What kind of folding does the embryo undergo in the fourth week?
What is the importance of lateral folding of the embryo?
- Creates ventral body wall
- Primitive gut becomes tubular
Draw a diagram illustrating lateral folding of the embryo.
Label the tubular primitive gut
What is the importance of craniocaudal folding of the embryo?
Creates cranial and caudal pockets from the yolk sac endoderm
What is the significance of the cranial and caudal pockets of the yolk sac endoderm?
It is the beginning of primitive gut development
Draw a diagram illustrating craniocaudal folding of the embryo
What is the primitive gut tube lined with?
What regions is the primitive gut tube divisible into?
What is the importance of the divisions of the primitive gut tube?
They remain fundamental to the anatomy of the adult GI tract, and determine its nerve and blood supply
When does the development of the primitve gut tube begin?
In the 3rd week
What is the first step in the development of the primitive fut tube?
It 'pinches off' from the yolk sac cavity
Where does the primitive gut tube run?
From the stomatodeum rostrally to the procotdeum caudally
What is the stomatodeum?
What is the proctodeum?
Where does the primitive gut tube have an opening?
At the umbilicus
What is the opening at the umbilicus in the primitive gut tube called?
The vitelline duct
Where does the vitelline duct lead to?
The yolk sac
Draw a diagram of the primitive gut tube
What is the internal lining of the primitive gut tube derived from?
The endoderm (future epithelial linings)
What is the external lining of the primitive gut tube derived from?
Splanchnic mesoderm (future musculature, visceral peritoneum)
What is the primitive gut tube suspended in?
What suspends the primitive gut tube in intraembryonic coelom?
A double layer of splanchnic mesoderm
What are the adult derivates of the foregut?
- Gall bladder
- Duodenum (proximal to entrance of bile duct)
What is the blood supply to the foregut?
What are the adult derivatives of the midgut?
- Duodenum (distal to entrance of bile duct)
- Ascending colon
- Proximal 2/3 transverse colon
What is the blood supply of the midgut?
- Superior mesenteric artery (SMA)
- Superior mesenteric vein
What is the parasympathetic innervation of the midgut?
What is the sympathetic innervation of the midgut?
Superior mesenteric ganglion and plexus
What are the adult derivatives of the hindgut?
- Distal 1/3 of transverse colon
- Descending colon
- Sigmoid colon
- Upper anal canal
- Internal lining of bladder and urethra
What is the blood supply of the hindgut?
- Inferior mesenteric artery (IMA)
- Inferior mesenteric vein (IMV)
What is the parasympathetic innervation of the hindgut?
Pelvic N. (S2/3/4)
What is the sympathetic innervation of the hindgut?
Inferior mesenteric ganglion and plexus
What is the mesoderm surrounding the gut split into?
- Somatic mesoderm
- Splanchnic mesoderm
What does the somatic mesoderm surrounding the gut develop into?
Muscles and fasciae of the abdominal wall
What does the splanchnic mesoderm surrounding the gut develop into?
The smooth muscles of the gut wall
What is the space created by the split in the mesoderm surronding the gut called?
The coelomic cavity
What is the significance of the coelomic cavity?
It is the forerunner of the pleural cavity and peritoneal cavity
What does the coelomic cavity surround?
The primitive gut
What happens as lateral folding of the embryo progresses?
The two sides of the developing anterolateral abdominal wall meet in the midline, forming the linea alba
How many openings are left once the linea alba has formed?
One, at the umbilicus
What is each side of the anterolateral abdominal wall formed of?
Three layers of muscle;
- External oblique
- Internal oblique
- Transversus abdominis
What is found anteriorly to the three layers of muscle forming the anterolateral abdominal wall?
A fourth muscle, the rectus abdominis
What is found deep to all the muscle layers in the anterolateral abdominal wall?
What lies externally to the anterolateral abdominal wall?
Superficial fascia and skin
What do the muscles and fascia of the abdoinal wall develop from?
Label this diagram
- A - Rectus abdominis
- B - Linea alba
- C - External oblique
- D - Internal oblique
- E - Transverse abdominis
- F - Transversalis fascia
- G - Peritoneum
What is the inguinal canal?
An oblique passage through the layers of the abdominal wall
What is the significance of the inguinal canal in males?
It allows the passage of the developing testis into the scrotum
Do the testis pierce the abdominal wall?
How do the testis get to their final position during development?
The abdominal wall pushes them out ahead of its passage
What does the abdominal wall pushing the testes out form?
The fascial coverings of the spermatic cord
What is the clinical significance of the inguinal canal?
It remains a potential site of weakness and hernia formation throughout life
What kind of malformation is an umbilical hernia?
What happens in an umbilical hernia?
The intestines protrude through the abdominal wall
In whom are umbilical hernias common?
Why are umbilical hernias common in babies?
Because the gut forms outside the abdomen and later returns through an opening that becomes the umbilicus. This opening is a potential site of weakness, so abdominal contents can push against it and herniate
What is the inguinal canal?
A passage through the layers of the abdominal wall
What does the mesoderm surrounding the gut split into?
Somatic and splanchnic mesoderm
What does the space between the somatic and splanchnic mesoderm form?
The coelomic cavity
How does the space between the somatic and splanchnic mesoderm form the coelomic cavity?
Via lateral folding
As what does the intraembryonic coelom begin?
One large cavity
What later subdivides the intraembryonic coelom?
The future diaphragm
What does the diaphragm divide the intraembryonic coelom into?
The abdominal and thoracic cavities
Draw a diagram illustrating the formation of the intraembryonic coelom
What does the peritoneal membrane do?
Lines the abdominal cavity and invests the viscera
What happens to the peritoneal membrane during its development?
It grows, changes shape, and specialises
Why is the peritoneal 'cavity' a potential space only?
Because under normal conditions, it should contain nothing
What is the entire developing gut attached to?
The roof of the abdominal cavity
How is the developing gut attached to the roof of the abdominal cavity?
By a fold of mesoderm known as the dorsal mesentery
What attaches to the floor of the abdominal cavity?
How is the foregut attached to the floor of the abdominal cavity?
Draw a diagram illustrating the attachments between the developing gut and the abdominal cavity
What do the dorsal and ventral mesenteries become?
The various peritoneal folds and reflections that suspend the gut and give passage to the vessels and nerves in the adult
What happens to the shape of the dorsal and ventral mesenteries?
They often become complex
Why does the shape of the dorsal and ventral mesenteries often become complex?
Due to the complexity of the changes in shape and position undergone by the developing gut
What do the dorsal and ventral mesenteries in the region of the foregut do?
Divide the cavity into left and right sacs
What does the left sac of the preitoneal cavity contribute to?
The greater peritoneal sac
What does the right sac of the peritoneal cavity contribute to?
The lesser peritoneal sac
Where does the lesser peritoneal sac come to lie?
Behind the stomach
Draw a diagram illustrating the greater and lesser peritoneal sacs, and the mesentery dividing them
What are the greater and lesser omenta?
Specialised regions of peritoneum
What is the greater omentum formed from?
The dorsal mesentery of the stomach
When would the greater omentum be the first structure to be seen?
When the abdominal cavity is opened anteriorly
What does the greater omentum connect?
The greater curve of the stomach to the transverse colon
What is the lesser omentum formed from?
The ventral mesentery of the stomach
What does the lessr omentum connect?
The lesser curve of the stomach to the liver
What are the greater and lesser sacs, and the omenta formed by?
The rotation of the stomach
What is the widest part of the foregut?
How does the shape of the stomach change during development?
Initially, it is symmetrical, and then as it enlarges in expands unevenly, mainly towards the left
What creates the greater curvature of the stomach?
The faster growth of the dorsal border
In what manner does the primitive stomach rotate?
In two directions, around the longitudinal axis and around the anteroposterior axis
What is the result of stomach rotation?
- The original left side becomes anterior
- The original right side becomes posterior
- Vagus nerves lie anterior and posterior instead of left and right
- Shifts cardia and pylorus from the midline, pushing the greater curve inferiorly
- Moves the lesser sac behind the stomach
- Creates the greater omentum
Draw a diagram illustrating the longitudinal rotation of the stomach
Draw a diagram illustrating the anteroposterior rotation of the stomach
What is peritoneal reflection?
A change in direction, from parietal peritoneum to mesentery, from mesentery to visceral peritoneum etc
Draw a diagram illustrating peritoneal reflection
Give two parts of the GI tract that remain suspended from the posterior abdominal wall?
- The jejenum
- The ileum
How do some parts of the GI tract remain suspended from the posterior abdominal wall?
What is the consequence of some parts of the GI tract remaining suspended from the psoterior abdominal wall?
They remain mobile
What are structures that are not suspended within the abdominal cavity called?
What is meant by retroperitoneal?
Were never in the peritoneal cavity and never had mesentery
Give 3 examples of structures that are retroperitoneal
- Vena cava
What is meant by secondary retroperitoneal?
Began development invested by peritoneum, and had mesentery, but with successive growth and development, the mesentery is lost through fusion with the posterior abdominal wall
Give three examples of structures that are secondary retroperitoneal
- Ascending colon
- Descending colon
What happens to the duodenum and its mesentery during development?
It is pushed against the posterior abdominal wall
Why is the duodenum and its mesentery pushed against the posterior abdominal wall during development?
Due to the rotation of the stomach and the large size of the liver
What produces fusion fascia?
The peritoneum of posteiror abdominal wall is 'squished together' with the duodenum and its mesentery, growing over it
Is fusion fascia vascular or avascular?
What is the consequence of fusion fascia being avascular?
You can remove it to make the duodenum mobile again without causing any damage
When does a respiratory diverticulum form?
In the 4th week
Where does a respiratory diverticulum form?
In the ventral wall of the foregut at the junction with the pharyngeal gut
What does the respiratory diverticulum become?
- The respiratory primordium ventrally
- The oesophagus dorsally
What divides the respiratory primordium and the oesophagus?
The tracheoesophageal septum
Draw diagrams illustrating the formation of the respiratory primordium and the oesophagus
What is the prevelance of oesophageal abnormalities?
Up to 1 in 3,000 live births
Draw a diagram illustrating the potential consequences of abnormal positioning of the tracheoesophageal septum?
Where are the foregut derived glands formed?
- In the ventral mesentery
- In the dorsal mesentery
What foregut derived glands are formed in the ventral mesentery?
- Biliary system
- Part of pancreas (ucinate process and inferior head)
What foregut derived glands are formed in the dorsal mesentery?
Pancreas (superior head, neck, body, and tail)
What is the earliest GI tract associated gland?
What does the liver develop from?
The hepatic bud within the ventral mesentery
What is the significance of the liver during development?
WIth regards to space
It occupies a large proportion of the abdomen
Draw a diagram illustrating the development of the liver
What happens to the lumen of the duodenum in the 5th and 6th weeks?
It is obliterated due to the lining growing so quickly
What happens to the duodenum by the end of the embryonic period?
It is recanalised
What is the effect of the rotation of the stomach on the duodenum?
It pushes it to the right, then back against the posterior abdominal wall
Label this diagram