23 Flashcards
What are the direct roles of the endoderm in embryonic development?
It forms the digestive system and respiratory system.
The liver, gallbladder, pancreas and lungs bud from the digestive system.
The pharynx develops from the most anterior region of the digestive tube, then small intestine and colon.
The tonsils, thyroid gland, thymus, and parathyroid develop from epithelial outpockets of pharynx.
What are the indirect roles of the endoderm in embryonic development?
It instructs the formation of tissues and organs from other germ layers, like the notochord, heart, and blood vessels.
When does the primitive gut form and what event causes it?
2 weeks as a result of embryonic folding after gastrulation.
What separates the anterior and posterior of the embryo during embryonic folding?
The primitive gut.
What does the allantoic diverticulum become?
Part of the umbilical cord.
How does the primitive gut influence embryonic folding?
It divides into heart progenitors which press up on the layer, causing folding and impacting the development of the notochord and an asymmetrical shape of the endoderm epithelially.
How does the midgut form?
By the anterior and caudal intestinal portals moving towards each other.
What are the divisions of the endoderm? What are they precursors to?
Midgut-Hindgut: colon, rectum, anus
Posterior foregut: Small intestine
Anterior foregut: lungs, mouth, pharynx
What is the mesoderm’s role in gut formation?
Mesenchyme from the splanchnic lateral mesoderm wraps around the endoderm (visceral mesoderm) and this forms the connective tissue that holds the digestive tube to the walls of the abdomen and the smooth muscles for peristalsis.
What is the dorsal mesentery and what is its role?
It is a derivative of the splanchnic lateral mesoderm that connects the gut to the body wall and drives the looping of the gut tube (along with endoderm growth).
What do hedgehog genes do to specify the gut?
The endoderm secretes Shh. Further from the endoderm, this generates smooth muscle cells because of a lower concentration. Closer to the endoderm, this induces BMP4 and the smooth muscle phenotype and myocardin are blocked. This undifferentiated mesenchyme forms the capillary network that takes food to the rest of the body.
Note that neural crest cells also secrete BMPs and inhib smooth muscle forming but there is a middle sweet spot where it does form.
Additionally, the differentiation of smooth muscle in mesoderm constricts underlying endoderm and mesenchyme, this causes the endoderm to buckle and forms villi, localizing ISCs! The buckling promotes Shh signaling at specific sites.
Describe the Wnt/BMP/Nodal endoderm gut specification pathway. How do concentration gradients play a part?
Nodal signaling activates epiblasts to assume a mesoendoderm fate, they migrate through the primitive streak.
If Nodal signaling remains high, these cells become definitive endoderm, marked by Sox17+ expression.
If Nodal signaling is lower, these cells receive uninhibited BMP and FGF and become mesoderm, marked by Brachyury+ expression.
Concentration gradients especially play a part in definitive endoderm specification, as where the cell is located on the anterior-posterior axis influences its exposure to BMPs, FGFs, and WNTs which are higher posteriorly.
Those most posterior cells become the Midgut-Hindgut cells and generate the intestines
Those most anterior cells become the Anterior foregut and generate the lung and thyroid.
Those in the middle become the posterior foregut and are the precursors to the liver (hepatocyte and cholangiocyte) and pancreas (exocrine and endocrine).
What determines if a PFG cell becomes a hepatoblast or a pancreas cell?
High MAPK, BMP, and Wnt signalling will produce a hepatoblast. MAPK and Inhibited BMP will produce a pancreas precursor.
What determines if a pancreas cell becomes exocrine or endocrine?
Inhibited Notch and TGFb pathways lead to the endocrine pancreas.
What is the role of the exocrine and endocrine pancreas?
The exocrine pancreas: digestive enzymes, the biggest part
The endocrine pancreas: creates insulin and glucagon to regulate energy storage and use
