Lecture 4: Development of the gut Flashcards
What are the foregut structures;
- Pharynx
- Lower resp. tract
- Proximal oesophagus
- Distal oesophagus
- Stomach
- Liver
- Biliary apparatus
- Pancreas
- Proximal 1/2 duodenum
What are the midgut structures?
- Distal 1/2 duodenum
- Jejenum
- Ilium
- Ceacum
- Appendix
- Ascending colon
- Proximal 2/3 transverse colon
What are the hindgut structures?
- Distal 1/3 transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Proximal 2/3 anal colon
What does the beginning of the gut development look like?
Movement from bilaminar disc to trilaminar disc.
Describe Bi-laminar to tri-laminar disc;
Bi-laminar; Epiblast and hypoblast
Epiblast -> Ectoderm
Hypoblast -> Endoderm
Mesoderm creates 3rd layer in-between
What are the two types of folding that occurs in the trilaminar disc?
Longitudinal and lateral folding occurs at the same time
What is lateral folding?
Lateral folding forms a tube
- Ectoderm and mesoderm fold laterally and ventrally
- Closes off endoderm, forming a separated gut tube structure
- Endoderm; forms epithelial lining of tube
- Mesoderm; gives rise to supporting structures and smooth muscle
- Space between mesoderm (body cavity)
What is longitudinal folding?
Formation of C-shape
- Gut tube to yolk sack closed (like purse string)
- Divides into Ant. intestinal portion (foregut) and post. intestinal portion (hindgut)
- Midgut remains open to yolk sac
In the formation of the gut tube how does the arterial supply relate to nerve and lymphatic?
Arterial supply shows you nerve and lymphatics
What is the arterial supply for the gut?
Foregut = Celiac trunk
Midgut = Sup. Mesenteric
Hindgut = Inf. mesenteric
What happens at the extremities of the tube?
At 4 weeks - 2 membranes closing off the tube;
- Cranial end; Oropharyngeal membrane (Ectoderm)
- Caudal end; Anal membrane (Ectoderm)
Wk4: Oropharyngeal membrane ruptures
Wk8; Anal membrane ruptures
What happens to the gut tube as development occurs?
1) Initially gut tube is patent
2) As epithelium proliferates - plugs up the lumen of the gut tube
3) ~Wk8 (end of embryonic period) - Recanalisation occurs via cavity formation in the epithelial plug
Describe how the oesophagus buds off the foregut;
- Ventral trachea bud forms and the oesophagus partitions itself to form two seperate structures.
- Starts off short and then rapidly elongates.
Successive stages in development of the resp. tract from the foregut, resulting in a seperate oesophagus and trachea.
What could go wrong with oesophagus formation?
Foregut->Oesophagus
Tracheosophogeal fistula/atresia (Connection/blockage) = Both occur as a result of incomplete partitioning.
Congenital hiatal hernia;
- Short oesophagus pulls everything up
= Displaces stomach cranially, herniates into thorax through oesophageal hiatus
Describe the formation of the stomach;
Stage 1; Gut tube starts to dilate (foregut suspended by ventral and dorsal mesentery)
Stage 2; Dilation continues, Rotation on long axis ~90 Deg. clockwise, Ant. mesogastrium move to right, posterior move left.
Stage 3; Dilation continues, rotation on coronal axis 90 deg. clockwise, right boundary becomes superior (lesser curvature, left boundaries becomes inferior (Greater curvature)
Describe the formation of the greater omentum;
- Dorsal mesogastrium dragged round by rotation of stomach
- Begins to weigh down under weight of gravity
- Both sides of the hanging fold fuse together (Making 4 layered peritoneal structure)
= Greater omentum
Whats a congenital malformation of the stomach?
Congenital hypertrophic pylorus stenosis
- Marked thickening of the muscular wall of the pyrlous (sphincter)
- Blocks exit of stomach into 1st part of duodenum
- Severe non-bilious vomiting i.e bile cant get back
Describe the formation of the liver and biliary tree from the foregut;
Wk 4; Ventral outgrowth (Hepatic diverticulum)
Divides in 2 as it grows
- larger, cranial = liver (On right side b/c gut rotation)
- Smaller, caudal = Biliary apparatus, attatches to ventral duodenum, as duodenum grows, ends up dorsal
Describe how the pancreas forms from the foregut;
- Develops b/w both layers of mesogastrium, 2 buds, (majority from the dorsal bud)
- Caudal part of foregut
- Ventral bud carried round as duodenum rotates
- Buds fuse, ducts anastomose
- Pancreatic duct = Ventral duct and distal dorsal duct
Describe the formation of the midgut;
Elongates forming U shape that herniates through the umbilical cord and rotates counter clockwise. (270 deg)
Eventually herniation retracts, midgut loop returns to abdominal cavity.
Ceacum and appendix rotate down to lower abdomen.
Cranial and caudal limbs named in relation to Superior mesenteric artery
What are some congenital malformations of the midgut?
Umbilical herniation/fistula
- Umbilical cord doesnt close, gut herniates out
Meckels diverticulum
- Ileal diverticulum
- ~2%
- Remnant of the yolk sac (Vitelline duct)
What occurs at the end of the hindgut?
Hindgut is not particularly exciting but;
The development of the cloaca;
- Expanded distal part of hindgut
- Divided into dorsal and ventral parts
- Mesenchymal urorectal septum
- As septum grows; Seperates rectum from urogenital sinus
Describe the formation of the rectum/anal canal in the hindgut;
Boundary between outer ectoderm and inner endoderm
- pectinate line denotes boundary (in anus)
Blood supply upper 2/3 is IMA.
Describe the features of the anus;
- Portal-systemic vessels anastomose
- Anal columns terminate at site of anal membrane
- Lymphatics change at anocutaneous line (White line) to inguinal nodes below.
