers of the dorsal mesogastrium
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Is a vascular lymphatic organ
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As the stomach rotates, the left surface of mesogastrium fuses with the peritoneum over the left
kidney—the fusion explains the dorsal attachment of the splenorenal ligament and why the adult
splenic artery follows a tortuous course posterior to the omental bursa and anterior to the left
kidney
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HISTOGENESIS OF SPLEEN
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Mesenchymal cells differentiate to form the capsule, connective tissue framework, and
parenchyma of the spleen
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PG. 224 accessory spleens/polysplenia
MIDGUT
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Derivatives are
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Small intestine, including duodenum distal to the opening of bile duct
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Cecum, appendix, ascending colon, and the right one half to two thirds of the transverse colon
These are supplied by the SUPERIOR MESENTERIC ARTERY, the midgut artery
As the midgut elongates, it forms a ventral U shaped loop of gut—the midgut of the intestine, projecting
into the remains of extraembryonic coelom in the proximal part of the umbilical cord
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This is a physiologic umbilical herniation
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It communicates with yolk sac through omphaloenteric duct/yolk stalk
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Occurs because there is not enough room in the abdominal cavity for the midgut
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Due to huge liver and kidneys at this point in development
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Omphaloenteric duct is attached to the apex of the midgut loop where the cranial and
caudal limbs join
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Cranial end grows rapidly and forms loops, but caudal end only develops cecal
swelling/diverticulum (primordium of cecum and appendix)
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Midgut loop is suspended from dorsal abdominal wall by mesentery
ROTATION OF MIDGUT LOOP
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Midgut loop rotates 90 degrees counterclockwise (looking from the ventral side) around the axis
of the SMA
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This brings the cranial limb (small intestine) of the midgut loop to the right and the caudal limb
(large intestine) to the left
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During rotation, the cranial end elongates and forms intestinal loops (primordial of jejunum and
ileum)
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RETURN OF MIDGUT LOOP TO THE ABDOMEN
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Not known what causes this to happen
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Small intestine returns first, passing posterior to the SMA and occupies central part of
the abdomen
As the large intestine returns, it undergoes a 180 degree counterclockwise rotation, and
later occupies the right side of the abdomen—ascending colon
FIXATION OF INTESTINES
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Rotation of stomach and duodenum causes the duodenum and pancreas to fall to the right
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Enlarged colon presses the duodenum and pancreas against the posterior abdominal wall, so
most duodenal mesentery is absorbed
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Therefore the duodenum, except for approximately the first part derived from foregut,
has no mesentery and lies retroperitoneally (posterior to peritoneum)
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Attachment of the dorsal mesentery to the posterior abdominal wall is greatly modified after the
intestines return to the abdominal cavity
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At first, the dorsal mesentery is in median plane
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As intestines enlarge and lengthen, their mesenteries are pressed against the posterior
abdominal wall
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Mesentery of ascending colon fuses with parietal peritoneum on this wall and
disappears—so ascending colon is also retroperitoneal
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Other derivatives of midgut loop retain mesenteries (jejunum and ileum)
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After the mesentery of the ascending colon disappears, the fan-shaped mesentery of the
small intestine acquires a new line of attachment that passes from the duodenojejunal
junction inferolaterally to the ileocecal junction
CECUM AND APPENDIX
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Primordium of these is cecal swelling
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Appendix eventually comes to enter the cecum’s medial side because of unequal growth of cecum
after birth
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Position of appendix is variable
PG. 228-235—more anomalies of the gut
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HINDGUT
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Derivatives are
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Left one third to one half of the transverse colon, the descending and sigmoid colon, the rectum,
and the superior part of the anal canal
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Epithelium of urinary bladder and most of the urethra
They are all supplied by INFERIOR MESENTERIC ARTERY, the artery of the hindgut
The junction between the segment of transverse colon derived from the midgut and that originating from
the hindgut is indicated by the change in blood supply from a branch of the SMA to a branch of the IMA
The descending colon becomes retroperitoneal as its mesentery fuses with the peritoneum on the left
posterior abdominal wall and then disappears
Mesentery of sigmoid colon is retained, but it is shorter than in the embryo
CLOACA
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Is the expanded terminal portion of the hindgut
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Lined with endoderm
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Contacts surface ectoderm at the cloacal membrane, which is composed of endoderm of cloaca
and ectoderm of the proctodeum/anal pit
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Cloaca receives the allantois ventrally—a fingerlike diverticulum
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PARTITIONING
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Divided into ventral and dorsal parts by urorectal septum—a wedge of mesenchyme
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Two parts
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Rectum and cranial part of anal canal dorsally
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Urogenital sinus ventrally
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Urorectal septum fuses with the cloacal membrane, dividing it into a dorsal anal
membrane and larger ventral urogenital membrane
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You see this fusion in the adult as the perineal body—tendinous center of the
perineum
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Several muscles converge and attach here
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urorectal septum also divides the cloacal sphincter into anterior and posterior parts
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posterior becomes external anal sphincter
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anterior part becomes superficial transverse perineal, bulbospongiosus, and
ischiocavernosus muscles
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this explains why the pudendal nerve supplies all these muscles
anal membrane is supposed to rupture by the end of the 8 th week, bringing the anal canal in
communication with amnionic cavity
ANAL CANAL
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Superior two thirds derived from hindgut
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Inferior third is from proctodeum
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Pectinate line indicates where epithelium derived from ectoderm joins with epithelium derived
from endoderm
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It is located at the inferior limit of the anal valves
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Approximates where the anal membrane was
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2 cm superior to anus is anocutaneous line where composition of anal epithelium changes from
columnar to stratified squamous
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Other layers of the wall of the anus are derived from splanchnic mesenchyme
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Superior two thirds of anal canal are supplied by SUPERIOR RECTAL ARTERY, continuation of the
IMA
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The hindgut artery
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Venous drainage of superior part is via SUPERIOR RECTAL VEIN, a tributary of IMV
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Nerves are from ANS
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Inferior third of anal canal, because it’s from proctodeum, is supplied by INFERIOR RECTAL
ARTERIES, branches of INTERNAL PUDENDAL ARTERY
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Venous drainage from inferior rectal vein, tributary of internal pudendal vein that drains
into internal iliac vein
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Nerves from inferior rectal nerve, sensitive to pain, temperature, touch, and pressure
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IMPORTANT when considering metastasis
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Characteristics of tumors of the two parts differ
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Tumors of superior part are painless and arise from columnar epithelium, while those in
inferior part are painful and arise from stratified squamous
MORE ANOMALIES PG. 238-241
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