Embryology of the GI Flashcards
Define gastrulation.
The bilaminar germ disk is covered into the trilaminar disk through the following process;
- Cells flow over primitive streak between two existing layers (ecto- and endoderm).
- A third embryonic germinal layer is formed, called the mesoderm.
- The ectoderm now lies dorsally, while the endoderm lies ventrally.
What occurs in Week 1 of embryonic development?
Fertilisation and implantation of the embryo in the uterine wall.
What occurs in Week 3 of embryonic development?
Trilaminar germ disk formation (gastrulation), CNS induction and neurulation.
Name two factors that can cause major abnormalities if present during organogenesis.
Drugs and viruses.
What occurs in Week 8 of embryonic development?
Organogenesis.
What does the embryonic ectoderm form?
Skin (epidermis, hair, nail), CNS, PNS, sensory epithelia of the eye, ear, nose & other structures (pituitary, mammary, sweat glands, enamel of teeth), and adrenal medulla.
What does the embryonic mesoderm form?
Muscle, bones, cartilage, dermis, vascular system (most of the CVS connective tissues, blood cells and bone marrow, vessels associated with organs and tissues), urogenital system except bladder, spleen, liver mesenchyme, and suprarenal gland cortex.
What does the embryonic endoderm form?
Epithelial lining of respiratory system, GIT, glands, liver (hepatocyte and biliary epithelium), pancreas, thyroid, parathyroid parenchyma, and bladder.
Name the components of the foregut.
Mouth, pharynx, oesophagus, stomach and upper duodenum.
Where does the foregut end?
The second section of the duodenum.
Is the duodenum intra- or retroperitoneal?
The first (superior) section is intraperitoneal while the other three sections are retroperitoneal.
Name the components of the midgut.
The distal duodenum, jejunum, ileum, caecum, appendix, ascending colon, and the proximal 2/3rds of the transverse colon.
Where does the midgut end?
Proximal to the end third of the transverse colon.
Describe the process of regulating gut formation.
SHH (Sonic Hedgehog) transcription factor is secreted from the gut endoderm –> induces HOX gene in gut mesoderm –> molecular regulation of craniocaudal organisation of gut
What structure does the foregut develop from?
The cranial part of the primitive gut tube.
What structure develops from the ventral wall of the foregut?
Laryngo-tracheal diverticulum (lung bud).
Describe the formation of the stomach.
Forms in the 4th week of embryonic development.
- Forms greater (GC) and lesser curvature (LC).
- Undergoes a 90° clockwise rotation around its own longitudinal axis (GC now on left side).
- Undergoes an anteroposterior to create final adult position.
Describe the formation of the omental bursa.
The stomach is attached to the dorsal and ventral wall by a mesentery called the mesogastrium.
Rotation along the longitudinal axis pulls the dorsal mesentery to the left, creating the mental bursa.
Define the transverse mesocolon.
The mesentery that attaches the transverse colon between the colic flexures..
What structure(s) does the duodenum form from?
The caudal part of the foregut and the cranial part of the midgut.
What arteries supply the duodenum?
Coeliac trunk and superior mesenteric artery.
What happens to the duodenum what the stomach rotates?
It become C-shaped and also rotates.
In what week of embryonic development is the liver bud formed?
Week 3.
What structure does the liver and biliary apparatus form from?
An outgrowth of endoderm (epithelia) in the distal end of the foregut.
Describe the formation of the liver and biliary apparatus.
- Liver bud grows into the septum transversum.
- Liver bud grows and connection between it and duodenum narrows –> forms bile duct.
- Ventral outgrowth from bile duct forms gallbladder and cystic duct.
When does the spleen form? What tissue type is it derived from?
Week 5 and is a mesodermal derivative (not parenchymal).
When do leukocytes colonise the spleen? What type of leukocytes are present?
Weeks 15-17: T-lymphocytes.
Week 23: B-lymphocyte precursors.
After this, the spleen can begin its lymphoid function.
Describe the formation of the pancreas.
- Forms from 2 bus that grow from endodermal lining of the duodenum (Week 5).
- As the duodenum rotates, the ventral bud also moves to lie close to the dorsal bud (Week 6).
- Dorsal bud –> inferior part of head of pancreas, ventral bud –> forms the rest.
Name all structures that develop from the foregut.
Oesophagus, lungs (bud), stomach, omental bursa, duodenum, liver and biliary apparatus, spleen and pancreas.
If the tracheo-oesophageal septum deviates incorrectly, then there is incomplete separation of laryngo-tracheal tube (lung bud). What abnormalities can result from this?
Oesophageal atresia (most common 90%) and trachea-oesophageal fistula.
Describe annular pancreas and its consequences.
The result of the ventral bud failing to migrate correctly.
This can cause duodenal stenosis or for pancreatic tissue to form in other areas of the foregut.
Describe the early development of the midgut.
Cephalic part --> distal duodenum, jejunum, proximal ileum Caudal part (below bile duct) --> distal ileum, caecum, appendix, ascending colon, proximal 2/3rds of transverse colon Both must undergo rotation and physiological herniation to reach adult pattern.
Describe the rotation of the midgut.
- In Week 6.
- Initial rotation pf primary intestinal loop occurs around the axis of superior mesenteric artery.
- 90° rotation anti-clockwise (think stomach formation).
- Cranial part of midgut carried to the right.
Describe how the physiological herniation of the midgut.
- Gut tube herniates into extra embryonic cavity in umbilical cord.
- Allows growth of gut tube (not enough room due to liver).
- Intestinal loops begin to move from umbilical cord back into abdominal cavity.
Describe omphalocele.
- Midgut anomaly.
- Herniation of intestinal loop through an enlarged umbilical ring.
- Viscera covered by layer of amnion.
Describe the midgut anomaly, remnants of vitelline duct.
- Small vitelline duct persists (2-4%)
-
