lecture 15 Flashcards
is there a central controller for development
no; everything happens in individual cells
what is a domino
a signaling/developmental process in a specific tissue
what is the first domino to fall
fertilization of the egg
what is inevitable after first domino (fertilization of egg)
proliferation, differentiation thru expression of unique transcriptional regulators, all the way till adult organism
what happens as you transition from one stage of development to another
expression of specific transcriptional regulators
what happens when a transcriptional regulator is experssed
not just one protein that gets expressed, but ALL the genes that are controlled by that transcriptional regulator can be turned on/off leading to cell specialization
what are those transcriptional regulators controlling (what are some dominos)
promote/inhibit cell-cell interactions, promote/inhibit cell movement, proliferation thru pathways, up/downregulating actomyosin contractility to help w tissue morphogenesis
is there a master organizer of development
no. no CNS dictating what happens when, no master cell or central microchip
what’s controlling all these things in individual cells
every cell acts on its own in response to changes in transcriptional regulators expressed
give an example of every cell acting on its own rather than a central controller
steps leading to formation of an eye, instructions are contained in the cells that express first step of differentiation
what happens if u take cells that express initial precursor transcriptional regulator (that is gonna put cell and its progeny into developmental path to become an eye) and put it anywhere else on the embryo
its gonna become an eye (maybe not functional, but will still form)
why would it become an eye if we put precursor cell at a different location
b/c all the info you need (cascade of dominos gonna fall) for embryonic cells to become an eye are all in the cells itself
describe image B (eye like structure forming on diff parts where they shouldn’t be
dissected out embryonic eye cells and transplanted them to diff parts of cell –> they became eyes even at the wrong location cuz its just a series of dominoes that fall (same events are gonna happen in that new location, now you’re going to get an eye like structure there)
describe the system that controls development
highly conserved –> same processes in mice, humans, flies, equid, etc.
what is Pax6
transcriptional regulator, when expressed initiates a series of dominoes that have to fall over to convert pax6 expressing cell into a fully differentiated eye cell
what happens when they expressed pax6 from a squid in flies (where its not supposed to be expressed)
still does the same thing –> makes an insect eye (even tho its squid Pax6)
what does this show
common origins of multicellular life
what is initial fertilization event
single cell undergoes cleavage (goes from one cell to many cells) and forms blastula
what is blastula
hollow sphere surrounded by cells or blastomeres
what is initial stage where you get series of cleavage events
fertilized egg to blastula
what happens as u go from a single cell to bastula
proliferation but not growth; size does NOT change
what happens in gastrulation
blastula is turned inside out to form primordial gut (blastula to gastrula)
what can you notice in gastrulation
it is noticeably getting bigger, growth is happening, cell movement is happening
how do we know cell movement is happening
it has turned inside out
what else kicks into play as you go from blastula to gastrula
initial differentiation (you get 3 early cell fates, mesoderm, endoderm, ectoderm) AND morphogenesis (tube is formed)
what are 3 fates of cells that go to form many differentiated cell tyeps
ectoderm, endoderm, mesoderm
what is ectoderm
remains on outside of gastrulation embryo
what does ectoderm give rise to
skin and nervous system
what is endoderm
cells on inside of gastrulated embryo
what does endoderm give rise to
primitive gut & associated organs –> digestive system, esophagus, stomach, lungs, pancreas, liver, etc.
what are endoderm and ectoderm
looking to form epithelial like tissue where cells are forming cell-cell junctions and love being in contact w/ each other
what kinda tissue are endoderm and ectoderm similar to
epithelial tissue
what else is happening in ectoderm and endoderm
cadherins holding them together, belt of actomyosin contractility near apical surface
what is mesoderm give rise to
everything else: muscles, heart, blood, connective tissues (extracellular matrix rich tissues), kidneys
describe mesoderm; what is it similar to
more mesenchymal; like fibroblasts
how is mesoderm like fibroblast
forms single cells, doesn’t like forming cell-cell contacts w/ each other
can you tell sides apart in blastula
no; looks same, top, bottom, left, right
can you tell sides apart in gastrula
yeah; can tell top from bottom
how can you tell top from bottom in gastrula
opening to embryonic/primordial gut (at bottom)
can u tell left from right in gastrula
no; its symmetrical
what happens a few hours after gastrulation
profound asymmetry
what is symmetry breaking or axis specification
step going from symmetrical to us being to morphologically tell front from back, head from tail, left from right
when does symmetry breaking/axis specification occur
early development
why is axis specification a critical step
allows subsequent steps of development to follow more of a pattern, to reinforce & expand on this body plan specification that happens early
what is front
anteriorw
what is back
posterior
what is top
dorsal
what is bottom
ventral
when is basic body plan established
early in development
does basic body plan persist or go away
persists for lifetime of organism
what happens to ectoderm
ectoderm is still in outer portion of embryo which is where skin is going to form
what else do we now see in ectoderm
neural tube; going to become spinal cord and CNS
what is neural tube gonna become
spinal cord and central nervous system
what is mesoderm now
layer below ectoderm
what is mesoderm gonna become
connective tissue, musclesw
where is endoderm
in the middle
what is endoderm forming
gut cavity lined by endoderm
are these different cell types segregated? and when
segregated at early stage
what gives rise to tissue patterning
controlled differentiation in space and time (like EVE protein)
what is proper tissue patterning responsible
shape and function of adult organism
what happens once you specify body plan
can tell it left from right, front from back –> further steps of differentiation and morphogenesis
in early embryo are all cells are same or are they expressing diff things
look same but expressing different transcriptional regulators, going to have different face
what is blue part going to form
head
what is red (middle part) going to form
thorax
what is green (end) part gonna form
abdomen
are these relative positions at early stage maintained or not
maintained through adult stagew
why does body plan specification happen so early
b/c you need a framework to begin to place these different embryonic cells in correct locations, so adult tissues will form in the correct locations
what is tissue patterning
above process; body plan specification happening so early (, spatially regulated phenotypic trajectories are imposed on identical cells to generate distinct phenotypic cell domains.)
what does tissue patterning represeng
one of the dominoes that can fall during development
what can tissue patterning be a result of
several diff upstream cell signaling processes
what is largely responsible for differences between cell types
regulatory DNA
in tissue patterning, why do diff cells differentiate into diff fates
b/c they express diff transcriptional regulators
describe DNA as it goes from precursor cell/embryo to ultimate fate (muscle cell)
DNA isn’t changed
so how is there diff cell types
differentiated cells have different transcriptional regulators resulting in unique subset of proteins expressed in each cell types
so basically why are there different cell types
different proteins being expressed
why are there different proteins expressed
activation/repression of different transcriptional regulators
what does early embryo maintain into adulthood
body plan
what is changing in the different examples of tissue patterning we’re going through
changing which transcriptional regulators are expressed in different cells
what happens to the developmental potential of cells
becomes progressively restricted
what is general principle of development
as u go from early embryo/embryonic cell, at the early stage cell has many diff possibilities it can become
what can fertilized egg cell become
any cell type in the body
what is this fertilized egg cell fate called
totipotent
what happens as these dominoes fall
cell divides, generates diff cell fates; basically each step farther down the dev. path where u go from totipotent fertilized egg, to mesoderm endoderm ectoderm, to skin, heart, etc.
can you go backwards in developmental path
no; one way trip in terms of developmental fate
progressive restriction meaning
as cells differentiate to become more specialized, each step of differentiation there’s less and less things it can be
what do cells undergo
one way process of development from pluripotency to terminal differentiation
blastomere
hollow blastula stage of development where all the cells are the same cell type
what options does blastomere have
endoderm, ectoderm, mesoderm [covers all the cell types in adult organisms]
what happens once it decides on endoderm
ectoderm and mesoderm are no longer an option
what happens after endoderm
becomes liver, lung, pancreas, all the way down to its final fate
why is each step of this pathway happening
b/c a different set of transcriptional regulators is being turned on or off to trigger differentiation
trigger differentiation of what to what
differentiation of cell from its earlier designation (like blastomere) to a more later (developmentally speaking) designation (like endoderm)
what is one of the reasons why it’s a one way trip
though cells dont have a brain, they have cell memory (can remember what they used to be)
what can happen thru cell memory
simple signals can generate complex patterns
what is cell memory
can be epigenetic DNA changes (histone acetylation)
what else can cell memory be
persistence of macromolecules like regulatory RNAs or transcriptional regulators
describe cell memory being an epigenetic change to DNA
at earlier step of development, cell had post-translational modifications made to histones associated w/ DNA
what is result of cell having post-translational histone modification
changes how the transcriptional regulators interact w/ that DNA
what is histone acetylation a way of
way for cell to remember what has happened
what does cell on the left receive
signal X, changes some transcriptional regulators and triggers acetylation of histones in nucleus (nucleus turns black)
what dos cell on right receive
signal Y (nucleus is white, changes trans. regulators, triggers histone acetylation in nucleus)
what happens when signal X is removed
no longer active in the tissues BUT the cell remembers signal that was there in the past
how does the cell remember
b/c it carries on histones (specific acetylation that was caused by presence of that signal)
what happens when signal C comes in
it’s going to have a very specific outcome because of histone acetylation
what happens if signal X never came in but signal C came in
no DNA modification and no memory, so it would trigger a different outcome in those cells
basically describe left vs. right in what signal C triggers
triggers different outcome in left vs right
what happens in signal Y and Y
both had some DNA modification; cell remembers b/c of acetylation changes to DNA; signal C thus triggers 2 diff outcomes
what is this [cell memory] an example of
one of the dominoes
why does this show how development can be complicated
way for signals to act on cells to get not just one option but many
what is another way for simple signals to generate complex patterns
combinatorial control (and cell memory)
what is combinatorial signal
allows you to get a specific differential outcome by combining signals so they’re there at the same time
what are signal A and B
paracrine signaling; growth factors expressed by neighboring tissues, acting on cells to trigger differentiation
what happens when signal A acts with B
you get a green cell; pancreatic duct cell (triggers specific susbet of transcriptional regulators activated that will make this a pancreatic duct cell)
what happens when signal A acts w/ signal C
triggers diff set of transcriptional regulators that give beta islet cell in pancreas
basically what is combinatorial signaling
way to get as many cell types from a limited subset of signals
what happens in an actual organism (w/r to combinatorial signaling)
cell doesn’t just have 3 but hundreds of thousands, putting them together in combos gives you millions of diff cell types
what does asymmetric cell division lead to
diversity
what are 2 ways to dictate how a common precursor can become two different cell types
molecular memory of previous signals, combinatorial signals actin on the same cell
what is another way to control cell differentiation
cell division
what does cell division lead to
cell can initiate a molecular program which can cause daughter cell to be 2 diff cell types
what does 2 diff cell types mean
diff set of transcriptional regulators that are activated
what is symmetric cell division
daughter cells are identical
what is asymmetric division
generates diversity;
describe how asymmetric division generates diversity
not everything is uniformly distributed in cytoplasm prior to anaphase and cytokinesis (red dots [ ] on one side of cell)
what are red dots
vesicles that contain a transcriptional regulator
what happens to red dots
kinesins w/ microtubules grab those vehicles and move them all to one side of cell b/c that’s where MTs are arranged
what happens to red dots after cytokinesis
red dots are found in daughter cells
describe result of asymmetric division
daughter cells are essentially born different
why are they considered 2 diff cell types
b/c they contain transcriptional regulators
what is symmetric cell division
cytoplasmic contents of the cell on the left are evenly mixed; when it divides –> 2 identical daughter cells
what has to happen in symmetric cell division to trigger differentiation
something to enable one cell to differentiate while the other one maintains its initial fate
how does this happen
even tho cells are the same they are in diff position within embryo; they aren’t on top of each other, diff location (like one closer to endoderm while other is not)
what does this different location lead to
opportunity for chem environment to be different (albeit small scale) –> triggers differentiation event
how can a cell undergoing symmetric division undergo 2 different chemical environments
inductive signal
what is process of induction
after cell division, it needs to differentiate. involves an inductive signal
what is inductive signal
growth factor expressed (by dark grey) & secreted into extracellular environment
how does inductive signal diffuse
short distance, binds onto receptors of light gray cells
what happens when inductive signal binds
triggers signaling pathway that changes which transcriptional regulators are expressed in those cells, thus triggering a SECOND differentiation of light gray into blue
basically describe induction
dark gray secretes GF short distance, binds onto light gray, triggers light gray’s differentiation into blue
simplified definition of inductive signaling
two cell types where one in the middle secretes a signal, acts on cells close by to trigger their differentiation to blue
why do only the close cells turn blue and not all of them
because [ ] of signal was highest there; signal decreases the further down you go
what are morphogens
long-range inductive signals that exert graded effects
describe gradient of signals
asymmetric [ ] over a distance; decreases the further you go
what can a magnitude of a signal induce
different transcriptional regulators and lead to diff cell types and patterning
where is the greatest [ ]
where it’s being produced; just diffuses thru tissues of developing embryo until it reaches 0
what does [ ] correlate with besides triggering differentiation like yes or no
instead [ ] correlates with the cell fate that is triggered
how can an embryo control where diff cell types are gonna form in space and time
tissue can be induced to divide into diff cell types based on [ ]
what does the concentration trigger
differentiation of diff cell types depending on how they respond to [ ] of morphogen
how can cells control shape of gradient
can make gradient sharper, make signal transient or stable
what is increased signal diffusion
can manipulate diffusion by controlling how much it interacts w/ matrix that’s surrounding these cells
what happens in increased signal diffusion
if it can’t bind to matrix well, it will float on by, increased signal diffusion
describe increased signal diffusion
lower [ ] at the source, extends farther out into the tissue
if we want increased diffusion do we make it bind to the matrix well or poorly
poorly; if it was bound 100% it could not leave matrix and diffuse
what’s another way to manipulate signal
increased signal stability
one way cells get rid of a morphogen in environment
endocytosis, and degrade it in lysosomes
what happens if you decrease endocytosis
increased signal stability
what happens if you increase endocytosis
lowers signal, and thus decreases signal stability
what does lateral inhibition do
generates pattern of diff cell types
what is lateral inhibition
stronger a signal a cell receives, weaker a signal it generates
what is transient bias
signal, force, ‘noisy’ signal
what’s another way for cells to control fates to go from one cell type to 2 cell types
lateral inhibitiion
describe lateral inhibitiion
negative feedback loop
describe lateral inhibition
protein X in cell 1 is trying to turn off protein X in cell 2, protein X in cell 2 is trying to turn off protein X in cell 1
what happens if both cells have exact # copies of X
nothing will change, both are equally inhibited, stable system
what happens if cell 1 has one more copy of X than cell 2
cell 1 wins the battle, turns off protein X in cell 2; means that cell 2 can’t turn it off in cell 1
what does this lead to
self-amplifying asymmetry in protein X; transient asymmetry where one cell is gonna win and thus 2 diff cell types
what actually ends up happening
bound to get this asymmetry just through random noise; almost guaranteed at one point to trigger self-amplifying symmetry
what possibilities are with lateral inhibition
can have noise or a very purposeful signal
either way what does lateral inhibition result in
stable differentiation event where red cell type is present on one side, yellow cell type on other side
what is short-range activation and long-range activation
combining short acting signal with a longer range signal to get complex cellular patterns
first step
short range signal (self-amplifying signal) [either thru lateral inhibition or random noise]
what happens with just short range signal
inevitably go from field of gray cells to field of yellow, because of doominos
what can you do to stop this (AKA second step)
longer range inhibitor of differentiation that keeps the cluster of differentiated cells limited in scope
what happens when you combine them
clusters of differentiated/specialized cells in an otherwise uniform field of same cell type
what is sequential induction
example of combinatorial signaling where depending on what’s present in the cell receiving the signal as well as what cell types produced in/producing the signal, can get rapid increase in complexity of cells within a tissue
sequential induction gives rise
from 2 cell types to 5 cell types (for example)
describe sequential induction steps
cell B secretes a signal that acts (short distance) cell A, triggers some of them to become cell C (so we go from A and B to A, B, C)
what happens next in sequential induction
cell B’s signal turns off, cell C secretes a signal; if cell A receives it turns into cell type D; if cell B receives it turns into cell E (basically whether the signal is received from cell A or B, will give rise to 2 diff cell types)
how do animals specify body plan / primary axes of polarization
diff mechanism; key first step is movement of transcriptional regulators thru cytoplasm
how does it work overall
initial dominoes that fall are gonna lead to all other asymmetric changes in developing embryo needed for successful developmental programming
what specifies front in back in xenopus development
where sperm fertilizes egg
what is the first domino that falls that reorients some components of cell (for xenopus)
sperm fertilizing egg
what happens when xenopus cell divides (after first domino falls)
asymmetric cell division, leads to more dominos falling, eventually adult organism
what is key first step / domino for drosophila
movement of transcriptional regulators is key in establishing the body plan
describe this process in drosophila
transcriptional regulators are already present in cytoplasm after fertilization, but the key thing is that they are concentrated on one side of embryo or the other
what do these processes in xenopus and drosophila have in common
diff processes, but the origins of the asymmetry which drive subsequent formation of embryonic body plan and establishment of primary axes of polarization are same
what are egg polarity genes
created from mothers genetic material, responsible for patterning drosophila embryo
what does bicoid create
anterior-posterior axis
what does toll create
dorsal-ventral axis
what does bicoid help do
helps control expression of Eve
what happens before bicoid controls expression of Eve
mRNA that expresses bicoid is concentrated on the side of the embryo fated to be the anterior/front
what & where is Nanos
Nanos is its partner transcriptional regulator, [ ] at posterior
what do bicoid and nanos do together
where they are [ ] specifies where the front and back are gonna be
what does Toll do
protein that controls formation of top-bottom axis
where is bicoid concentrated
anterior
once you form these differentiated tissues, how do you maintain an even boundary b/w diff segments of developing embryos?
positive feedback loop
what does positive feedback triggered by engrailed do
helps maintain the segmented pattern
is the positive feedback loop always active or only sometimes
always; that’s why there’s such sharp boundary between cells
what is one cell expressing
protein called wingless
what does cell w/ wingless do
acts on its neighbor to activate the engrailed protein
what is the engrailed protein
transcriptional regulator that enters nucleus, produces hedgehog protein
what does hedgehog protein do
acts on first cell to produce more wingless protein –> positive feedback loop
what does the positive feedback loop do
maintains sharp boundary, keeps them stable in their two different fates
what would happen w/o positive feedback loop
even lines (in EVE protein in drosophila) would not be as sharp or stable throughout development
what does Toll pathway control
controls asymmetric activity of Dorsal protein
what does D-V signaling genes do
creates a gradient of transcription regulator Dorsal
what does toll receptor activation in ventral cells trigger
translocation of Dorsal protein into nuclei on the same side of embryo, leading to asymmetry in DV axis
why is it called dorsal protein
b/c its recognized at where the protein is activated (dorsal)
what does dorsal protein do and in response to what
transcriptional regulator; enters nucleus in response to toll receptor activation
what does toll pathway ultimately lead to
controls Dorsal protein so it only enters nucleus of cells located on the future ventral side of embryo
what happens if you manipulate dorsal protein to be localized in every nucleus of embryo
bad things; no dorsal-ventral axis formed; it thinks its all ventral
describe the animal body plan
ancient & conserved
what are cellular memory and inductive signalig
important mechanisms of pattern formation
what does bicoid form
AP axis
what does toll form
DV axis
