Embryology Flashcards
Describe the basic development of the gastrointestinal tract
- Develops from ventral invaginations at either end of the embryo
- Elongate and fuse along ventral midline = straight tube
- Mesogastrium (and mesentery) forms along with tube forming gut - infolding of visceral layer (of lateral plate mesoderm and associated endoderm)
What structures are derived from the endoderm?
Intestinal epithelium
What structures are derived from the mesoderm?
MSK system
What structures are derived from the ectoderm?
Epithelium of extreme ends of tract
Outer epithelium and nervous system
What structures are derived from the paraxial mesoderm?
Skeletal muscle
What structures are derived from the intermediate mesoderm?
The kidneys
What structures are derived from the lateral plate mesoderm?
Muscles of the body wall
Wrapping of gut
Describe blastocyst development of the gastrointestinal system
- Mesoderm continues to grow, forms sac arround yolk sac
- Folds dorsally to form amniotic folds (custard splash)
- Mesoderm expands around, squeezes between endo and ectoderm
- End up with space,
- Edges of tissue come up to meet = cavity that embryo sits in (quadrupeds)
- True placenta from allonatois (makes up part of bladder wall
Describe the formation of the gut and extra embryonic membranes
- Components of GIT develop by proliferation and differentiation from a simple tube
- Signals from adjacent mesoderm, and between epitehlial cells, involved in embryonic development
determine normal development/differentiation, homeostasis and apoptosis in adults - Smooth muscle from splanchnic mesenchyme surrounding primitive gut and its derivatives
- In response to signals from endoderm myoblasts elongate and develop contractile elements
- Cavity between mesoderm layers
- Get yolk sac formation
- Embryo grows quicker than membranes so rolls up
- As rolls around, pinches off area not growing as quickly
Forms the gut
Describe the development of the mesoderm in the initial layers of the embryo
- Squeezes out of embryonic disk between ecto adn endoderm
- Carries blood vessels
- Is how embryo gets blood supply
What do the placenta and membranes form from?
The ectoderm, mesoderm and endoderm outside the embryonic disc
Describe the formation of the buccopharyngeal and coacal membranes
- 2 small circular areas of embryo do not have mesoderm
- Makes 2 areas where ectoderm adn endoderm are in direct contact
- Gaps form where mesoderm isn’t present
- Are at head and tail of embryo
- Form the buccopharyngeal and cloacal membranes
- inside the mesoderm have developing notochord
- (Mesoderm is of ectodermal origin and become mesodermal cells by formation of the primitive streak)
Describe the formatio fo the yolk sac and allantoic sac
- Yolk sac formation as embryo rolls up (links to midgut and becomes choriovitelline placenta)
- Allantoic sac froms as outgrowth of hindgut
- Root with gut becomes divided by urorectal septum to create hindgut and bladder
- Becomes chorioallantoic placenta
Describe the formation of the bladder and rectum
- Growth of urorectal septum divides the cloaca ino bladder and rectum
- Is barrier between anal membrane and urogenital membrane
Describe the formation of the ventral abdominal wall
- Ventral mesentery breaks down except for lesser omentum and falciform ligament
- Mesenteries allow blood and nerves to reach the organs suspended in them
- Serosal surfaces are covered by mesothelium (simple squamous epithelium derived from mesoderm)
Describe the vitelline artery in blood supply to the gut
- Initially supplies yolk sac, forms plexus around gut
- Vitelline plexus forms arterial blood supply to the gut
- Coeliac artery
- Cranial and caudal mesenteric arteries
What organs are supplied by the coeliac artery?
- Stomach (part)
- Duodenum (part)
- Pancrease (part)
- (Liver and spleen)
What organs are supplied by the cranial mesenteric artery?
- Stomach/duodenum/pancreas (part)
- Jejunum
- Ileum
- Large intestine (part)
What organs are supplied by the caudal mesenteric artery?
- Rest of the large intestine (the areas not supplied by the cranial mesenteric)
What is the mesentery of the GI system?
The supporting membrane containing blood vessels
What organs have both a ventral and a dorsal mesentery?
- Stomach
- Liver
- (most organs only have the dorsal mesentery, some share mesenteries)
Describe the embryological formation of the mesentery of the GI system
- Made up of 2 layers of mesothelium and mesoderm between those layers
- All organs have dorsal, some lose ventral
- Liver and pancreas share with stomach
- Small intestine has no ventral mesentery
What are the adult derivatives of the embryological foregut?
- In embryo from buccopharyngeal membrane to septum transversum
- Forms:
- Oral cavity, tongue, pharynx
- Upper respiratory tract
- oesophagus
- Stomach
- Duodenum (part)
- Liver biliary tract
- Pancreas
- lower respiratory tract
- tonsils, salivary gland
- anterior pituitary (adenohypophysis)
- thyroid, parathyroid
- thymus
What are the adult derivatives of the embryological midgut?
- In embryo communicates with vitelline tube and yolk sac
- Forms:
- rest of small intestine
- large intestine (part)
What are the adult derivatives of the embryological hindgut?
- In embryo communicates with allantoic diverticulum and extends to cloacal membrane
- Forms rest of large intestine
Describe the development and rotation of the simple stomach
- One aspect of the stomach wall grows quicker, gives curved shape
- Rotates on one axis, rolled on its own axis
Describe the rotation of the gut loop tube
- Small intestine
- Jejunum undergoes lengthwise growth
- Gradually coils on itself as is drawn into the abdomen
- Coiling draws duodenum and large colon anticlockwise (ventral view)
- Cranial mesenteric artery is the centre of rotation
- Colon and duodenum share mesocolon (mesentery)
- Fixed at oesophagus and anus so can only roll and rotate anticlockwise
Describe the formation and development of the gut tube in the ruminant in relation to the position of the structures in the adult
- All pre-abomasal structures are part of foetal stomach (not oesophagus)
- Cranio-dorsal part of embyronic stomach enlarges
- As enlarges, becomes left-sided and more ventral to become rumen
Describe the formation and development of the caecum, ascending and descendinc colon in the horse in relation to the position of these structures in the adult
- Caecum is blind ending sac between small and large intestine
- Grossly enlarged in horses
- Rolls anticlockwise
- Ascending colon develops as large double loop
- Have R and L ventral and R and L dorsal colon
- LDC is narrow
- Very short transverse colon
- Descending colon leads to the rectum
List some common developmental defects of the gut tube
- Stenosis
- Atresia
- Umbilical hernia
- Formation of clefts
Briefly describe umbilical hernias
- Not a fault of intestinal development
- Faulty closure of abdominal wall leaves large opening
- Herniation of fat then SI through umbilicus
- Hereditary
Briefly describe the formation of clefts
- Persistence of embryonic/foetal clefts due to abnormal growth process
- e.g. spina bifida
- Palatoschisis (cleft lip and palate)
- Schistosome reflexum - open body wall and dislocation of organs
Briefly describe stenosis
- Cells in lumen of tube not removed
- thinner lumen
Briefly describe atresia
- Cells in lumen not removed
- Connective tissue completely blocing tube
Describe the origin and formation of the liver
- Endodermal and mesodermal origin
- Inductive and growth factor signalling from septum transversum and cardiac mesenchyme (to establish final liver architecture)
- Splanchnic mesoderm contributes to sinusoids, connective tissue stroma, liver capsule, falciform ligament, muscle of biliary tract
- liver bud grows from gut, fuses with diaphragm
- Final liver made from glandular tissue from gut and connective tissue from diaphragm
- Embryo ends swing around, umbilical cord formed, septum transversum (diaphragm and connective tissue of liver) formed
- Coming out of newly formed gut have buds - will form secretory part of liver
- Endoderm forms finger-like cords to interdigitate with cords of ST connective tissue
- Early hepatoblasts bipotential - hepatocytes or cholangiocyte
Describe the action of FGFs on the embryological development of the liver
- Produced in cardiac mesenchyme
Interact with hypatocyte nuclear factors in adjacent endodermal foregute - Lead to proliferation and differentiation of liver tissue
- Position of the heart leads to FGFs
Describe the formation of the blood supply to the liver
- Primitive branching plates develop in the mesenchyme alongisde adjacent portal vein branches
- Branching of blood vessels, aorta stimulated
- Blood drainage from gut (venous) does not go back to main system, goes via hepatic portal veins to liver
- own arterial supply plus venous drainage from gut = 2 blood supplies, then out via hepatic vein
Describe the origin and formation of the pancreas
- made up of dorsal and ventral bids
- Both from embryonic foregut
- Fuse, associated with rotation of duodenum
- As intestines roll into body, dorsal and ventral more L shaped
- Acinar and islet cells both arise from foregut endoderm
- One bud trapped in bend of duodenum
- Origianlly has 2 ducts, sometimes fuse (no fusion in dog and horses)
Describe the origin and formation of the spleen
- Mesenteries allow blood vessels andnerves to reach organs suspended in them
- Spleen appears within dorsal mesentery near stomach
- Mesenchyme cells seem to get signals but don’t know where from
- Spleen is filter for blood
- Is already in supporting system and has blood supply - no tricks needed
What are the branches of the coeliac artery?
- Left gastric
- Hepatic
- Splenic
What are the branches of the cranial mesenteric artery?
- Dominant blood suply to GI tract
- Caudal pancreaticoduodenal
- jejunal (has 15-19 more branches)
- ileo colic
What are the branches of the caudal mesenteric artery?
- Left colic
- Cranial rectal
Outline the basics of foetal circulation in the GI system
- Minimal supply to the guts
- Blood from placenta bypasses liver via ductus venosus (from umbilical vein into general circulation)
What are the branches of the hepatic artery?
- Hepatic branches
- Cystic branches
- Right gastric
- Cranial pancreatico-duodenal
- Right gastro-epiploic
What are the branches of the splenic artery?
- Pancreatic arches
- Splenic branches
- Short gastric
- Left gastro-epiploic
What are teh branches of the ileo colic artery?
- Middle colic
- Right colic
- Colic
- Ileal
- Caecal
When was your last tea/coffee break?
Doesn’t matter, you should probably take another one
What is aplasia?
- Absence of part or all of an organ
What is hypoplasia?
- Reduced development of an organ
What is atrophy of an organ?
Acquired reduction in size of an organ
Describe a congenital porto-systemic shunt
- Failure of ductus venosus to close
- Allos portal vein blood to continue to bypass liver in adult life
- Failure of normal perfusion of liver may lead to failure of development of intrahepatic vasculature
- Get small liver and weak animal
Describe an acquired porto-systemic shunt
- Obstruction of normal hepatic venous outflow e.g. diffuse liver fibrosis
- Leads to establishment of collateral venous drainage
- Utilises pre-existing channels e.g. spleen/omentum
- Scarring in liver, restricts blood flow to liver, blood tries to find another way
- Small anastomoses become enlarged
- Miss out liver and go via the spleen
Discuss the timing of gut closure in the neonate
- Gut closure is the point at which no more passive immunity can be acquired
- Begines between 4-8 hours, ends within 24 hours
- Need to get antibodies (in colostrum) before this time
Discuss the duration of passive immuntiy in the neonate
- Passive immunity replaced by puppy’s own antibodies
- Window of susceptibility where both passive and own are at their lowest
- More antibodies taken in initially give shorter window of susceptibility (more time for puppy to develop own antibodies)
Discuss the features of the neonatal GI tract that facilitate immunoglobulin absorption
- Less acidic environment, lack of proteases
- Immunoglobulins not destroyed
- Can absorb Ig better than adults
