Endoderm and derivatives

Question 1

Expain early divisions of blastomeres and how do we get different endoderms.

Answer 1

blastomere -> trophoectoderm + inner cell mass
inner cell mass -> primitive endoderm + epiblast

primitive endoderm => parietal + visceral
epiblast => definitive endoderm, mesoderm, ectoderm

Question 2

Which of the endoderms becomes a 3D gut tube?

Answer 2

definitive

Question 3

Endoderm and mesoderm both arise from a common precursor: mesoendoderm. What causes the commitment to one or the other fate?

Answer 3

high Nodal = endoderm

low = mesoderm

Question 4

In endoderm, Nodal signaling leads to expression of a conserved network of TFs. Which are they and what are their functions?

Answer 4

Mixl1
Foxa2
Sox17

commit cells to endodermal fate, work in a regulatory network (not in a linear fashion)

Question 5

Which are the steps of gut formation from endoderm formation to organs?

Answer 5

endoderm formation
endoderm patterning -> morphogenesis to become tube; patterned along the AP axis by surrounding mesoderm
organ specification
organ buds

Question 6

What is the ground state/initial specification of gut and which gradient is important for proper formation?

Answer 6

initial = anterior fate (trachea, stomach..)

Wnt gradient (max @ posterior end)
-> Wnt KOs = missing posterior parts of the gut or shortened or smth

Question 7

Respiratory tube is an outgrowth of the digestive tube (DV patterning plays a role). To achieve this patterning, a TF is expressed in mesenchyme around the tube that influences if the tube will develop into squamos esophageal epithelium or cilliated respiratory epithelium. Which TF is that and which major signaling pathway it influences?

Answer 7

Barx1 -> Wnt inhibition

Question 8

What is the expression pattern of Shh and Wnt in:

a) stomach
b) dorsal pancreas
c) intestine

Answer 8

a) Shh, no Wnt
b) no Shh, Wnt
c) Shh and Wnt

Question 9

Regarding foregut organ induction: which gradient is important for development of different organs (pancreas, liver, thyroid and lungs)? Where does this gradient originate?

Answer 9

Fgf2 from cardiac mesoderm

Question 10

Which TF is present in pulmonary endoderm (-> lungs)?

Answer 10

Nkx2.1

Question 11

Which primordia does pancreas develop from?

Answer 11

dorsal and ventral pancreatic bud fuse to form adult pancreas

Question 12

Describe pancreas organogenesis.

Answer 12

• establishing pre-pancreatic domain in the dorsal endoderm -> Pdx1 expression
• bud outgrowth and proliferation
• 2 pancreatic buds created subsequently -> elongate alongside the presumptive duodenum and eventually fuse into one organ because of gut rotation

To get the morphology:

• in the bud, multipotent pancreatic progenitors expand from the bud and are polarized
• some cells initiate apical polarization and constiction -> rosete formation
• budding structure organizes in tip (multipotent progenitors) and trunk region
• it remodels and grows, gets lateral branches and splitting
• in the end, tip region loses potency and becomes acinar cells

Question 13

In pancreas development initiated by Pdx1, some cells become tip cells and some trunk. Which TF determines that? How do then some cells become ductal and some endocrinal?

Answer 13

no Notch = tip = acinar cells

where Notch is: graded signal
high = ductal fate (Hes1)
low = Sox9 -> Ngn3 = endocrine fate

Question 14

In development of endocrine cells in the pancreas, they differentiate from the pancreatic progenitor cord because Ngn3 becomes expressed there. How?

Answer 14

Ngn3 targets Snail TF which leads to EMT

Question 15

Pancreas has exocrine and endocrine function. Which cells are responsible for exocrine function and what is it? Where are they found in the pancreas?

Answer 15

acinar
ductal cells

secrete pancreatic juice with enzymes which is transported into duodenum and digests nutrients

acinar cells cap the endings of small terminal ducts

Question 16

Pancreas has exocrine and endocrine function. Which cells are responsible for endocrine function and what is it? Where in the pancreas are they found?

Answer 16

endocrine cells
alpha, beta, delta, epsilon, PP cells

release hormones into the blood stream to maintaint glucose homeostasis

clustered in islets of Langerhans

Question 17

Which transcription factor, which is also very important in development (like formation of node and notochord, neural tue patterning, gut morphogenesis) plays a role in pancreas in postnatal life? What does it do? How is it called and why?

Answer 17

Foxa2
controls glucose metabolism through the regulation of target genes like proglucagon in pancreas

pioneer TF, because it binds alone, independently directly to nucleosome or chromatin (precedes other TFs)

Question 18

Why are pioneer TFs important?

Answer 18

initiate cell-fate changes by binding to silent target genes (key regulatory sites) and displace nucleosomes -> open chromatin so that other TFs can bind

Question 19

What is diabetes mellitus and which types are there?

Answer 19

disease characterized by progressive loss/dysfunction of the insulin producing beta cells in pancreas
results in chronic hyperglycemia -> systemic metabolic complications -> multiple organ damage in long term (nephropathy, retinopathy, enteropathy)

type 1 = autoimmune (cytotoxic T cells destroy beta cells)
type 2 = adult onset, consequence of genetic predisposition, obesity, lack of physical exercise

Question 20

Pancreatic transplants are not the ideal solution for diabetes. What other main strategies are being expored to replace lost/dysfunctional beta cells?

Answer 20

1. in vivo differentiation of beta cells from stem cells (neogenesis, redifferentiation, transdifferentiation)
2. endogenous beta cell regeneration (self-replication)

Question 21

Which part of the intestine is derived from endoderm?

Answer 21

epithelium only

Question 22

Which germ layer are smooth muscle, vasculature, lymphatics and immune cells in intestine derived from?

Answer 22

mesoderm

Question 23

Which are 5 main cell types in the intestine?

Answer 23

enterocytes (absorb nutrients)
goblet cells (secrete mucus)
Paneth cells (secrete lysozyme)
enteroendocrine cells (secrete hormones)
tuft cells (secrete opioids)

Question 24

Where in the intestine (morphology) are intestinal stem cells found?

Answer 24

in the crypts

Question 25

Which are the 4 main stages of intestinal development (morphologically)?

Answer 25

1. intestinal segment patterning
2. intestinal growth and elongation
3. epithelial reorganization and vilification, cytodifferentiation
4. crypt formation and acquisition of adult properties

Question 26

At some point in intestinal formation, vilification happens. How?

Answer 26

epithelium acquires polatized form because of signaling from mesoderm beneath it
(BMP and PdgfA from mesoderm, PdgfA and Hh from prospective crypt)

BMP signaling leads to:

• Sox9 in intervillus regions
• B-catenin in villus(?)

crypt require Foxl1 and Wnt from the mesoderm to maintain proliferative areas

crypts form postnatally

Question 27

Which factors mediate regional functional identities of the intestinal tract?

Answer 27

GATA factors

Question 28

Which pathway is used to single out secretory progenitors from absorptive progenitors in the intestine?

Answer 28

Notch (on absorptive) and delta (secretory progenitors)

Question 29

Ablation of which TF in the gut leads to production of functional insulin producing cells and how?

Answer 29

Foxo1

no Foxo1 = no inhibition of Ngn3
Ngn3 -> upregulation of Pdx1, Pax4, Nkx2.2… -> insulin