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
E. Liver and Biliary Tree
III. Development of the Midgut (5th – 10th Week)
A. Development of the Intestines
IV. Development of the Hindgut (5th – 7th Week)
From the Powerpoint presentation
From the lecturer
From other sources (textbook, Internet, etc.)
Coordinator: Dr. Angela Salvaña
LO: Kei Genuino
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
Appearance of amniotic cavity
Formation of primary yolk sac
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
Secondary yolk sac – formed after degeneration of primary yolk sac
Allantois – out pouching of the secondary yolk sac
Initially, the gut is closed on both ends (fusion of the endoderm and ectoderm)
The cephalic end has the buccopharyngeal membrane (prechordal plate – thickened circular area, indicates future head region), the future site for the mouth
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):
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
Gives rise to the genitourinary system
Lateral plate mesoderm(focus of the lecture)
Spaces formed within the lateral plate mesoderm gives rise to intraembryonic coelom/abdominal cavity
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 (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
A partition called the tracheoesophageal septum separates the lung bud from the foregut, bringing about:
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 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.
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.
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 stomachgrows faster than the ventral wall, resulting in the formation of the curvatures of the stomach
Dorsal/posterior wall – greater curvature
Ventral wall – lesser curvature (anterior side grows slower)
The stomach then rotates 90° clockwise along its longitudinal axis, in effect the:
Lesser curvature (from ventral side) becomes situated in the right side; right side faces posteriorly
Greater curvature (from dorsal side) becomes situated in the left side; left side faces anteriorly
This explains how the left vagal trunk supplies anterior structures and the right vagal trunk supplies posterior structures
LARP – Left goes Anterior, Right goes Posterior
It then rotates in an anteroposterior (AP) axis
Cranial region or cardiac portion moves inferiorly & to the right
Caudal region or pyloric part moves superiorly & to the left
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
Figure 8. Rotation of stomach and formation of the curvatures.
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 9. Movement of spleen and pancreas due to stomach rotation.
First ascending (T12-L1)
Terminal ascending (L1-L2)
Derived from the:
Terminal part of the foregut
Cephalic part of the midgut
With the rotation of the stomach, the duodenum becomes a C-shaped 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
The lumen of the duodenum is obliterated by the proliferation of cells, and then is recanalized (no recanalization leads to atresia/stenosis)
Figure 10. Development of the duodenum.
E. LIVER AND BILIARY TREE
The liver bud develops at the distal end of the foregut
Hepatic cell cords grow towards the septum transversum
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
The lesser omentum between esophagus, stomach, and upper part of duodenum to liver, composed of the hepatogastric and hepatoduodenal portions
The falciform ligament between the liver and ventral body wall, containing the umbilical vein (ligamentum teres hepatis or round ligament of the liver)
Figure 11. Development of the pancreas.
The pancreas is the last organ that develops in the foregut
The pancreas originates as two pancreatic buds:
The dorsal pancreatic bud is in the dorsal mesentery
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.
Ventral pancreatic bud becomes the uncinate process and head of the pancreas!
Also, as a result of the rotation of the duodenum, the pancreas becomes retroperitoneal as well.
Development of the Pancreatic Duct
Main duct/Duct of Wirsung: entire ventral pancreatic duct + distal part of the dorsal bud duct
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
III. DEVELOPMENT OF THE MIDGUT (5th – 10th week)
Common bile duct divides proximal and distal part of duodenum.
Parts of the duodenum proximal to common bile duct come from the foregut and is supplied by the celiac trunk
Distal to the duodenum are from midgut and is supplied by SMA
Derivatives of the midgut include:
Small intestines including most of the duodenum
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:
Cranial limb – from which the following structures are derived:
distal duodenum, jejunum, proximal ileum
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.
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.
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
As herniation occurs, the loop undergoes 90o counterclockwise longitudinal rotation about the SMA
Associated with other malformations: cardiac anomalies, neural tube defects and chromosomal abnormalities
Poorer prognosis than gastroschisis
Ileal or Meckel’s Diverticulum
Portion of the vitelline duct fails to degenerate
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 18. Omphalocoele.
Figure 19. Meckel’s Diverticulum.
Associated with great risk for volvulus followed by severe pain and necrosis of the twisted segment
Non-rotation – occurs when the intestines do not rotate 180 degrees as it returns into the abdominal cavity; the large intestine 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:
Distal third of the transverse colon
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:
Rectum and anal canal dorsally
Tip of urorectal septum becomes the central point perineum
Urogenital sinus ventrally, which develops into the:
Prostatic and membranous urethra
Whole female urethra
Figure 20. Development of the hindgut.
Failure of urorectal septum to divide the cloaca
Defective migration of the neural crest cells into the hindgut
Also called aganglionic megacolon
Presents with failure to pass meconium
Neural crest cells
Starts with the formation of neural tube
Neural crest cells detach from the neural tube and migrate towards the entire body
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-mega-embryo trans with orientation box.
Marvin: Happy Valentine! :D Sino ang gigising ng Friday morning mo? :>