Cell Differentiation Lecture Sep 30

Question 1

What are te 3 main goals of a fertilized egg durin gearly development?

Answer 1

1. increase # of cells via proliferatoin
2. increase number of cell types by differentiation
3. Generate polarity to establish body axes

Question 2

In monozygotic twinning, what will happen if:

the split occurs before trophoblast formation?

the split occursat the early inner cell mass stage?

the split occurs later than that?

Answer 2

before trophoblast: there will be 2 placentas and 2 amnions.

during early inner cell mass: 1 placenta and 2 amnions

later: 1 placenta and 1 amnion

Question 3

what increases the risk of conjoined twins?

Answer 3

the later the split, the more likely they won’t separate entirely

Question 4

What is regulative development?

WHat is mosaic development?w

Which do humans use?

Answer 4

Regulative development: the blastomeres have similar developmental potencies with each capable of giving rise to a complete embryo. DIfferentiation is responsive to environmental signals.

Mosaic Development: cell fat is already assigned during cleavage and a strict developmental pplan is in place whereby removal of one or more sells results in an incomplete embryo.Cell fate is determined by differential inheritance of specific factors among daughter cells via asymmetric cell division?

Humans do mainly regulative development with some mosaic

Question 5

Patterns of gene expression can be influenced by what two methods? Which one goes with regulative development? Which one goes with mosaic development?

Answer 5

Asymmetric cell division (sister cells are born different - they get different factors). This one goes with mosaic development in lower animals

Induction is the ability of one cell to influence the deelopment of another cell, so daughter cells are identical and then altered by receiving distinct envrionmental signals. THis one goes with regulative development.

Question 6

What 5 basic mechanisms underlie the ability of cells ot produce different sets of functional proteins?

Answer 6

1. differential gene expression
2. selective nuclear RNA processing
3. Selective mRNA translation
4. Selective mRNA degradation
5. Differential protein modification

Question 7

During development when does the first differentiation decision ocur? How about the second?

Answer 7

First decision: inner cell mass or trophoblast?

Second decision: epiblast or hypoblast?

Question 8

What signalling pathway is involved int he first decision: inner cell mass or trophoblast?

Answer 8

Hippo signalling with Mst1 or Mst2, Yap and Tead4.

Question 9

How does Hippo signalling results in differentiation between the inner cell mass and trophoblast?

Answer 9

1. Signals from the outer cells inhibit Mst1/2 phosphorylation of the transcription factor Yap. This allows Yap to translocate to the nucleus and activate Tead4 transcription factors which will activate genes associated with the formation of the trophoblast (these include Cdx2).
2. Inner cells are unpolarized and do not inhibit Mst1/2. so Yap is phosphorylated and cannot translocate into the nhcleus to activate Tead4, you don’t get Cdx2, and they don’t develop into trophoblast.

Question 10

What transcription factors drive the second decision: epiblast or hypoblast?

Answer 10

Oct4

Nanog

Cdx2

GATA

Question 11

What are the steps leading to the separation of epiblast or hypoblast?

Answer 11

1. at first, all blostomeres express Oct4 and Nanog, which are trx factors associated with pluripotency and stem cell fucntion
2. In the 1st decision, Cdx2 is expressed in trophoblast cells only bia inhibition of Hippo
3. Inner cell mass cell maintain Hippo signaling and express Oct4, Nanog and Gata
4. Oct4/Nanog and Cdx2/GATA inhibit each other, so inner cells express EITHER Oct4/Nanog OR Gata.
5. Gata-positive cells segregate out to form the hypoblast.

Question 12

What two fundamental processes occur to early development?

Answer 12

1. a progressive restriction of cell fate
2. extensive cell motility and migration events

Question 13

What are totipotent cells capable of?

Answer 13

they can give rise to all embryonic and extraembryonic cell types and structures

Question 14

WHat are pleuripotent cells capable of?

Answer 14

can give rise to all embryonic cell types and structures

Question 15

What are multipotent cells capable of?

Answer 15

can give rise to multiple, but not all, cell types

Question 16

what are unipotent cells capable of?

Answer 16

can only give rise to one cell type

Question 17

What is the potency of epiglast cells?

Answer 17

they are pleuripotent because they can give rise to all embryonic cell types and structures, but CANNOT give rise to the trophoblast anymore

Question 18

WHat is the differnce between when a cell is committed and when a cell is determined?

Answer 18

A committed cell gains a general type of identity like a neural or epidermal cell, but the precise cell type is not yet defined

A determined cell is defined and further development is independent of envrionmetnal signals.

This usually occurs in sequence: commitment followed by determination

Question 19

What is the embryonic genome activation?

Answer 19

On day 3 the mother’s mRNAs have mostly been degraded and the embryonic genome itself begins to be expressed in the cells

Question 20

Cell differentiation relies on the emergence of differential gene expression patterns which alrgely involve what process?

Answer 20

Epigenetics

• modification of histones to silence some genes
• induction of transcription factors
• chromatin remodelling

Question 21

Describe Waddington’s “epigenetic landscape” model of cell fate determination.

Answer 21

• a pleuripotent cell is like marble tolling down a landscape that separates into different grooves that branch out and branch out until you get differentiated cells.
• if a cell wants to be reprogrammed back to plleuripotency, it will have to foll the marble back upt he hill, then it could roll back down a new groove
• OR you could get direct conversion (bypassing pleuripotency) - called transdifferentiation – if the cell is stimulated to jump over the different fate hills into a new groove. This doesn’t really happen though.

Question 22

Describe the alternative “epigenetic disc” model of cell fate determination.

why is it better?

Answer 22

Direct conversion is in fact definitely possible.

The pluripotent state is mtastable and just one of any number of states - it’s not necessarily the top of a hierarchy.

Conversion to a different fate can occur via extinsic factors that “tilt” the disc in a certain direction

Trx facotrs can serve as “guide rails” to channel specific fates.

Question 23

What is induction?

Answer 23

the process wehre on cell or a group of cells changes the developmental fate of another group

Question 24

What are signals that alter cell fate called>

what are cells producing the signals called?

Answer 24

morphogens

inducers

Question 25

Induction can be defined in terms of distance between cels and by behavior of cells. give examples of each.

Answer 25

Distance: paracrine, jusxtacrine, or autocrine

Behavior: instructive (cell A gives signal causing specific differention of cell B) and permissive (call B is specified, but the signal from A alows differentiation to proceed)

Question 26

How are most embryonic inductions mediated?

Answer 26

through secretion of signalling factors

Question 27

What are the 6 ways to develop gradients and other patterns of signalling?

Answer 27

simple diffusion or more complex interactions between activators and inhibitors:

1. inductive signalling
2. gradient signalling
3. antagonist signalling
4. Cascade signalling
5. Combinatorial signalling
6. Lateral signaling

Question 28

How do inductive interactions give rise to bronchial branching?

Answer 28

Through the fibroblast growth factor signalling system:

Mesenchyme cells secrete FGF10

FGF10 receptors are located on the bud of the epithelium cells

Binding of FGF10 to the receptor results in proliferation of the bud that has the receptors AND it induces the secretion fo sonic hedgehog by the epithelial cells at the to of the growing bud.

The Shh diffuses back to the mesenchym cells directly across and inhibits FGF10 secretion

THe areas on the sides will still secrete FGF10, however, which will go back and stimulate FGF10 receptors on the two sides, causing cell proliferation and BRANCHING.

This cycle contines until you get bronchioles branched.

Question 29

Why is gradient signaling so important?

Answer 29

DIfferent concentrations of a single morphogen can generate different cellstypes

Question 30

Describe sequential induction’s role in eye development.

Answer 30

There are two parts involved: surface ectoderm of head (lens and cornea) and neural ectoderm (retina and optic nerve)

1. optic vesicle neural ectoderm induces part of the surface ectoderm to invaginate inward and pinch off cells that form the lens (through FGF8 requiring Pax-6 expression first - only in the head)
2. The lengs now is lying between the optic cub and surface ectoderm and can therefor induce the neural ectoerm to form the retina AND the overlying surface ectoderm to form the cornea

Question 31

What are the three body axes?

Answer 31

Dorsal ventral: head to feet

Left right

Anterior-posterior: butt to front

Question 32

What deterines the dorsal ventral axis formation?

Answer 32

We don’t know what determines this early on, but…

WIthn the neural tube, cells are first commited but notyet specified.

1. the notochodr induces the ventral floor plate to secrete Shh
2. the overlying ectoderm induces the dorsal roof plate to secrete BMP
3. opposing gradiet formthrough the neural chord
4. the gradient instructs the cells in between to express particular transcription factors and adopt specific fates

This results in different neurons forming

Question 33

What gives rise to the left right asymmetry/axis?

Answer 33

THe cells in the primitive node have cilia and will beat in the exact same direction to the left.

Fluid flow tot he left increases the expression of the signaling molecule NODAL on the left side of tne embryo

Nodal on the left stimulates production of more NODAL and another signalling molecule LEFTY (which is antagonistic to nodal)

The high levels of nodal on the left make it impossible for lefty to completely antagonize nodal on the left, but it can completely antagonize it on the right, making Nodal practically only on the left side

Nodal ont he left stimualtes Pitx2 , a trx factor that modulates gene expression patterns associated with left-right asymmetry.

Question 34

What mediates the anterio-posterior axis formation?

Answer 34

Homebox genes are organized in a gene complex of 13 genes

the different versions of hox encode transcription factors that will result in different cell differentiation patterns

this means the order of hox down the anterior=posterior axis has to be highly regulated

interestingly, the order of the Hox genes along the axis is the same as the order of the genes on the chromosome

Question 35

What protein is involved in regulation of Hox expression?

Why can this be an issue sometimes?

Answer 35

Endogenous retinoic acid is involved in regulatin Hox expression at the appropriate time during development

Retinoic acid is a key incregient in many acne medications, and exogenous retinoic acid can distrup normal patterns of Hox gene expression, making it a teratogen

Question 36

What are the “big five” signalling pathways involved in development?

Answer 36

1. TGF pathway (transforming growth factor which induces TGF and BMP proteins)
2. Hedgehog pathway (often turns on cell proliferation in mammals)
3. FGF pathway (fibroblast growth factor which incudes a number of FGFs)
4. Wnt pathway
5. Notch pathway

Question 37

BRiefly describe the TGF-betal pathway?

Answer 37

receptros include type 1 and 2

bindig of the ligand (either TGF-B or BMP) incudes assembly of receptor homodimers

results in phosphlrylation of the type 1 receptor by the type 2 dimer

phosphorylate typ 1 dimer phosphorylates Smad proteins

Smaad then acts as a transcriptional factor

Question 38

What is a mutation in a ligand for the TGF pathway associated with?

Answer 38

brachydactyly

acromesomelic dysplasia

chondrodysplasia

(progressive skeletal disorders)

Question 39

What does hedge hog ultimately activate?

Answer 39

GL1, which is a transcription factor

Question 40

What three disorders will arise from mutations in hedgehog signaling?

Answer 40

Grieg cephalopolysyndactylyl (megacephaly, broad thumb ad digit fusion, duplicated big toe)

Pallister hall SYndrone (polydactyly)

Cyclopia (cyclopia, cleft palate and single inscisor)

Question 41

What happens in the FGF pathway?

Answer 41

The FGF binds to its tyrosine kinase receptor

receptor dimerizes and autophosphorylates

activates Ras

activates MAP kinase cascade

MA{K enter nucleus an dphosphorylates transcription factors to stimulate gene expression

Question 42

What will mutations in the FGF signalling pathway cause?

Answer 42

Crouson’s syndrome

Apert Syndrome

Pfeiffer syndrome

Question 43

What do mutations in the Wnt pathway cause?

Answer 43

colon cancer

Question 44

Mutations in notch result in what?

Answer 44

alagille syndrome

Question 45

What mediates cell migration (both collective and single)?

Answer 45

The Epithelial to Mesenchymal Transition (TGF, FGF, Wnt, and Notch)

and the Mesenchymal to Epithelial Transition (BMP)

FOr cells that become neural crest tubes, they undergo the EMT to become mesenchymal cells

as mesenchymal cells they can migrate around int he body as free cells

they will then come together and aggregate in new areas of the forming body and undergo an MET back to epithelial cells

Question 46

Inappropriate EMT will result in what?

Answer 46

cancer!

You don’t want epithelial cells going back to mesenchymal cells and traveling through the body because they’ll restuls in epithelial tumors and metastases

Question 47

The EMT and MET signalling pathways activate expression of transcription factors like Snail and slug, which do what?

Answer 47

they inhibit cadherin production so you don’t get as much cell cell adhesion and metastases develop

Question 48

WHat are the three characteristic hallmarks of COLLECTIVE cell migration?

Answer 48

1. cells remain physically and funtionally connected
2. Multicellular polarity and supracellular actin organization drive motility
3. migrating cells modify the environment (clearing and altering of the ECM)

The development of mammary glands is an example

Question 49

How is selective cell-cell adhesion mediated during development, giving rise to tissues?

Answer 49

different cadherins are expressed in tissue specific patterns and cadherin will only bind to identical cadherin.

So cells expressing E-cadherin will only bind to other cells withe E-cadherin - ectoderm

N-cadherin with N-cadherin: neural tube

etc.

Question 50

How does the formation of the zonulae adherens help lead to epithelial folding?

Answer 50

epithelial folding during development is primarily pushed by cell proliferation, but the oriented contraction of apical bundles of actin filaments at the zonulae adherens will also cause a narrowing of cells at their apical ends and helps the epithelial sheet to roll into a tube.

Question 51

What mediates single cell migration?

Answer 51

Epithelial to mesenchymal transition (EMT)

Snail represses caherin, claudin and occludin expression which results in loss of adherens and tight junctions

matrix metalloproteinase and vimentin expression is increased to promote the detachment of cells from an epithelium and convert them into mesenchymal like cells to invade the extracellular matrix.

Note that going backwards means you block snail for MET