The document describes the development of the gastrointestinal tract (GIT) and associated organs from the embryonic stage through fetal development. It discusses how the foregut, midgut, and hindgut form the initial gut tube and how rotation and elongation result in the adult anatomy. Key developmental processes include formation of the dorsal mesentery and its roles, rotation of the stomach and duodenum, development of the liver, pancreas, and other digestive organs from embryonic buds, and the return of the intestinal loop from the umbilical herniation. The document provides detailed information on the embryonic origin and development of every part of the GIT.

1. • The Digestive System
• General Overview
• Parts of the GIT
• GIT Glands

4.  The GIT (gastrointestinal tract) is formed by incorporation of a portion of the yolk sac into the embryo
during embryonic folding; this results in formation of the blind-ended foregut and hindgut, and the
midgut which remains in communication with the yolk sac through the vitelline duct
 Endoderm forms the epithelial lining of the GIT and the parenchyma of the derivative glands; the
stroma and muscle components are derived from the splanchnic mesoderm

5.  Pharyngeal gut (or pharynx) extends from buccopharyngeal membrane to respiratory diverticulum;
then foregut to the liver bud; then midgut to the distal third of adult transverse colon; then hindgut to
the cloacal membrane

6.  As GIT bulges into the peritoneal cavity, the double-layered connection between the surrounding
splanchnic mesoderm and somatic mesoderm is referred to as dorsal mesentery, with specific names
as follows: dorsal mesogastrium or greater omentum (for stomach), dorsal mesoduodenum (for
duodenum), dorsal mesocolon (for colon), and mesentery proper (for the jejunoileum)
 An organ completely covered by peritoneum (i.e. having a mesentery) is called intraperitoneal while
one covered only on the anterior surface is called retroperitoneal

7.  An organ completely covered by peritoneum (i.e. having a mesentery) is called intraperitoneal while
one covered only on the anterior surface is called retroperitoneal

8.  A ventral mesentery is formed from the terminal part of esophagus down to the initial portion of the
duodenum; this is formed when the liver penetrates the mesenchyme of septum transversum, causes
it to bulge down and form the mesentery; the liver divides this mesentery into two parts: the dorsal
part connecting the liver to the terminal part of esophagus down to the initial part of duodenum
(called the lesser omentum) and a ventral part connecting the liver to the ventral body wall (called
the falciform ligament)

9.  When the tracheoesophageal septum appears, it separates the foregut into a ventral trachea and a
doral esophagus; upper two-thirds of esophagus has striated muscle (derived from the caudal
pharyngeal arches) so it is supplied by vagus nerve; the lower third has smooth muscle supplied by
autonomic nerves

10.  The stomach appears as a fusiform dilation of the foregut; the stomach rotates 90o clockwise so that
its original left surface now lies anteriorly and its original right side lies posteriorly; thus the vagus
nerves follow a similar course; during this rotation, the original posterior surface grows more rapidly
forming the greater curvature

11.  The stomach also rotates anteroposteriorly so that its caudal end (the pyloric end) moves upwards
and to the right, while the cranial end (the cardiac end) rotates downwards and to the left; thus the
axis of the stomach runs from above left to below right

12.  As a result of the rotation of the stomach, the greater omentum is pulled to the left, forming a space
behind the stomach (the lesser sac or omental bursa), and also bulges down in front of the transverse
colon; later its two layers fuse to form a single sheet that also fuses with the transverse mesocolon

13.  As a result of the rotation of the stomach, the greater omentum is pulled to the left, forming a space
behind the stomach (the lesser sac or omental bursa), and also bulges down in front of the transverse
colon; later its two layers fuse to form a single sheet that also fuses with the transverse mesocolon
 When spleen primordium appears in the dorsal mesogastrium, the portion behind it becomes the
lienorenal ligament (or splenicorenal ligament) and the anterior part forms the gastrolienal ligament

14.  As a result of the rotation of the stomach, the greater omentum is pulled to the left, forming a space
behind the stomach (the lesser sac or omental bursa), and also bulges down in front of the transverse
colon; later its two layers fuse to form a single sheet that also fuses with the transverse mesocolon
 When spleen primordium appears in the dorsal mesogastrium, the portion behind it becomes the
lienorenal ligament (or splenicorenal ligament) and the anterior part forms the gastrolienal ligament

15.  As a result of the rotation of the stomach, the greater omentum is pulled to the left, forming a space
behind the stomach (the lesser sac or omental bursa), and also bulges down in front of the transverse
colon; later its two layers fuse to form a single sheet that also fuses with the transverse mesocolon

16.  The liver divides the ventral mesentery into three parts:
 Thin peritoneal lining covering the liver
 Falciform ligament connecting the liver to the ventral body wall, containing the ligamentum teres hepatis
 Lesser omentum connecting liver to the stomach and upper duodenum, composed of the hepatoduodenal
ligament inferiorly and the hepatogastric ligament superiorly; the hepatoduodenal ligament contains the portal
triad and forms the roof of the epiploic foramen of Winslow (connecting the greater and lesser sacs)

17.  Lesser omentum connecting liver to the stomach and upper duodenum, composed of the
hepatoduodenal ligament inferiorly and the hepatogastric ligament superiorly; the hepatoduodenal
ligament contains the portal triad and forms the roof of the epiploic foramen of Winslow (connecting
the greater and lesser sacs)

19.  The duodenum develops from the caudal part of the foregut and the cranial end of the midgut (and is
thus supplied by both celiac and superior mesenteric arteries) and their junction lies below the origin
of the liver bud; it has a solid phase followed by recanalization
 With rotation of the stomach and growth of head of pancreas, duodenum becomes C-shaped, rotates
to the right and is placed more towards the left; the dorsal mesoduodenum degenerates except at
the duodenal cap, thus the duodenum is almost entirely (secondarily) retroperitoneal

21.  The liver bud (or hepatic diverticulum) appears from the caudal end of foregut (i.e. duodenum); it
penetrates the septum transversum and divides the ventral mesentery; meanwhile its connection
with the duodenum narrows to form the bile duct; later an outgrowth from the ventral wall of the
bile duct forms the cystic duct and gallbladder; as the duodenum rotates, the entrance of bile duct
shifts from its anterior position to a posterior one and the duct passes behind the duodenum
 Liver cells (parenchyma) and lining of biliary ducts is derived from the endoderm; the hemopoietic
stem cells, Kupffer cells and connective tissue are derived from splanchnic mesoderm

23.  Because of hemopoietic function, at ten weeks the liver is about 10% of body weight; this function
subsides in the last two months and the liver weighs 5% of body weight at birth; bile secretion begins
at twelve weeks and enters the GIT giving meconium its color
 The mesenchyme of septum transversum which now surrounds the liver differentiates into visceral
peritoneum except at the cranial end where it contributes to central tendon of diaphragm formed by
the original septum transversum; this area of liver, called bare area, thus is not covered by peritoneum

26.  The pancreas develops from two buds: a dorsal bud from the duodenum, and a ventral bud from the
origin of the liver bud; as the duodenum rotates to the right, the bile duct and the ventral pancreatic
bud also rotate until they lie posteriorly; the ventral bud thus lies below and behind the dorsal bud
 The ventral bud forms the uncinate process and part of head of pancreas; the rest is formed by the
dorsal bud; the main pancreatic duct is formed from the entire ventral duct and the distal part of the
dorsal duct; the proximal part of the dorsal duct often forms an accessory pancreatic duct
 Islets of Langerhans form from pancreatic parenchyma (derived from endoderm) in the 3rd month;
insulin secretion begins in the 5th month; connective tissue is derived from the splanchnic mesoderm
 Pancreas becomes secondarily retroperitoneal as its mesentery fuses with the posterior abdominal
wall

32.  Elongation of the midgut forms the primary intestinal loop which is connected at its apex to the
vitelline duct; the superior mesenteric artery forms the axis of the loop; the cephalic limb of the loop
later forms the rest of the duodenum, jejunum, and part of ileum; the caudal limb forms the rest of
the ileum, caecum and appendix, ascending colon, and proximal two-thirds of transverse colon
 The loop elongates rapidly, especially at the cephalic limb; because of enlargement of liver, the
abdominal cavity temporarily cannot accommodate the loop and it herniates into the extraembryonic
cavity in the 6th week (called physiological umbilical herniation)

33.  During the herniation, the loop rotates 90o counterclockwise (when viewed ventrally); later the loop
returns to the abdominal cavity; the cause for this return may be attributed to degeneration of
mesonephric kidneys, reduced growth of the liver, and expansion of abdominal cavity; during the
retraction, the loop rotates another 180o counterclockwise

36.  The first part of the loop to return is the proximal portion of the jejunum, which lies on the left side;
subsequent parts lie more on the right; the last part to return is the cecal bud which forms as a
conical dilation at the distal end of the caudal limb at its antimesenteric border; it first lies in the right
upper quadrant but later descends, placing the ascending colon and right colic flexure on the right
 During the descent of the colon, the diverticulum for the appendix appears at its caudal end;
postnatally, unequal growth in the wall of the cecum places the appendix posteriomedially; since the
appendix appears during the descent, its definitive position can vary (retrocecal, retrocolic, etc.)

38.  The mesentery of the midgut twists around the origin of the superior mesenteric artery as the midgut
rotates; when the ascending and descending colons obtain their definitive positions, their
mesenteries fuse with the posterior abdominal wall and they become (secondarily) retroperitoneal
 Mesentery of jejunoileum (called mesentery proper) has a line of attachment that runs from the
duodenojejunal junction to the ileocecal junction; sigmoid colon, lower part of cecum, and appendix
retain their mesenteries

39.  The mesentery of the transverse colon fuses with the greater omentum but retains some mobility and
has a line of attachment that runs from the right to the left colic flexures

40.  The mesentery of the transverse colon fuses with the greater omentum but retains some mobility and
has a line of attachment that runs from the right to the left colic flexures

41.  The hindgut forms the distal third of the transverse colon, descending colon, sigmoid colon, rectum,
and upper part of anal canal (in addition to the internal lining of the bladder and most of urethra)
 The distal end of the hindgut is dilated, forming the cloaca; the boundary between the cloaca and the
ectoderm is the site of the cloacal membrane; the anterior portion of the cloaca receives the
allantois; mesoderm from around the allantois and yolk sac forms the urorectal septum, which
separates the region of the allantois from that of the rest of the hindgut
 Later the tip of the septum comes close to the cloacal membrane, though they never make contact;
the cloacal membrane ruptures at the end of the 7th week forming a ventral opening for the
urogenital sinus and a dorsal opening for the anorectal canal, separated by the urorectal septum (later
forming the perineal body)

45.  Proliferation of the ectoderm closes the caudal end of the anal canal; recanalization of this ectoderm
adds an additional part to the anal canal which is thus derived from ectoderm; thus this part is
supplied by the inferior rectal arteries (from internal pudendal) while the upper part is supplied by
the superior rectal arteries (from inferior mesenteric); this also has clinical significance

46.  The endodermal-ectodermal junction in the adult anal canal is indicated by the pectinate line which
lies approximately at the site of the fetal cloacal membrane below the anal columns in the adult; the
change in epithelium, however, occurs lower, at the anocutaneous line (white line)