S2-3) Development of the Gut (look at powerpoint too) Flashcards
In the 4th week, the embryo folds.
Describe the result of its lateral folding
Lateral:
- Creates ventral body wall
- Primitive gut becomes tubular
In the 4th week, the embryo folds.
Describe the result of its craniocaudal folding
Craniocaudal:
- Creates cranial & caudal pockets from yolk sac endoderm
- Beginning primitive gut development
- cranial leads to the foregut (mouth and heart)
- caudal leads to the hind gut (anus)
What is the gut tube?
- The gut tube is an endoderm-lined tube
- It runs the length of the body and opens at the umbilicus
Describe some features found on the primitive gut tube
- Blind pouches at the head and tail ends
- Splanchnic mesoderm covering
Identify and describe the divisions of the gut tube
- Foregut and hindgut begin as blind diverticula
- Midgut has an opening at first and is continuous with the yolk sac
Identify the derivatives of the foregut
- Oesophagus
- Stomach
- Pancreas, liver & gall bladder
- Duodenum (proximal to entrance of bile duct)
Identify the derivatives of the midgut
- Duodenum (distal to entrance of bile duct)
- Jejunum
- Ileum
- Cecum
- Ascending colon
- Proximal 2/3 transverse colon
Identify the derivatives of the hindgut
- Distal 1/3 transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Upper anal canal
- Internal lining of bladder & urethra
Explain how the embryonic divisions have implications for blood supply in the adult
- Arterial supply reflects embryonic development
- Each embryonic segment receives blood supply from a distinct branch of the abdominal aorta
The structures that develop close to the junction between foregut and midgut will have mixed blood supply.
Relate this to the duodenum
- Proximal to entry of bile duct: gastroduodenal artery & superior pancreaticoduodenal artery (CT)
- Distal to entry of bile duct: inferior pancreaticoduodenal artery (SMA)
The structures that develop close to the junction between foregut and midgut will have mixed blood supply.
Relate this to the head of the pancreas
- Superior pancreaticoduodenal artery (CT)
- Inferior pancreaticoduodenal artery (SMA)
Describe the formation of the intraembryonic coelom
- Formed as the embryo folds
- Begins as one large cavity
- Later subdivided by the future diaphragm into abdominal and thoracic cavities
Explain the specialisation of intraembryonic coelom and its membrane
- One membrane lining the whole intraembryonic cavity
- Specialises as the cavities specialise:
I. Pericardium
II. Pleural membrane
What are mesenteries and why are they needed?
- Mesenteries are a double layer of peritoneum suspending the gut tube from the abdominal wall
- Purpose:
I. Allow a conduit for blood and nerve supply
II. Allow mobility where needed
How are mesenteries formed?
The new primitive gut is suspended within the intraembryonic coelom:
- Splanchnic mesoderm surrounds new gut
- Mesentery formed from a condensation of this mesoderm
Where are the mesenteries?
- Dorsal mesentery suspends the entire gut tube from the dorsal body wall
- Ventral mesentery is only in the region of the foregut
Explain the contribution of the dorsal and ventral mesenteries to the lesser and greater sacs of the peritoneal cavity
Dorsal and ventral mesenteries divide the cavity into left and right sacs (in foregut only):
- The left sac contributes to the greater sac
- The right sac becomes the lesser sac
What are omenta?
Omenta are specialised regions of peritoneum
Describe the formation of the greater omentum
- Formed from the dorsal mesentery
- First structure seen when the abdominal cavity is opened anteriorly
Describe the formation of the lesser omentum
- Formed from the ventral mesentery
- Free edge conducts the portal triad
What is meant by the term peritoneal reflection?
A change in direction:
- From parietal peritoneum to mesentery
- From mesentery to visceral peritoneum, etc
What influences the position of the greater and lesser sacs?
Rotation of the stomach during development
Structures that are not suspended within the abdominal cavity are retroperitoneal.
What does this mean?
Retroperitoneal structures were never in the peritoneal cavity & never had a mesentery
What is meant by the term secondarily retroperitoneal?
Secondarily retroperitoneal structures are structures which developed intraperitoneally whose mesentery was lost to fusion with posterior abdominal wall parietal peritoneum due to massive expansion of GI tract during development
Identify some secondarily retroperitoneal structures of the foregut
- Duodenum
- Pancreas
Describe the location of the foregut
Extends from the lung bud to the liver bud
What happens in the 4th week of development in relation to the foregut?
A respiratory diverticulum forms in the ventral wall of the foregut at the junction with the pharyngeal gut:
- Respiratory primordium (ventrally)
- Oesophagus (dorsally)
A variety of oesophageal abnormalities occur as a consequence of abnormal positioning of the tracheoesophageal septum.
Identify two
Describe which mesentery is responsible for the development of different areas of the pancreas (foregut-derived gland)
- Ventral mesentery: uncinate process and inferior head
- Dorsal mesentery: superior head, neck, body & tail
Describe the formation of the primary intestinal loop
The midgut elongates enormously then makes a loop that:
- Has the SMA as its axis
- Is connected to the yolk sac by the vitelline duct
- Has cranial & caudal limbs
How does physiological herniation occur?
- Primary intestinal loop elongates rapidly and liver also grows rapidly
- Abdominal cavity is too small to accommodate both & intestines herniate into the umbilical cord
What immediately follows the rotation of the gut?
The descent of the caecal bud
How does incomplete rotation occur and what is the result?
- Midgut loop makes only one 90° rotation
- Result: left-sided colon
How does reverse rotation occur and what is the result?
- Midgut loop makes one 90° rotation clockwise
- Result: transverse colon passes posterior to the duodenum
The vitelline duct can persist resulting in a number of different abnormalities.
Identify 3 of these
- Vitelline cyst
- Vitelline fistula
- Meckel’s diverticulum
How does a vitelline cyst form?
Vitelline cyst – vitelline duct forms fibrous strands
Describe the features of a vitelline fistula
Vitelline fistula – direct communication between the umbilicus & intestinal tract
What is pyloric stenosis?
- Pyloric stenosis is hypertrophy of the circular muscle in the region of the pyloric sphincter
- It is not a recanalisation failure
What is gastroschisis?
- Gastroschisis is the failure of closure of the abdominal wall during folding of the embryo
- Gut tube & derivatives are outside the body cavitiy
What is omphalocoele?
Omphalocoele is the persistence of physiological herniation
How does the omphalocoele differ from an umbilical hernia?
Umbilical hernias have the covering of skin and subcutaneous tissue
How is the anal canal divided?
The anal canal is divided by the pectinate line into histologically distinct superior and inferior parts
What is the proctodeum?
The proctodeum is the junction between two embryonic germ layers (anal pit)
Identify 3 different hindgut abnormalities
- Imperforate anus
- Anal / anorectal agenesis
- Hindgut fistulae
