Embryology gut Flashcards
Broadly describe gut formation
starting at gastrulation in humans.
Differentiation and pattern formation occur by a variety of mechanisms:
- Concentration gradients starting with the asymmetric egg.
- Transcription factor cascades, and chemical signals.
- Physical stimuli
**Developmental genes may have multiple functions depending on circumstances.
REview gastrulation
- Implantation in first week (blastocyst - hollow ball with ICM)
- second week - bilayer developed
- Inner cell mass develops at 2-3 weeks
- The primitive streak develops on the dorsal surface.
- Cells migrate from the primitive streak, and the ultimate outcome is a three-layered embryo:
- ectoderm
- mesoderm
- endoderm – makes gut and gut-derived structures. It encloses the secondary yolk sac which then tubularises to form the gut and its derivatives
Significance of neurulation for gut formation
- The notochord and neural tube have developed, and signals from the notochord induce gut differentiation in its various parts.
So we get the clustering of features of the VACTERL -association.- due to cross signalling from notochord to gut formation - Humans are chordates – we have a notochord transiently.
- Signals from the notochord are important in determining many gut-derived structures. For instance lung and liver are gut-derived.
- Problems with this process are thought to contribute to the condition of oesophageal atresia and tracheo-oesophageal fistula.
(Parts of the VACTERL association.)
DESCRIBE THE FOREGUT, derived structures, and developemnt and blood supply
- Function: transport and storage primarily, although the duodenum is, of course, involved in absorption. Stomach secretes acid and digestive enzymes, both sterilizing the food and initiating digestion.
- Derived structures: branchial structures (thymus, thyroid, parathyroid), lungs, esophagus, stomach, first part duodenum. Liver, gallbladder, pancreas.
- Local growth and differential growth create a large capacity part of the gut called the stomach. Differential growth causes it to rotate so that the left vagus comes to lie anteriorly, and the right posteriorly.
- Endodermal buds from the duodenum:
- Grow into the septum transversum to create the bile ducts and to interact with mesenchyme of septum transversum to form the liver (i.e. develops in ventral mesentery).
- Grow ventrally and dorsally to form pancreatic buds. Differential growth brings both buds together as pancreatic ducts on the left of the retroperitoneum. Generally, the two ducts fuse to form a single pancreatic duct, supplying a left-pointing retroperitoneal pancreas. The pancreatic head, uncinate process and its duct are the remains of the ventral duct and pancreatic bud. The dorsal duct forms the rest of the pancreas.
- The main blood supply of the foregut is the coeliac axis.
Lit derived structures of foregut
Derived structures: branchial structures, lungs, esophagus.
- Six branchial arches e.g. hyoid, malleus, incus, stapedis, thyroid, thymus… (of which 1 is vestigial) form around developing pharynx.
- Mesoderm-derived. Endodermal coverings form thyroid, thymus, parathyroid, etc.
- Important in head and neck development.
- Lung formation
- laryngo-tracheal groove separates from oesophagus by end of wk 4 to make lung buds
- by day 20 neurenteric canal closes and notochord forms
- “H” TOF - an issue: persistent connections between trachea and oesophagus, or interruption of oesophagus where distal oesophagus is continuous with trachea
Describe how stomach forms
Local growth and differential growth create a large capacity part of the gut called the stomach. Differential growth causes it to rotate so that the left vagus comes to lie anteriorly (ventrally), and the right posteriorly.
- Ventral, becomes right-sided.
- Dorsal, becomes left-sided.
when is the solid cord stages of gut developemnt
Solid cord stage of gut development: 6 – 8 weeks.
- huge proliferation of tissue in duodenum
- dudodenum becomes solid
- lumen re-develops by 8 weeks
list issues of foregut mal development
Foregut mal-development:
- Duodenal atresia - solid stage persists
- Failure of recanalization of the solid cord stage - thought to be
- Why? Local Wnt or Hox gene defect?
- Assoc with Down syndrome (~40%)
describe gastic rotation a nd growth of midgut, and provide an overview for the development of the pancreas and gallbladder
- day 30: early bile duct and pancreas development
- Pancreatico-biliary budding
- 6 weeks: dorsal and ventral pancreatic buds fuse
- 8-11 weeks: physiological hernia - midgut grows into base of umbilical cord (Stomach rotation, midgut expansion into cord, lesser sac formation)
Describe midgut, derived structres, function and developemnt
- Function: absorption of food.
- Derived structures: small bowel from the second part duodenum to the ileo-caecal valve and colon to mid transverse. Vermiform appendix. Pathologically, may include Meckel’s diverticulum.
- The main job of the small bowel is nutrient absorption, and to do this job properly given humans complex diet, we need length. During the first trimester, the midgut elongates greatly, protruding out from the abdominal cavity into the umbilical cord making the “physiological hernia”. Towards the end of the first trimester, the abdominal cavity grows to re-engulf the midgut, and as the midgut re-enters the abdominal cavity, it rotates around the axis of its artery 270° anticlockwise. This is the normal midgut rotation. It is surprisingly important.
- The main blood supply around which the midgut rotates is the superior mesenteric artery.
List issues of the midgut
- Patent vitellointestinal duct. leads to …. the (usually asymptomatic) Meckel’s diverticulum.
Midgut “Physiological hernia”: - persists
Usually gut returns to the abdomen in the third month, with a relative decrease in the size of the liver and kidneys. In Meckel’s diverticulum, outpouching of abdominal contents remains due to persistent vitellointestinal duct.
Two possible presentations:
- Gastroschisis
The gut has returned (?), then come out again.
- Exomphalos
The gut has never been back in.
- Stability and the root of the mesentery: small bowel is not fixed, but stable so long as length is relatively maintained (to root of mesentery)BEWARE GREEN VOMITING!
DESCRIB e hindgut, dunction, derived structures and development
- 12 weeks: ganglion cell migration reaches the anus – enteric nervous system
- Function: salt and water reabsorption. Faecal storage until release is convenient.
- Derived structures: Mid transverse colon to the bottom of the rectum.
Sigmoid and rectum involved in storage pending defaecation.- Bladder and urethra are endodermal derived, the urogenital sinus being divided from the rectum by the uro-rectal septum.
- Bladder originally drains into the allantois.
- The main hindgut blood supply is the inferior mesenteric artery.
Hindgut: cloacal region
The hindgut is progressively separated from the primitive urogenital sinus by an ingrowing fold – the uro-rectal septum, which grows from cranial to caudal to reach the cloacal membrane which then breaks down.
NB. Urogenital sinus also lined by endoderm.
List hindugt malformations
- Rectal atresia
- Cloacal anomaly with haemangioma.
- anterior ectopic anus
Descrube the ENS
- The cells of the ENS migrate into the gut from the neural crest, specifically largely from the vagal neural crest at the cranial end.
- The ENS is the largest extracranial collection of nerve cells in the body. It encompasses the whole gut, and coordinates movement, secretion, sensation, and seems to have a role in immune response.
- As for the CNS, numerous developmental malfunctions are possible
describe the neural crest and state its signficance to gut formation
- Ectodermal ridges form alongside the neural tube.
- Supply cells widely including to:
- ENS, autonomic ganglia, branchial arch derivatives, pigment cells of skin, heart…
