The cortex is made in an ____ manner, meaning the _____ layers are made first, while the _____ layers are made last.
The cortex is made in an inside out manner, meaning the innermost layers are made first, while the outermost layers are made last.
What is the order in which the layers are made?
Layer 1, followed by 6,5,4,3, and 2.
How is layer 1 unique?
It is cell sparse, and contains axonal fibers from distant regions of the brain, dendrites from cells of the lower layers, and what few cells it has are mostly interneurons.
To figure out the time-course of how the cortex is formed, what is injected and where is it injected?
Scientists injected a radioactive nucleotide, 3H-Thymidine into the ventricular zone.
How can you distinguish where newly divided cells have migrated to?
- When there is new synthesis of DNA, along with normal thyminide, the DNA will incorporate 3H-Thyminide, meaning that a significant portion of the newly synthsized DNA will be radioactive. As the cell continues to divide, radioactivity will decrease since 3H-Thyminide is applied once.
- Thus: with every new division, the amount of 3H-Thyminide in each cell originating from the original parent cell will be reduced by about half.
When you inject 3H-Thyminde in embryonic mice, older embryos (E15) have new neurons going to which layers compared to younger embryos (E11)?
- Older embryos (towards embryonic day 15-E15) have new neurons going to more superficial layers (like layer 2/3) than younger embryos (toward embryonic day 11-E11) where new neurons will go to deeper layers (like layer 6).
- Cortex appears to grow in inside out manner.
What is it called when a single progenitor cell can only create cells in in one layer?
Lineage model: there is a strict lineage of cells that a progenitor can be, and it's already been determined by the time the progenitor cell is created.
What is it called when a single progenitor can create any of the cortex layers?
Instructional model: a progenitor can be instructed from outside signals to become several different cell types.
Using _______, a technique similar to 3H-Thyminide, but unique in that it traces all the daughter cells of a given cell, we see that _________________________. What model does this point to?
- Using retroviral lineage tracing, a technique similar to 3H-Thyminide, but unique in that it traces all the daughter cells of a given cell, we see that a single progenitor can give rise to neurons in all of the cell layers.
- This points us towards the instructional model, but it does not prove it.
Experiment 1: What happens when you take progenitor cells in the ventricular zone (VZ) of an E29 ferret, cells that will most likely become layer 6 cells, and put them immediately into the VZ of a postnatal day 1 (P1) ferret, where cells found there naturally will turn into layer 2/3 cells?
- The transplanted cells will turn into layer 2/3 cells.
- The cells adopt the fate of the environment and turn into cells with a P1 fate.
Experiment 2: What happens when you take cells from the VZ of an E29 ferret, and culture them with other E29 VZ tissue for about 4-8 hours, and then put them into the VZ of a P1 ferret?
- These cells will create layer 6 cells.
- They will adopt the fate of the E29 cells.
Experiment 3: What happens if you take cells from the VZ of an E29 ferret, and culture them at a very low density, so it won't be surrounded by other cells (kind of like when we cultured the dissociated animal cap and those cells became neurons), then you place those in the VZ of the P1 ferret?
They will turn into layer 2/3 cells, adopting P1 fate.
Experiment 4: What happens if you take cells from the VZ of a P1 ferret, and put them into the VZ of an E29 ferret?
The cells will turn into layer 2/3 cells, keeping the fate of the P1 cells.
What are all the experiments involving the ferrets trying to determine?
- The experiments are trying to determine what differentiates Layer 6 and Layer 2/3 and
- Whether cells that turn into layer 6 cells turn into layer 2/3 cells if they're in the right environment, or is it once you're destined to be a layer 6, you'll stay layer 6.
What are the results of all the experiments involving the ferrets? Where/when do cells appear to adopt the fate of the environment?
- The cells are multipotent, and the local environment influences the determination of their development.
- Cells appear to adopt the fate of the environment where they have their last division.
What does a cell require when it adopts the fate of the environment where they have their last division?
- Signaling from cells surrounding
- It needs cell-cell contacts.
What does a cell lose when it decides to not become layer 6, and become a layer 2/3 cell?
- It loses its competence to go back to being a layer 6 cell.
- A cell can lose potential, and its fate can be restricted over time.
Which model do the experiments involving the ferrets point to?
The instructional model.
What gives rise to cells in the peripheral nervous system including melanocytes, neurons, Schwann cells, and adrenal chromaffin cells?
Neural crest cells.
Neural crest cells give rise to cells in the _____ including ____, _____, _____, and ____.
Neural crest cells give rise to cells in the peripheral nervous system including melanocytes, neurons, Schwann cells, and adrenal chromaffin cells
Similar to other neurons throughout development, the type of cells neural crest cells become depend on what?
- The signals they receive.
- The signals they receive will change depending on where the cells migrate to, whether it's the dorsal root ganglion, sympathetic ganglion, or the adrenal medulia.
Cell fates of the neural crest cells will initially be determined by what?
Cell fates of the neural crest cells will initially be determined by the basic helix-loop-helix (bHLH) domain trascription factors and to which location they go, the dorsal root ganglia or the sympathetic ganglia.
Neural crest cells going to the dorsal root ganglia will become what? Which bHLH TFs will they be exposed to?
- Neural crest cells going to the dorsal root ganglia will become sensory cells
- They will be exposed to bHLH TFs like neuregulin 1/2 and FoxS1.
Neural crest cells going to the sympathetic ganglion will become what? Which bHLH TFs will they be exposed to?
- Neural crest cells going to the sympathetic ganglion will become sympathetic neurons.
- They will be exposed to bHLH TFs like Mash1, Phox2 and Gata 2/3.
Cells that have migrated towards the sympathetic ganglion route must decide between which two things?
Between staying at the sympathetic ganglion or continuing onto the adrenal glands.
Cells migrating to the adrenal glands are exposed to what? What does this induce?
- Cells migrating to the adrenal glands will be exposed to glucocorticoids, which
- Will induce their differentiation into chromaffin cells.
What do chromaffin cells secrete?
What are cells at the sympathetic ganglia are exposed to what? What does this induce?
- Cells at the sympathetic ganglia are exposed to FGF (fibroblast growth factor) and NGF (nerve growth factor).
- This will induce the migrating cells to become adrenergic neurons.
What do adrenergic neurons make at their synaptic terminals?
They make norepinephrine at their synaptic terminals.
What is the main point of the diagram in your GSIs notes? Where could the sympathoadrenal precursor cell be located?
- It's more to paint the picture that when you don't go down to the dorsal root ganglia, you could become a sympathetic neuron or an adrenal chromaffin cell.
- The sympathoadrenal precursor cell could be located upstream (to the left) of the sympathetic precursor, or ti could just be a different name for the sympathetic precursor.
The sympathetic neurons can then go on to release norepinephrine or acetylcholine depending on what?
Depending on their projection targets.
If the sympathetic neurons projects to ____ it will still release norepinephrine.
A muscle cell.
What happens when sympathetic neurons projects to gland cells?
There will be retrograde signaling from the gland cells onto the neuron, and induce it to release Ach. It does this by releasing LIF and CNTF.
What technique is used to figure out how cells are specified to different layer fates?
Neuronal birthdating with 3H-thymidine
What technique identifies the time when the cell underwent its last cell division?
What is tritium?
3H thymidine-precursor for thymidine that's incorporated into the DNA.
In which phase is 3H-thymadine incorporated in the DNA?
In which phase does replication of the DNA occur?
Which technique can you distinguish mother from daughter cells?
How can 3H thymidine be detected? What forms?
- By autoradiography.
- Silver precipitate after being exposed to radiation.
What happens to thymidine if cell underwent its last division? What happens to thymidine as long as the cell is alive?
- If cell underwent its last division, thymidine would stay in cell until it dies.
- As long as cell is alive, thymidine content doesn't change.
What happens to 3H thymidine if only applied in a single pulse?
- It will disappear in approximately 2-4 generations.
- Can detect 1st and 2nd generation, but then signal is lost due to dilution of label through DNA replication.
When can this method (3H thymidine) to label cells be used? What happens to cells that continue to proliferate?
- Can therefore use this method to label cells that are undergoing their terminal (final) mitosis.
- Cells that continue to proliferate will not be detected.
Purpose of experiment involving injecting pregnant female mice with 3H thymidine?
Label embryos with 3H thymidine at different times in development.
The later the injection of thymidine, the more ____ the labeling.
What does the superficial labeling of later injections of thymidine suggest about the pattern of cortical neurogenesis?
This suggests there is an inside out pattern of cortical neurogenesis.
Which layers does the inside-out pattern apply to? What is the formation (order?) of the layers?
Only applies to layers 2-6. Layer 1 is formed first. So formation goes layer 1, layer 6, layer 5, layer 4, layer 2/3.
What type of developmental time does each cortical layer have?
Each cortical layer, had a relatively restricted period of developmental time, over which it is normally generated.
Image/diagram- Which layers are formed in an inside out order? What is the outside called (in parenthesis) and the inside called (in parenthesis)
- Layers 2-6 are formed in an inside-out order.
- Outside (pial surface). Inside (ventricle).
What differentiates layer 2/3 cells from layer 6 cells? (What two theories accounts for the differentiation of layer 2/3 cells from layer 6 cells?
- 1. Lineage model: there are layer specific progenitors (cells that give rise only to cells in one layer)
- 2. Instructional model: There is a common progenitor for cells of all layers. The fate of each particular cell is determined by the environment.
In which model is there layer specific progenitors (cells that give rise only to cells in one layer)?
In which model is there a common progenitor for cells of all layers? In this model, the fate of each particular cell is determined by _____.
- Instructional model.
- Determined by the environment.
Which technique is used to look at layer specific lineage?
Lineage tracing with virus
What is used for lineage tracing? This cannot replicate and infect other cells, but it can still do what?
- Use replication incompetent retrovirus.
- Cannot replicate and infect other cells, but can still be incorporated into a replicating cell's DNA.
In what amount will the virus DNA be inherited by all the daughter cells?
- It will be inherited by all the daughter cells in the same amount.
- It doesn't become diluted.
Where does the retrovirus insert its DNA? This is also known as a ____ for cell and all its descendants.
- Retrovirus inserts its DNA into the chromosome of the cell it infects.
- AKA a permanent marker for cell and all its descendants.
Image/diagram- In the example, what does lineage tracing in the retina reveal?
The presence of multipotent progenitor cells.
Diagram/image- In the lineage vs instructional experiment testing that used retroviral lineage tracing to label single neurons and follow their progenies, where was retrovirus injected? What happened, what was the result and which model does this result support?
- Retrovirus was injected into the ventricular space.
- Individual progenitor cells can give rise to cells in every layer of cortex.
- Single cells can rise to neurons in different layers.
- Thus, this result disqualifies the lineage model and supports the instructional model, although it doesn't prove it.
(Slide- What differentiates layer 2/3 cells from layer 6 cells?) During the experiment of heterochronic transplants- in a ferret, which cells are born at E29 and which cells are born at P1?
In ferret, layer 6 cells (deep layer cells) are born at E29 and layer 2/3 cells are born at P1.
In the heterochronic transplants experiment, mitotic cells are labeled with 3H-Thyminide. What is harvested and where is it transplanted? What is the result?
- Harvest and immediately transplant E29 cells (still in S phase) into P1 animal.
- The cells adopt the fate of the P1 cells (layer 2/3).