OS 214 [A]: Digestion and Excretion
Lec 01.1: Embryology of the Digestive Tract
February 10, 2014
Blesile S. Mantaring, MD
TOPIC OUTLINE
I.
Review of Embryonic Body Cavity Formation
A. Second Week of Gestation: Bilaminar Embryonic Disc
B. Development of Intraembryonic Mesoderm
C. Formation of Intraembryonic Coelom
D. Folding of the Embryo
E. Dev’t of Mesentery 2° to Lateral Folding of Embryo
F. Development of the Primitive Gut
G. Recanalization of The Gut
II. Development of the Foregut
A. Esophagus
B. Stomach
C. Spleen
D. Duodenum
E. Liver and Biliary Tree
F. Pancreas
III. Development of the Midgut (5th – 10th Week)
A. Development of the Intestines
IV. Development of the Hindgut (5th – 7th Week)
1
o Secondary yolk sac – formed after degeneration of primary yolk
sac
o Allantois – out pouching of the secondary yolk sac
 Initially, the gut is closed on both ends (fusion of the endoderm and
ectoderm)
o The cephalic end has the buccopharyngeal membrane
(prechordal plate – thickened circular area, indicates future head
region), the future site for the mouth
o The caudal end has the cloacal membrane, the future site for the anus
B. DEVELOPMENT OF INTRAEMBRYONIC MESODERM
 This leads to the formation of 3 parts (from medial to lateral):
o Paraxial mesoderm (medial)
 Differentiates and begins to divide into paired cuboidal bodies,
somites
 Gives rise to axial skeleton and skeletal muscles
 The more somites there are, the older the embryo
o Intermediate mesoderm
 Gives rise to the genitourinary system
o Lateral plate mesoderm (focus of the lecture)
 Spaces formed within the lateral plate mesoderm gives rise to
Legends:
From the Powerpoint presentation
From the lecturer
From other sources (textbook, Internet, etc.)
intraembryonic coelom/abdominal cavity
ORIENTATION
Coordinator: Dr. Angela Salvaña
LO: Kei Genuino
Grading System:
http://uvle.up.edu.ph/mod/page/view.php?id=43587
Exemption: exemptions from finals for those with grades of 60% (final
raw score) or better for the two long examinations with no failing for
either.
ATTENDANCE will be checked and at least 80% is required to complete
the course.
T/N: Hello! This trans will be largely similar to Block B’s trans (TY, Block B!)
with our notes as well. Dr. Mantaring doesn’t give her PPT, and she hasn’t
given us her outline as of print time, so we took from Block B’s resource
folder. 
I. REVIEW OF EMBRYONIC BODY CAVITY FORMATION
A. SECOND WEEK OF GESTATION: BILAMINAR EMBRYONIC DISC
 During the first week of life, the zygote undergoes divisions, forming first
the morula, then the blastocyst, which is composed of the embryoblast
and the trophoblast (forms placenta and membranes)
 Embryoblast differentiates into a bilaminar embryonic disc
o epiblast
o hypoblast
 Appearance of amniotic cavity
 Formation of primary yolk sac
Figure 2. Schematic drawings of the human embryo during the 3rd & 4th
weeks. LEFT: dorsal views of the developing embryo illustrating early
formation of the brain, intraembryonic coelom, and somites. RIGHT:
schematic transverse sections illustrating formation of the neural crest,
neural tube, intraembryonic coelom, and somites.
C. FORMATION OF INTRAEMBRYONIC COELOM
 Isolated spaces appear in the lateral mesoderm and cardiogenic area
Figure 1. Formation and development of embryonic disc.
 Development of the 3 germ layers (ectoderm, mesoderm, endoderm all
come from epiblast) through migration of mesenchymal cells from the
primitive pit
 Blastocyst cavity  primary yolk sac  secondary yolk sac
 Formation of the following:
o Amniotic cavity – appears within the epiblast
o Primary yolk sac – from blastocyst cavity, associated with the
hypoblast
o Chorionic cavity – from the hypoblast
Marvin, Reggie, Sha




(superior to prechordal plate; mesodermal cells cannot infiltrate the
prechordal plate so it proceeds superior to it)
The heart develops superior to the prechordal plate
Spaces coalesce to form a flat U-shaped cavity, the intraembryonic
coelom
Intraembryonic Coelom has three parts:
o Pericardial cavity
o Pericardioperitoneal canal – becomes pleural cavity
o Peritoneal cavity – abdominal cavity
The intraembryonic coelom divides lateral mesoderm into 2 layers:
o Somatopleure (somatic or parietal layer), which is continuous with
the extraembryonic mesoderm lining the amnion
Page 1 / 6
Lec 01.1: Embryology of the Digestive Tract
OS 214
o Splanchnopleure (splanchnic or visceral layer), which is
continuous with the extraembryonic mesoderm covering the yolk
sac
 Intraembryonic coelom communicates with the chorionic cavity
D. FOLDING OF THE EMBRYO
 Occurs in the median and horizontal plane, secondary to the rapid
growth of the embryo leads to the:
o Formation of the head fold and tail fold in the median plane
(much faster)
o Formation of the lateral fold in the horizontal plane
o Embryo becomes more cylindrical
SEQUELAE OF BODY FOLDING
1. Remodeling of the intraembryonic coelom
2. Formation of the primitive gut from the secondary yolk sac
3. Ventral body wall is established
4. Formation of the mesentery that surrounds the primitive gut
CONSEQUENCES OF LONGITUDINAL/MEDIAN FOLDING
 Formation of the head fold and tail fold
 Formation of the primitive gut
o With folding, the superior portion of the secondary yolk sac
becomes incorporated within the embryo, forming the primitive
gut
o Formation of vitelline duct or yolk stalk due to constriction of
secondary yolk sac – connection of the primitive gut to secondary
yolk sac
o Foregut and hindgut – blind ended tubes
 Remodeling of the intraembryonic coelom
o The intraembryonic coelom changes from a flat U-shaped cavity
to a bent one with the pericardial cavity pushed to the ventral side
o Sequelae of longitudinal body folding is the formation and
development of the head fold and tail fold
o Upper portion of secondary yolk sac gradually forms into the
primitive gut
o Intraembryonic coelom loses its connection with the chorionic
cavity and becomes a closed cavity
o Ventral movement of pericardial cavity due to the formation of the
head fold
Figure 4. Lateral folding.
E. DEV’T OF MESENTERY SECONDARY TO LATERAL FOLDING
DORSAL MESENTERY




Suspends the gut from the posterior abdominal wall
Extend from caudal part of foregut, midgut & major part of hindgut
From esophagus to colon
Parts:
o Greater omentum or dorsal mesogastrium in stomach
o Mesoduodenum in the duodenum
o Dorsal mesocolon in the colon
o Mesentery proper in the jejunal and ileal loops
VENTRAL MESENTERY
 Suspends the gut from the anterior abdominal wall
 Gradually degenerates except in the foregut - distal esophagus,
stomach, and upper portion of the duodenum
 Becomes part of the lesser omentum
 Foregut to duodenum
Figure 5. Mesenteries.
SUMMARY OF THE SEQUELAE OF FOLDING PHENOMENON
 Several movements affect the final position of the mesenteries
o Rotation of the stomach
o Movement of the duodenum
o Reabsorption of the gut
Thoracic
Cephalic
fold
Lateral fold
Lateral fold
Abdominal
wall
Figure 3. Longitudinal folding.
CONSEQUENCES OF LATERAL FOLDING
 Flat embryo becomes more cylindrical
 Ventral body wall is established
o Two edges of the embryo meet at the ventral side, forming the
ventral body wall
 Development of the mesentery
o The mesentery surrounds the primitive gut, and suspends the gut
from the posterior and anterior abdominal wall
o Fusion of the splanchnic mesoderm around the gut secondary
to lateral folding leads to development of the primary mesentery,
which hangs the primitive gut in the abdominal wall
o Two types of mesentery:
 Dorsal mesentery – ex. Mesocolon, mesogaster
 Ventral mesentery – ex. Falciform ligament, lesser omentum
(gastrohepatic & hepatoduodenal ligaments)
o The mesentery divides the coelom into right and left halves, until
the ventral mesentery degenerates
Marvin, Reggie, Sha
Epigastric
Close midgut and
form the lateral
abdominal walls
Hindgut
Bladder
Caudal fold
Hypogastric wall
Figure 6. Formation of abdominal wall.
BODY WALL DEFECTS
 Due to a failure of progression of the body folding
 Examples:
o Cleft sternum – may result to ectopia cordis; thoracic area
o Gastroschisis
 Herniation of abdominal contents directly into amniotic cavity
 Usually occurs on the right lateral to the umbilicus
o Bladder and cloacal exstrophy: hypogastric area
Page 2 / 6
Lec 01.1: Embryology of the Digestive Tract
F. DEVELOPMENT OF THE PRIMITIVE GUT
Primitive gut arises from superior part of the secondary yolk sac!
G. RECANALIZATION OF THE GUT
 Starts with a canal  proliferation of epithelium resulting to a solid
structure formation of spaces (secondary cellular death due to
apoptosis) recanalization (secondary formation of a canal)
 Occurs in the entire primitive gut
 No recanalization causes formation of atresia or stenosis which can
occur in any part of the GI tract
II. DEVELOPMENT OF THE FOREGUT





Pharnyx & derivatives (oral cavity, tongue, tonsils, salivary glands)
Esophagus
Stomach
Duodenum proximal to bile duct
Arises as diverticula from foregut:
o Tracheobronchial tree
o Liver
o Biliary apparatus
o Pancreas
o Respiratory system
OS 214
 The stomach then rotates 90° clockwise along its longitudinal axis,
in effect the:
o Lesser curvature (from ventral side) becomes situated in the right
side; right side faces posteriorly
o Greater curvature (from dorsal side) becomes situated in the left
side; left side faces anteriorly
o This explains how the left vagal trunk supplies anterior structures
and the right vagal trunk supplies posterior structures
o LARP – Left goes Anterior, Right goes Posterior
 It then rotates in an anteroposterior (AP) axis
o Cranial region or cardiac portion moves inferiorly & to the right
o Caudal region or pyloric part moves superiorly & to the left
o Long axis in the stomach becomes nearly transverse
MOVEMENT OF THE MESENTERY
 Rotation of the stomach about the longitudinal axis pulls the dorsal
mesentery (originally midline) to the left and forms the omental bursa or
lesser peritoneal sac (opening: Foramen of Winslow)
 Rotation about the AP axis pulls the dorsal mesentery down, forming
the greater omentum
A. ESOPHAGUS
 A partition called the tracheoesophageal septum separates the lung




bud from the foregut, bringing about:
o Dorsal esophagus
o Ventral trachea and lung buds
Displacement/deviation of septum may cause atresia or stenosis
Elongates secondary to the descent of the heart and the lungs into the
pleural cavity
7 weeks AOG – esophagus has reached its final relative length. Its
epithelium (from endoderm) proliferates until the lumen is obstructed
By the end of 8 weeks AOG – esophagus undergoes recanalization to
reform the lumen
Figure 8. Rotation of stomach and formation of the curvatures.
C. SPLEEN
 Mesodermal proliferation in the dorsal mesogastrium
 Because of the rotation of the stomach, the spleen which was also in
the midline moves to left
 Persists as an intraperitoneal organ
Figure 7. Successive stages in the development of the
tracheoesophageal septum during the 4th and 5th weeks. A, B, C are
lateral views showing the partitioning of the foregut into the esophagus
and laryngotracheal tube. D, E, F, are transverse sections showing
formation of the tracheoesophageal septum and showing how it separates
the foregut into the laryngotracheal tube and esophagus.
Figure 9. Movement of spleen and pancreas due to stomach rotation.
COMMON ESOPHAGEAL ANOMALIES
 Esophageal atresia with or without tracheoesophageal fistula –
tracheoesophageal septum is displaced
 Esophageal stenosis – more posterior displacement of septum
 Short esophagus leading to congenital hiatal hernia – esophagus
did not lengthen
 As heart grows, esophagus is pulled down. Congenital diaphragmatic
hernia is a defect wherein the esophagus did not come down.
Atresia/fistula is a defect wherein the septum did not divide.
B. STOMACH
 The stomach begins as a fusiform dilatation in the foregut, initially a
midline structure. It is attached to the body wall through the dorsal
(mesogastrium) and ventral mesentery
 The dorsal or posterior wall of the stomach grows faster than the
ventral wall, resulting in the formation of the curvatures of the
stomach
o Dorsal/posterior wall – greater curvature
o Ventral wall – lesser curvature (anterior side grows slower)
Marvin, Reggie, Sha
D. DUODENUM
 Parts
1. First ascending (T12-L1)
2. Descending (L1)
3. Transverse (L3-L4)
4. Terminal ascending (L1-L2)
 Derived from the:
o Terminal part of the foregut
o Cephalic part of the midgut
 With the rotation of the stomach, the duodenum becomes a Cshaped loop
 Rotation of stomach pushes duodenum to the left and to the back
 It swings from its initial midline position to the left side of the abdominal
cavity and adheres to the body wall then loses its mesentery and
becomes retroperitoneal
 First 5 cm will still have the mesentery (intraperitoneal) and is still
movable
 The lumen of the duodenum is obliterated by the proliferation of cells,
and then is recanalized (no recanalization leads to atresia/stenosis)
Page 3 / 6
Lec 01.1: Embryology of the Digestive Tract
OS 214
 Also, as a result of the rotation of the duodenum, the pancreas
becomes retroperitoneal as well.
 Development of the Pancreatic Duct
o Main duct/Duct of Wirsung: entire ventral pancreatic duct + distal
part of the dorsal bud duct
o Accessory duct of Santorini: proximal part of dorsal bud duct
ANOMALY: ANNULAR PANCREAS
 Sometimes the ventral bud may have two separate buds. One bud
moves back while the other moves front, embracing the duodenum and
forming an annular pancreas
Figure 10. Development of the duodenum.




E. LIVER AND BILIARY TREE
The liver bud develops at the distal end of the foregut
o Hepatic cell cords grow towards the septum transversum
o Cell cords surround vitelline veins which form hepatic sinusoids
The septum transversum is a mesodermal tissue that grows from
the ventral wall, which separates the heart from the liver
As the hepatic cells penetrate the septum, the connection between the
liver bud/hepatic diverticulum and the duodenum narrows to become
the common bile duct. The common bile duct has a small ventral
outgrowth, which later on becomes the gallbladder. (Cystic duct –
constriction between bile duct and gallbladder)
The liver divides the ventral mesentery
o The lesser omentum between esophagus, stomach, and upper
part of duodenum to liver, composed of the hepatogastric and
hepatoduodenal portions
o The falciform ligament between the liver and ventral body wall,
containing the umbilical vein (ligamentum teres hepatis or round
ligament of the liver)
III. DEVELOPMENT OF THE MIDGUT (5th – 10th week)
 Common bile duct divides proximal and distal part of duodenum.
o Parts of the duodenum proximal to common bile duct come from the
foregut and is supplied by the celiac trunk
o Distal to the duodenum are from midgut and is supplied by SMA
 Derivatives of the midgut include:
o Small intestines including most of the duodenum
o Cecum
o Appendix
o Ascending colon
o 2/3 of the transverse colon
 Supplied by the superior mesenteric artery (SMA)
A. DEVELOPMENT OF THE INTESTINES
DEVELOPMENT OF THE INTESTINAL LOOP
 The midgut elongates forming the primary intestinal loop, with two
limbs:
o Cranial limb – from which the following structures are derived:
distal duodenum, jejunum, proximal ileum
o Caudal limb – from which the following structures are derived:
distal part of ileum and proximal part of the large intestines
(Cecum, appendix, ascending colon, proximal 2/3 of the
transverse colon). Note that a cecal bud develops from the
caudal limb that becomes the cecum.
 The vitelline duct connects apex of the intestinal loop to yolk sac.
Note that the duct lies within the umbilical cord.
F. PANCREAS
Figure 12. Intestinal loop and its rotations.
PHYSIOLOGICAL UMBILICAL HERNIATION
 The intestinal loop will start to grow longer but due to the very small
abdominal cavity, the developing mesonephric kidney, and liver, there is
no space for intestinal loop to grow; hence, there is physiologic
umbilical hernia.
 The abdominal organs grow much faster than abdominal cavity!
 At about 6-8 weeks AOG, physiological umbilical herniation occurs.
o The primary intestinal loop is forced to herniate into the
extraembryonic cavity through the umbilicus into the umbilical
cord (physiological hernia) because there is not enough space to
grow intraembryonically
o As herniation occurs, the loop undergoes 90o counterclockwise
longitudinal rotation about the SMA
Figure 11. Development of the pancreas.
 The pancreas is the last organ that develops in the foregut
 The pancreas originates as two pancreatic buds:
o The dorsal pancreatic bud is in the dorsal mesentery
o The ventral pancreatic bud is near the bile duct
 When the duodenum rotates and becomes C-shaped, the ventral
pancreatic bud moves dorsally, caudally and posterior to the dorsal
bud. The buds then fuse.
o Ventral pancreatic bud  becomes the uncinate process and
head of the pancreas!
o Dorsal bud  body and tail of the pancreas!
Marvin, Reggie, Sha
Figure 13. Physiological umbilical herniation.
Page 4 / 6
Lec 01.1: Embryology of the Digestive Tract
 Rotation of the midgut – the midgut loop undergoes a 90°
counterclockwise rotation around the axis
RETURN TO THE ABDOMINAL CAVITY
 At 10-11 weeks AOG, the intestine loop retracts back into the
abdominal cavity since it is now big enough to accommodate the
abdominal organs, with cranial limb returning first. This leads to:
o Regression of the mesonephric kidney
o Reduced growth of liver
o Increased size of the abdominal cavity
 As the colon returns, the loop undergoes further rotation of 180°
counterclockwise around the SMA from the initial rotation (total
rotation: 270o)
OS 214
FURTHER DEVELOPMENT AND FIXATION
 At 11 weeks AOG, the ascending colon elongates and the cecal bud
appears in the caudal limb (the cecum descends and settles in the
right lower quadrant)
 Vitelline duct regresses and disappears!
 Finally, ascending and descending colon become retroperitoneal.
Their mesenteries fixate/fuse to the posterior abdominal wall.
Figure 17. Fixation of the intestines.
DEVELOPMENTAL ANOMALIES OF THE INTESTINES
Figure 14. First stage of rotation (90°).
 The cranial limb retracts first going towards the left side of the
abdominal cavity
 The caudal limb is pulled next and settles to the right of the cranial
limb
o The first part that enters is the jejunum, followed by the ileum, then
lastly, the cecal bulb!
 Atresia and stenosis
o During development, the gut undergoes a solid phase
o If recanalization is incomplete then there are abnormalities
 Omphalocoele
o Congenital anomaly when the intestines fail to return to the
abdominal cavity in the 10th-11th week
o Herniated loops produce swelling; covered only by amnion
o Differentiated from gastroschisis where ventral wall failed to
close
o Associated with other malformations: cardiac anomalies, neural
tube defects and chromosomal abnormalities
o Poorer prognosis than gastroschisis
 Ileal or Meckel’s Diverticulum
o Portion of the vitelline duct fails to degenerate
o Different presentations include:
 Appendix-like structure in the ileum
 Fibrous cord attached to the umbilicus
 Patent vitelline duct with potential for fecal matter to leave ileum
through the diverticulum and through the umbilicus (vs. patent
urachus which manifests as urinal discharge in umbilicus)
Figure 15. Second stage of rotation (additional 180°)  midgut at 10
weeks AOG.
Figure 18. Omphalocoele.
Figure 19. Meckel’s Diverticulum.
Figure 16. Second stage of rotation (additional 180°)  midgut at 11
weeks AOG with all intestines having returned to the abdominal cavity.
Marvin, Reggie, Sha
 Malrotation
o Associated with great risk for volvulus followed by severe pain and
necrosis of the twisted segment
o Kinds
 Non-rotation – occurs when the intestines do not rotate 180
degrees as it returns into the abdominal cavity; the large intestine
Page 5 / 6
Lec 01.1: Embryology of the Digestive Tract
OS 214
is located at the left of the body and small intestine is on the right
side
 Reversed rotation – occurs when the primary loop rotates 90
degrees clockwise; transverse colon passes behind the
duodenum
IV. DEVELOPMENT OF THE HINDGUT (5th – 7th Week)
 Derivatives of the hindgut include:
o Distal third of the transverse colon
o Descending colon
o Sigmoid
o Rectum
o Upper part of the anal canal
 The cloaca is the dilated terminal chamber of the hindgut. The
urorectal septum, which is a mesodermal plate, divides it into dorsal
and ventral parts:
o Rectum and anal canal dorsally
o Tip of urorectal septum becomes the central point perineum
o Urogenital sinus ventrally, which develops into the:
 Urinary bladder
 Urethra
- Prostatic and membranous urethra
- Whole female urethra
Figure 20. Development of the hindgut.
DEVELOPMENTAL ANOMALIES
 Failure of urorectal septum to divide the cloaca
 Hirschprung’s Disease
o Defective migration of the neural crest cells into the hindgut
o Also called aganglionic megacolon
o Presents with failure to pass meconium
o Neural crest cells
 Starts with the formation of neural tube
 Neural crest cells detach from the neural tube and migrate
towards the entire body
 Imperforate Anus
o Results from the failure of dilated cloacal membrane to rupture
END OF TRANSCRIPTION
Reggie: Opening OS 214 with a bang! Have yourself a two-part-megaembryo trans with orientation box.
Marvin: Happy Valentine! :D Sino ang gigising ng Friday morning mo? :>
Marvin, Reggie, Sha
Page 6 / 6