2b & 3a.) Embryology Flashcards
What causes the primitive gut tube to form?
Cephalocaudal and lateral folding
Cephalocaudal and lateral folding form the primitive gut tube; explain, more specifically, what each of the foldings does
- Lateral:
- Creates ventral body wall
- Makes primitive gut tube become tubular
- Cephalocaudal
- Makes cranial and caudal pockets from yolk sac endorm
State the three divisions of the primitive gut tube
- Foregut
- Midgut
- Hindgut
Is the primitive gut tube open at any points?
- Midgut remains temporally connected to yolk sac by vitelline duct (yolk stalk) at the umbilicus
- Foregut and hindgut are blind-ended (blind divertiula)
Explain, based on embryonic derivation, why visceral pain is poorly localised yet parietal is localised?
Viscera is derived from splanchnic mesoderm. Parietal derived from somatic mesoderm.
State the structures of foregut
State the structures of the midgut
State the structures of the hindgut
State the exact point of the junction between the foregut and midgut
Where bile duct enters duodenum
Describe the arterial supply of each of the divisions of the primitive gut
NOTE: structures that develop close to junction between foregut and midgut have mixed blood supply (e.g. duodenum and pancreas)
Given that the duodenum has mixed blood supply since it is close to the foregut-midgut junction; state it’s blood supply
- Proximal to entry of bile duct: gastroduodenal & superior pancreaticoduodenal (celiac trunk branch)
- Distal to entry of bile duct: inferior pancreaticoduodenal (SMA branch)
Given that the pancreas has mixed blood supply since it is close to the foregut-midgut junction; state it’s blood supply
- Head: superior pancreaticoduodenal (celiac trunk branch)
- Tail: inferior pancreaticoduodenal (SMA branch_)
When does devlopment of primitive gut tube begin?
Week 3
The internal lining of gut tube derived from ______ and becomes ______?
Endoderm
Epithelia
The external lining of gut tube derived from _____ and becomes ______?
Splanchnic mesoderm
Peritoneum
The mesoderm surrounding gut is split into two layers; state what each layer becomes
- Somatic: abdominal wall
- Splanchnic: smooth muscle of gut wall
Describe how the intraembyronic coleom is formed
Formed as the embryo folds and parietal mesoderm moves down and gused to pinch off section of yolk sac
Describe what happens to the intraembryonic coelom as develoment continues
Begins as one large cavity but subdivided by future diaphragm (septeum transversum) into abdominal and thoracic cavities.
There is one membrane lining the whole intraembryonic cavity and this membrane specialises as the cavities specialise into: pericardium, pleural membrane and peritoneum
Describe what a mesentery is and what their purpose is
Double layers of peritoneum that suspend the gut tube and organs from the abdominal wall. Allow:
- Passage of blood vessesl & nerves
- Mobility
Describe how the dorsal mesentary forms
Splanchnic mesoderm surrounds primitive gut tube and suspends it from the abdominal wall. The splanchnic mesoderm surrounding primitive gut tube meets as the point where it suspends gut tube to form a double layer of peritoneum (dorsal mesentery)
Where does the dorsal mesentery exist from and to?
Where does the ventral mesentery exist from and to?
- Dorsal: lower end of oesohagus to cloaca. Dorsal attaches gut tube to roof of abdo cavity
- Ventral: terminal part of oesophagus to duodenum. Ventral attaches foregut to floor of abdo cavity
Where is the ventral mesentery derived from?
Septum transversum
Describe how the cavity surrounding the foregut is divided into left and right sacs and explain why only the foregut is divided into such sacs
Dorsal & ventral mesentaries divide the cavity into left and right. Only in foregut as only the foregut has a ventral mesentery
What are omenta?
State the two omenta in abdominal cavity and what they are derived from
Fold of peritoneum connecting stomach to other abdominal organs
- Greater: dorsal mesentery
- Lesser: ventral mesentery
What does the free edge of the lesser omentum conduct?
The cavity surrounding the foregut is divided into left and right sacs; state what each of these sacs becomes
- Left sac: greater sac
- Right sac: lesser sac
Growth of the liver divides the ventral mesentery into what two structures?
- Lesser omentum
- Falciform ligament
Describe the process that happens to result in the digestive system being in contact with external environment
Oropharyngeal and cloacal membranes break down at future mouth and anus
How are the greater and lesser sacs of peritoneal cavity formed?
Rotation of the stomach 90 degrees clockwise on it’s longitudinal axis
State 5 consequences of rotation of the stomach
- Puts vagus nerve anterior and posterior to stomach (left= anterior, right= posterior)
- Shifts cardia and pylorus from midline
- Stomach comes to lie obliquely
- Contributes to moving lesser sac behind stomach
- Creates greater omentum
What is a peritoneal reflection?
Describe the difference between retroperitoneal and secondary retroperitoneal
- Retroperitoneal: were never in peritoneal cavity and never had a mesentery
- Secondary retroperitoneal: began development surrounded by peritoneum and therefore had a mesentery but with growth and devlopement almost pushed back towards posterior abominal wall and the mesentery is lost through fusion with posterior abdominal wall
Omental bursa is an alternative name for what?
The lesser sac of peritoneal cavity
Describe what happens in the 4th week to create the oesophagus and repsiratory primordium
- Respiratory diverticulum forms in ventral wall of foregut at junction of pharyngeal gut.
- Tracheoesophageal septum then separates the respiratory primordium from the oesophagus
Describe some anatomical consequences of abnormal positioning of tracheoesophageal septum
What causes the stomach to obtain its characteristic shape?
Dorsal border develops faster giving the stomach it’s characteristic shape with greater and lesser curvature
What do the left and right vagus nerves innervate in terms of abdominal wall?
- Left: anterior abdominal wall
- Right: posterior abdominal wall
After rotation of the stomach on it’s longitudinal axis what causes the stomach to assume it’s final positon (think about further rotation)?
Stomach rotates on antero-posterior axis so that caudal/pyloric part moves right and upward and the cephalic part moves left and downwards
State which mesentery the liver (and its biliary system) and pancreas develop in
- Liver & biliary system: ventral mesentery
- Pancreas
- Unicate process and inferior head: ventral
- Superior head, neck, body and tail: dorsal
Describe and explain what determines the shape of the duodenum
Rotation of stomach determines shape; it pushes duodenum to the right and then against posterior abdominal wall so it becomes secondary retroperitoneal
Is duodenum secconary retroperitoneal?
Yes, except the duodenal cap
Describe what happens to the duodenums lumen over the course of development
- Lumen obliterated in 5th and 6th week
- Recanalised by end of embryonic period (~8 weeks)
What are the:
- Stomatodeum
- Proctodeum
- Stomatodeum= future mouth
- Proctodeum= future anus
Briefly describe development of the liver
- Liver bud grows as an outgrowth of foregut
- Penetrates septum transversum
- Connection between liver bud and foregut narrows to form bile duct
- Outgrowth of bile duct froms gallbladder and cystic duct
- Mesoderm of septum transversum becomes membranous forming lesser omentuma and falciform ligament
Describe the looping of the midgut and explain why it loops
Midgut makes a loop with cranial and caudal limbs with superior mesenteric artery as it’s axis
At what week does physiological herniation of the intestines occur?
Week 6
Why does the midgut herniate through the umbilicus?
Liver and intestines grow too quickly and abdominal cavity cannot accomodate them
Describe the rotation of the midgut, include:
- Angle
- Direction
- What happens to proximal and distal part at same time
- 270 degrees rotation around axis formed by SMA
- When viewed from front is counterclockwise, from prospectus of fetus it is clockwise
- Proximal part becomes convulted, distal part develops caecal bulge
When does the midgut return to the abdomen after physiological herniation?
Week 10
Describe how the midgut re-enter the abdomen after physiological herniation and the consequences of this for anatomy
Proximal part of jejenum is first pat to enter and comes to lie on the left side. The later returning loops settle more on the right.
Once the midgut has returned to the abdominal cavity, describe what happens to the caecal bud
Caecal bud decends into the right iliac fossa. During this process distal end of caecal bud also forms narrow diverticulum which forms appendix
Describe the anatomical consequences if the following malrotations occured:
- Midgut loop only makes one 90 degree rotation
- Midgut loop makes one 90 degree rotation clockwise (viewed from front rotation should be anticlockwise)
- One 90 rotation: when this happens the colon and caecum are first to return to abdomen hence get left-sided colon
- One 90 rotation clockwise: transverse colon passes behind duodenum
Define volvulus
Twisiting of the intestines
State some potential consequences of malrotation (hint: consequences of the anatomical abnormalities caused by malrotation)
Volvulus leading to:
- Ischaemia
- Strangulation
Describe the 3 abnormalities that can occur if the vitelline duct fails to degnerate
- Vitelline cyst: both ends of duct form fibrous cords and middle forms large cyst
- Vitelline fistula: duct remains patent across entire length
- Meckel’s diverticulum: small portion of duct persists (other part of duct may degenerate or form fibrous cord)
Describe the rule of 2’s for Meckel’s diverticulum
- 2% population
- 2 feet from ileocaecal valve
- Usually detected in under 2’s
- 2:1 ratio for male:female
Does Meckel’s diverticulum usually cause symptoms?
Not usually, however it can contain ectopic gastric or pancreatic tissue which can cause ulceration, perforation or bleeding
State possible symptoms of vitelline cyst
Fibrous cords traveerse peritoneal cavity hence intestinal loops may twist around the strands and become obstructed leading to volvulus or strangulation
State possible symptoms of vitelline fistula
Faecal discharge found at umbilicus
In some gut structures, cell growth becomes so rapid that lumen is partially or completed obliterated and recanalisation is later required to restore the lumen; state some structures which this occurs in
- Oesophagus
- Bile duct
- Small intestine
Define:
- Atresia
- Stenosis
- Atresia: complete blockage of bowel lumen
- Stenosis: narrowing/partial blockage of bowel lumen
Where do most cases of atresia or stenosis occur?
Duodenum
For pyloric stenosis, describe:
- What it is
- Whether it is classed as recanalisation failure
- Symptoms
- Hypertrophy of circular muscle in region of pyloric sphincter
- NOT a recanalisation failure
- Projectile vomitting in infants
For gastroschisis, describe:
- What it is
- If viscera are covered in peritoneum
- If it is an isolated defect
- Can you detect on ultrasound
- Failure of closure of abdominal wall during folding
- Viscera not covered by peritoneum hence bowel may be damaged by amniotic fluid
- Isolated defec/not related to other severe defects
- Detect on ultrasound around 20 weeks
State an alternative name for omphalocoele
Exomphalos
For omphalocoele, describe:
- What it is
- Are viscera covered in peritoneum
- Is it associated with other defects
- Persistence of physiological herniaton
- Viscera covered in peritoneum
- Associated with other severe defects hence has high mortality
State the 5 hindgut derivatives
- Distal 1/3 transverse colon
- Descending colon
- Rectum
- Superior part anal canal
- Epithelium of urinary bladder
Where is the pectinate line and what is it’s purpose and significance?
Pectinate line divides anal canal into superior and inferior parts; these parts have different vasculature, nerve supply and lymphatic drainage
Describe where each of the following is derived from:
- Anal canal superior to pectinate line
- Anal canal inferior to pectinate line
- Superior: hindgut
- Inferior: ectoderm
For the anal canal superior to pectinate line, state:
- Arterial supply
- Innervation
- Epithelia
- Lymphatic drainage
- IMA
- S2,S3,S4 pelvic parasympathetics
- Columnar epithelium
- Internal iliac nodes
For the anal canal inferior to the pectinate line, state:
- Arterial supply
- Innervation
- Epithelia
- Lymphatics
- Pudenal artery
- S2,S3,S4 pudenal nerves
- Stratified epithelium
- Superficial inguinal lymph nodes
Given that the anal canal is divided into two parts according to position relative to the pectineal line the different parts have different innervations; state the consequences of this
- Superior to pectinate line: only sensation possible is strech as have parasympathetics (autonomic)
- Inferior to pectinate line: sensitive to temp, touch, pain etc… as somatic innervation
Describe what happens to the terminal portion of the hindgut
A layer of mesoderm called the urorectal septum separates the allantois and hindgut and comes to lie close to cloacal membrane. When cloacal membrane ruptures in 7th week it creates the anal opening for hindgut and ventral opening for urogential sinus. The tip of the urorectal septum forms the perinal body
Describe 3 hindgut abnormalities which can arise from defects in urorectal septum
- Imperforate anus: failure of anal membrane to rupture
- Anal/anorectal agenesis: high blind ending rectum
- Hindgut fistulae: connection between gut and urethra or vagina in females
State structures of gut which are:
- Intraperitoneal
- Secondary retroperitoneal
