Cell Differentiation Lecture Sep 30 Flashcards
What are te 3 main goals of a fertilized egg durin gearly development?
- increase # of cells via proliferatoin
- increase number of cell types by differentiation
- Generate polarity to establish body axes
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?
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
what increases the risk of conjoined twins?
the later the split, the more likely they won’t separate entirely
What is regulative development?
WHat is mosaic development?w
Which do humans use?
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
Patterns of gene expression can be influenced by what two methods? Which one goes with regulative development? Which one goes with mosaic development?
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.
What 5 basic mechanisms underlie the ability of cells ot produce different sets of functional proteins?
- differential gene expression
- selective nuclear RNA processing
- Selective mRNA translation
- Selective mRNA degradation
- Differential protein modification
During development when does the first differentiation decision ocur? How about the second?
First decision: inner cell mass or trophoblast?
Second decision: epiblast or hypoblast?
What signalling pathway is involved int he first decision: inner cell mass or trophoblast?
Hippo signalling with Mst1 or Mst2, Yap and Tead4.
How does Hippo signalling results in differentiation between the inner cell mass and trophoblast?
- 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).
- 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.
What transcription factors drive the second decision: epiblast or hypoblast?
Oct4
Nanog
Cdx2
GATA
What are the steps leading to the separation of epiblast or hypoblast?
- at first, all blostomeres express Oct4 and Nanog, which are trx factors associated with pluripotency and stem cell fucntion
- In the 1st decision, Cdx2 is expressed in trophoblast cells only bia inhibition of Hippo
- Inner cell mass cell maintain Hippo signaling and express Oct4, Nanog and Gata
- Oct4/Nanog and Cdx2/GATA inhibit each other, so inner cells express EITHER Oct4/Nanog OR Gata.
- Gata-positive cells segregate out to form the hypoblast.
What two fundamental processes occur to early development?
- a progressive restriction of cell fate
- extensive cell motility and migration events
What are totipotent cells capable of?
they can give rise to all embryonic and extraembryonic cell types and structures
WHat are pleuripotent cells capable of?
can give rise to all embryonic cell types and structures
What are multipotent cells capable of?
can give rise to multiple, but not all, cell types
what are unipotent cells capable of?
can only give rise to one cell type
What is the potency of epiglast cells?
they are pleuripotent because they can give rise to all embryonic cell types and structures, but CANNOT give rise to the trophoblast anymore
WHat is the differnce between when a cell is committed and when a cell is determined?
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
What is the embryonic genome activation?
On day 3 the mother’s mRNAs have mostly been degraded and the embryonic genome itself begins to be expressed in the cells
Cell differentiation relies on the emergence of differential gene expression patterns which alrgely involve what process?
Epigenetics
- modification of histones to silence some genes
- induction of transcription factors
- chromatin remodelling
Describe Waddington’s “epigenetic landscape” model of cell fate determination.
- 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.
Describe the alternative “epigenetic disc” model of cell fate determination.
why is it better?
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.
What is induction?
the process wehre on cell or a group of cells changes the developmental fate of another group
What are signals that alter cell fate called>
what are cells producing the signals called?
morphogens
inducers
Induction can be defined in terms of distance between cels and by behavior of cells. give examples of each.
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)
How are most embryonic inductions mediated?
through secretion of signalling factors
What are the 6 ways to develop gradients and other patterns of signalling?
simple diffusion or more complex interactions between activators and inhibitors:
- inductive signalling
- gradient signalling
- antagonist signalling
- Cascade signalling
- Combinatorial signalling
- Lateral signaling
How do inductive interactions give rise to bronchial branching?
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.
Why is gradient signaling so important?
DIfferent concentrations of a single morphogen can generate different cellstypes
Describe sequential induction’s role in eye development.
There are two parts involved: surface ectoderm of head (lens and cornea) and neural ectoderm (retina and optic nerve)
- 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)
- 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
What are the three body axes?
Dorsal ventral: head to feet
Left right
Anterior-posterior: butt to front
What deterines the dorsal ventral axis formation?
We don’t know what determines this early on, but…
WIthn the neural tube, cells are first commited but notyet specified.
- the notochodr induces the ventral floor plate to secrete Shh
- the overlying ectoderm induces the dorsal roof plate to secrete BMP
- opposing gradiet formthrough the neural chord
- the gradient instructs the cells in between to express particular transcription factors and adopt specific fates
This results in different neurons forming
What gives rise to the left right asymmetry/axis?
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.
What mediates the anterio-posterior axis formation?
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
What protein is involved in regulation of Hox expression?
Why can this be an issue sometimes?
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
What are the “big five” signalling pathways involved in development?
- TGF pathway (transforming growth factor which induces TGF and BMP proteins)
- Hedgehog pathway (often turns on cell proliferation in mammals)
- FGF pathway (fibroblast growth factor which incudes a number of FGFs)
- Wnt pathway
- Notch pathway
BRiefly describe the TGF-betal pathway?
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
What is a mutation in a ligand for the TGF pathway associated with?
brachydactyly
acromesomelic dysplasia
chondrodysplasia
(progressive skeletal disorders)
What does hedge hog ultimately activate?
GL1, which is a transcription factor
What three disorders will arise from mutations in hedgehog signaling?
Grieg cephalopolysyndactylyl (megacephaly, broad thumb ad digit fusion, duplicated big toe)
Pallister hall SYndrone (polydactyly)
Cyclopia (cyclopia, cleft palate and single inscisor)
What happens in the FGF pathway?
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
What will mutations in the FGF signalling pathway cause?
Crouson’s syndrome
Apert Syndrome
Pfeiffer syndrome
What do mutations in the Wnt pathway cause?
colon cancer
Mutations in notch result in what?
alagille syndrome
What mediates cell migration (both collective and single)?
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
Inappropriate EMT will result in what?
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
The EMT and MET signalling pathways activate expression of transcription factors like Snail and slug, which do what?
they inhibit cadherin production so you don’t get as much cell cell adhesion and metastases develop
WHat are the three characteristic hallmarks of COLLECTIVE cell migration?
- cells remain physically and funtionally connected
- Multicellular polarity and supracellular actin organization drive motility
- migrating cells modify the environment (clearing and altering of the ECM)
The development of mammary glands is an example
How is selective cell-cell adhesion mediated during development, giving rise to tissues?
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.
How does the formation of the zonulae adherens help lead to epithelial folding?
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.
What mediates single cell migration?
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
