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RICHA MCDB 138 Set 3
Terms in this set (110)
What is the most dorsal component of the dorsal ectoderm, and why (xenopus)
Notochord, because it is made in the region with lowest levels of BMP
What is the most ventral component of the ventral ectoderm, and why (xenopus)
Epidermis, because it is expressed in regions with the lowest levels of BMP
What does the Nieuwkoop Center induce?
Induces overlying animal cap cells to become the most dorsal mesoderm (makes sense because the most dorsal region of mesoderm has lowest levels of BMP, plus the Nieuwkoop Center produces the highest levels of Xnr)
Where does the Organizer come from?
The newly formed dorsal mesoderm, induced by Nieuwkoop Center
What does the Organizer produce a gradient of, and how?
BMP, by producing BMP antagonists
How does the organizer affect gastrulation in Xenopus?
It makes a gradient of BMP (by producing BMP antagonists), and thus further defines the DV axis
How does the organizer affect the formation of neural tissue?
Neural tissues form in the dorsalmost side, where BMP activity is lowest, because it forms just above the organizer,
How does the organizer affect the formation of the epidermis?
Epidermis forms in ventralmost side, where organizer activity is lowest (so it has the highest levels of BMP)
What cell movement happens at the organizer (xenopus)?
It's the site of involution. This leads to changes in the cell-cell interactions and brings the mesoderm in direct contact with the ectoderm. Note that the organizer is also the dorsal lip of the blastopore.
How do we know what parts of the ectoderm become the neural cells and what become the epidermis?
Make a fate map; dye the portion just above the organizer or the portion on the opposite side of the embryo (of the ectoderm) and see what the dyed cells turn out to be.
Are ectoderm cells already determined to become neural or epidermal cells? How do we know this?
No; do a transplantation experiment. Dye cells from the two presumptive regions of the early embryo different colors, then transplant them into each other's regions.
Is any part of the early gastrula determined?
Do an experiment: transplant the dorsal lip into the region of the presumptive epidermis.
Results: The transplanted dorsal lip gives rise to a new axis.
Conclusion: The cells are not determined, but the dorsal lip can induce a neural fate in the overlying ectoderm.
Is the neural inducing signal from the dorsal lip a direct cell-cell interaction signal or a secreted signal (Xenopus)?
Experiment: put a nitrocellulose filter between dorsal lip tissue and ectoderm, and see if the neural fate is still induced
Results: It is
Conclusion: the neural inducing signal is secreted by dorsal lip (note: it's an antagonist of BMP)
What are the antagonists of BMP secreted by the dorsal lip?
follistatin, chordin, noggin
How did we find the secreted factors of the dorsal lip?
1. Make a cDNA library of all the genes in the dorsal lip.
2. insert each of these into the ventral side of the embryo and see if it can induce a new blastopore
How do we know that it's the BMP antagonists are the ones responsible for the activity of the organizer?
Experiment: inject morpholinos against all three BMP antagonists.
Results: There is no neural induction, and no formation of dorsal mesoderm either
Conclusion: BMP antagonists are necessary for neural induction, and also important to dorsal mesoderm formation
What would happen if BMP signaling was lost?
Prediction: low levels of BMP would lead to loss of ventral tissues and overall dorsalization of embryo.
Experiment: inject morpholinos against BMPs
Results: you get a loss of ventral tissue (epidermis) and an excess of dorsal tissue (neural tissue)
Conclusion: BMP gradient leads to patterning of ectoderm and mesoderm along the DV axis in vertebrates
Does the dorsal lip affect the AP axis?
Experiment: transplant the dorsal lip to a host embryo
Results: a secondary embryo is induced in the host embryo, with a recognizable AP axis
Conclusion: Organizer establishes the AP axis as well as the DV axis
When is the AP axis established in an embryo?
Experiment: transplant the organizer from the dorsal to ventral side in an early embryo and a late embryo
Results: in an early embryo you get a whole new axis, but the older an embryo is the more posteriorly restricted the new axis is
Conclusion: the inductive properties of the organizer change over time, and the AP axis emerges during gastrulation
How does the organizer relate to the expression of Hox genes?
Cells that pass through the organizer early express 3' Hox genes, and cells that pass through later express more 5' Hox genes
Where do the cells that pass through the organizer end up?
Dorsal mesoderm cells pass through the organizer and migrate to sit under the dorsal ectoderm (which becomes the neural tissue).
What gradient affects the pattern of AP development in Xenopus?
What is Wnt?
A secreted protein and the vertebrate homolog of Wingless (a segment polarity gene)
How do we know that Wnt activity affects the AP axis?
Experiment: inject gastrula with mRNA encoding a Wnt antagonist
Results: embryo lacks posterior structures
Conclusion: high Wnt levels induce posterior structures, low levels lead to anterior structures
What creates the Wnt gradient?
The organizer produces Wnt antagonists.
Describe the patterns of gradients created by the organizer.
Low BMP: Dorsal
High BMP: Ventral
Low Wnt: Anterior
High Wnt: Posterior
What part of the gastrula (other than the organizer) causes the AP patterning of the CNS?
Anterior Neural Ridge and Midbrain-Hindbrain Boundary
How do we know the dorsal mesoderm induces the AP pattern of the neural tissue?
Experiment: transplant dorsal mesoderm along different regions of the AP axis (derived from organizer) to a host early gastrula. Depending on where the transplanted tissue was from, a different part of the AP axis will be induced in the host embryo
How are cells sorted in the neural ectoderm based on cadherins?
N-cadherin is upregulated and E-cadherin is downregulated
What is a neuroblast?
A cell that will eventually differentiate into a glial cell or a neuron. It has left the ventricular zone and has been committed to differentiate, as it expresses more Delta than its mother cell.
Where do neuroblasts come from?
Stem cells in neural tissue give rise to neuroblasts over time, during development and into adulthood.
How do neuroblasts become specified?
Through lateral inhibition with Delta-Notch signaling
What is lateral inhibition?
It regulates whether and when an individual stem cell differentiates into neuroblast
How does Notch signaling work?
It inhibits expression of proneural genes (which lead to formation of neuroblasts) by inhibiting neurogenin
How does Delta-Notch signaling lead to neuroblast formation?
An individual cell will start to produce more Delta, which leads to more Notch expression in neighboring cells. The higher Notch levels lead to suppression of neurogenin, which then makes the neighboring cells stop producing Delta. Thus the neighboring cells stop laterally inhibiting your cell, which can differentiate into neuroblasts.
How does the Xenopus embryo differ from the bird and mammalian embryo in early development?
1. Before gastrulation, Xenopus embryos are spherical, but bird and mammal embryos are flat sheets
2. In Xenopus, germ layers are specified before gastrulation movements. In birds and mammals, this is not so.
3. Xenopus undergoes involution to internalize mesoderm and ectoderm, but birds and mammals undergo ingression to do so.
What does the primitive streak do? What cell movements does it involve?
In birds and mammals, initiates gastrulation. It establishes the AP and DV axes.
It is formed by convergent extension of epiblast cells towards the midline and causes ingression of future mesoderm and endoderm.
How does the primitive streak establish the germ layers?
Once cells ingress through streak, they become mesoderm and endoderm
Where does the primitive streak originate?
The posterior marginal zone
What is the posterior marginal zone?
Analogous to the Nieuwkoop center in amphibians. Characterized by high Wnt(ß-catenin) activity. In birds, its position is established by gravity, but we don't know in humans
How is the primitive streak initiated?
Wnt(ß-catentin) in posterior marginal zone induces Nodal signaling in adjacent epiblast cells; Nodal (and FGF) induce the mesoderm
Describe what happens when the primitive streak extends.
1. The primitive streak extends towards the anterior part of the embryo via convergent extension
2. Mesoderm induction and patterning happens along the streak while it is forming
3. The node (Hensen's node) forms at the most anterior end of the fully extended streak and expresses the highest level of Nodal in the streak.
4. Once the node forms, cells begin to form.
Describe what happens when the primitive streak regresses.
1. Once the streak is fully extended, the mesoderm and endoderm begin ingressing.
2. After ingression is done, the streak regresses back to the posterior.
3. The notochord forms from the cells that ingress through the node (as it moves posteriorly) and move anteriorly.
What does the notochord do?
induces the overlying ectoderm to become neural tissue (compare with organizer and dorsal mesoderm in Xenopus)
Are mammalian/bird nodes organizers? Why or why not?
Yes, because they can induce new AP and DV axes, and the inductive properties change over time (early node can induce anterior and posterior structures, later ones can only induce more posterior structures).
How is the DV axis established by the primitive streak?
Cells that ingress through the node either move forward to become the notochord or laterally to become the mesoderm. The movement of the streak is the dorsal axis, and away from the streak is the ventral axis.
What are the key molecules of gastrulation in birds and mammals?
The same as in Xenopus: Loss of nodal (and nodal-related) leads to loss of mesoderm, loss of BMP antagonists leads to a ventralized embryo and the loss of dorsal mesoderm derivatives.
Where does mesoderm induction and patterning take place in birds and mammals?
It takes place in the primitive streak, where the node produces chordin to create a BMP gradient. This makes it so that the location of the streak becomes dorsal, and everything far from it becomes ventral.
Where does neural induction occur in birds and mammals?
In the epiblast cells overlying the developing notochord, which develops from cells that ingress through the node and move anteriorly. These cells produce high levels of chordin, which is why the notochord develops there (it is the most dorsal part of the ectoderm)
How do the cells in the mammalian/bird nodes and adjacent epiblast have stem cell properties?
They are self-renewing and multipotent
What does the axial (dorsal) mesoderm become in birds/mammals?
The notochord. The axial mesoderm induces the neural plate, which becomes the notochord, which becomes the intervertebral discs of the spine
What does paraxial mesoderm become in birds/mammals?
The somites, the dermatome (later becomes dermis), mytome (muscle), sclerotome (axial skeleton)
What does the intermediate mesoderm become in birds/mammals?
kidneys and reproductive organs
What does the lateral plate (ventral mesoderm) become in birds/mammals?
heart, blood vessels, blood, smooth muscle, limb bud
What is the timing of somite formation?
Somites are derived from the paraxial mesoderm and form progressively from anterior to posterior
Does somite formation rely on external signals or is
it an autonomous process in the presomitic mesoderm?
Experiment: rotate the presomitic mesoderm
Results: somites form in their original order (so now posterior to anterior)
Conclusion: temporal order of somite formation is already determined in the presomitic mesoderm
What controls the timing of somite formation?
FGFs, which are secreted factors, are secreted by the newly formed mesoderm. FGFs maintain the presomitic mesoderm in an undifferentiated state. As the node regresses, previously ingressed mesoderm goes outside the range of FGF and differentiates into somites
What are the two models of somite formation?
clock and wavefront
What is the clock model of somite formation?
A molecular clock is in the presomitic mesoderm which oscillates the delta/notch activity. Negative feedback loops are what cause the clock.
Give an example of a gene that is caused by a mutation in FGFs
Delta-like 3 (DII3). A mutation causes fewer or no somites formed, which leads to fused and deformed vertebrae. In humans, it causes a condition called Spondylcostal dysotosis
What is the sclerotome?
The vertebrae and ribs, comes from ventro-medial cells of the somite
What induces sclerotome formation?
The notochord and ventral neural tube secrete sonic hedgehog (SHH), which which induces somites that would have become the dermomyotome to become sclerotome
What is the dermomyotome induced by?
Signals form the dorsal neural tube and epidermis
Briefly describe the regionalization of the neural tube along the AP axis
From anterior to posterior: forebrain, midbrain, hindbrain
What is the evidence for segmentation of the brain and neural tube?
The brain arises in a series of discrete bulges in the anterior part of the neural tube, and the hindbrain is divided into rhombomeres
What is a rhombomere?
Separate compartments, the boundaries of which are the same as the boundaries of Hox gene expression, that give rise to similar structures
What is the evidence that Hox genes pattern the rhombomeres?
Hox gene expression boundaries are the same as the boundaries of each rhombomere, and knocking out Hox genes affects the rhombomere development in the most anterior position that the Hox gene acts
How does the anterior region of the brain acquire AP patterning information?
Local signaling center pattern the neural tube in places anterior to the range of Hox gene expression
What is the region of the neural tube in which Hox genes pattern the AP axis?
The hindbrain, or everything under the MHB (midbrain hindbrain boundary). The midbrain and forebrain are given their AP axes by FGF8, which is produced by the MHB and the anterior neural ridge (ANR)
How are different TFs expressed to develop different regions of the brain?
The different concentrations of different secreted factors by signaling centers activate different TF expression
Do FGFs from the ANR regulate the formation and patterning of the anterior neural tube?
Experiment: Use Cre/LoxP to knockout FGF receptors in the forebrain
Result: When cells are unable to receive FGF8 signals, the anterior forebrain doesn't develop
Conclusion: anterior forebrain development and patterning requires FGF8 signaling from the ANR
What is SHH produced by?
It's a secreted protein produced by the notochord and floorplate (both on the ventral side of the neural tube)
How do we know that SHH is a ventralizing signal in the neural tube?
Experiment: inject SHH at a more dorsal position in the neural tube
Results: Ventral cell types, like motor neurons and the floorplate, form at the site of injection. Dorsal cell types are also repressed
Conclusion: SHH is a ventralizing signal and acts as a morphogen
What leads to the differentiation of different types of neurons along the DV axis of the nervous system?
Different combinations of transcription factors activated by the level of expression of SHH in that region
To review, what is the main dorsal signal and the main ventral signal of the neural tube?
BMP is the dorsalizing signal and SHH is the ventralizing signal
Briefly describe the differentiation of cells along the apical/basal axis of the ventral neural tube
The notochord is on the basal side, the neural plate is on the apical side. The soon-to-be neural crest cells are on either side, and after the ingression into an actual tube, these cells go through an epithelial-to-mesenchymal transition, and the apical cells are part of the lumen of the neural tube.
What kinds of cells are in the apical side of the the neural tube?
Ventricular zone progenitor cells, which is a proliferative layer of cells that will give rise to all the neurons and supporting cells
How do neurons get to every part of the body?
Neurons arise in the ventricular zone and migrate outward, to the basal side, with temporal and spatial controls.
Describe the difference between symmetric and asymmetric cell divisions
Symmetric: give rise to two neurons that both stay in the ventricular zone
Asymmetric: give rise to two neurons, one stays in the ventricular zone and and one that leaves and differentiates
What signaling factor controls the stem cells of the ventricular zone?
Delta-Notch signaling and lateral inhibition. (remember, Delta promotes differentiation).
Why do the cells that leave the ventricular zone differentiate?
There is an asymmetric distribution of Delta and Notch when cells divide asymmetrically. The one that will stay has more Notch, and the one that will leave has more Delta. When cells divide symmetrically, they get an equal distribution of Delta and Notch
Describe, using an experiment, how Delta-Notch cells control cell fate decisions by regulating when neuroblasts are born.
If Delta is overexpressed, there is too much lateral inhibition and the cells never form into neurons (all become glial cells). If Delta is underexpresed, there is no lateral inhibition, and the cells differentiate too early and all become neurons.
How does lateral inhibition control when neuroblasts are born?
Each neuroblast inhibits progenitors from differentiating, so the progenitors can only differentiate after the neuroblasts have left the
Briefly describe the growth patterns of neurons and axons.
A growth cone extends out from the born neuron and extends towards the target cells. The axons grow along the path of the growth cone
Briefly describe how growth cones extend out.
Filopodia made of microfilaments (actin) scope out the path, and lamellipodia made of microtubules help the cell migrate.
What is axonal pathfinding?
A directional cell migration
How is a growth cone like a migrating cell?
It samples the environment around it to determine which direction to travel in
What is the purpose of filopodia and lamellipodia?
To maximize the amount of cell membrane available to sample the environment. They are created by changes in the cytoskeleton
How do cells know where to go?
The ECM only binds to certain receptors on the cell surface. So if the cell surface is binding to the ECM, it will shift it's entire body to move to where the surface of the filopodia and lamellipodia are bound.
What provides the link between the ECM and the cytoskeleton?
Integrins, mainly, which specifically link to laminins
How do integrins work to help the cell migrate?
Once they attach to the ECM, they can trigger a signaling pathway, like by focal adhesion kinase, and they can thus contract actin filaments or undo actin filaments, etc.
How does the ECM regulate movement of neurons
Some parts of the ECM permit movement (like parts with laminin), and some won't (like collagen)
Give an example of cell-cell contact that gives rise to guidance cues for neuron migration
Neurons migrate from the ventricular zone along the surface of glial cells, involves homophilic interactions between N-cadherins
Give an example of a long- range attractive cue
Netrin, which attracts commisural neurons. They project their axons ventrally from the dorsal side to the floorplate, then cross the floorplate, then back to the dorsal side.
What are commisural neurons useful for?
They coordinate left and right sides of the nervous system
How do we know that netrin is a long-range attractive cue secreted by the floorplate?
Do a kind of "move it" experiment.
Experiment: explant pieces of dorsal neural tissue next to a floorplate (or extracted floorplate proteins), purify the attractant from the floorplate
Results/conclusin: netrin is an attractant, netrin attracts neurons to the midline (floorplate), and netrin is a diffusible long-range cue
How do we know netrin is required for the guidance of commisural neurons?
a netrin knockout leads to abnormal growth of commisural neurons
What cues allow the commisural neuron to cross the midline at the floorplate?
Netrin, which is an attractive cue, and slit, which is a repulsive cue. Robo 1/2, a receptor for slit, is always on the growth cone. Robo 3, a receptor that overcomes slit, is only on until the growth cone goes into the midline, then it is downregulated and Robo 1/2 allows the neuron to pushed out of the midline.
what keeps the neurons that have crossed the floorplate from migrating back up to the roofplate?
Semaphorins, a long range repulsive cue. Commissural neurons start becoming sensitive to semaphorins after crossing the floorplate.
Where are semaphorins expressed?
Broadly in the neural tube, all over the place, but low near the floorplate.
Is retinotectal mapping a result of long-range or short-range cues?
short-range cues from the tectum
How do we know that guidance cues in the tectum are what cause the precise connections established between the retina and the tectum?
Experiment: alter the spacial relationship between the retina and the tectum by rotating the retina
Results: the same connections are established as before, just with different trajectories
conclusion: the cues from the tectum are what guide the connections between the tectum and the retina
What is the nature of the guidance cues in retinotectal mapping?
There is a graded concentration of a few cues, and the mapping is dependent on repulsive cues present in the posterior membrane
What is the molecular basis of the Eph/ephrin system of retinotectal mapping?
There is a concentration of receptor (Eph) in the retina and a mirror image concentration gradient of ligand (ephrin) in the tectum
Name the derivatives of neural crest cells
dorsal root ganglia (sensory neurons),
Where do neural crest cells arise?
Junction of neural plate and epidermis, where BMP signaling is at an intermediate level. When the neural tube folds, the neural crest cells undergo an EMT at the dorsal-most basal side of the neural tube.
What is Treacher Collins syndrome?
When you have underdevelopment (hypoplasia) of neural crest-derived facial bones
What is downregulated in the EMT of NCCs?
Cadherins. E-cadherins because of the EMT and N-cadherins because of the fact that they are leaving the neural tube
How are peripheral nerves derived from the neural crest arranged in a segmented pattern?
Discrete migratory streams of NCCs from the neural tube
What half of the somite do NCCs avoid when migrating, and why
The posterior half, because it produces ephrin (a repulsive signal) and the NCCs produce ephrin receptors
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