Module 1 Flashcards
what are the five phases of neurodevelopment? (in order)
Induction of the neural plate, neural proliferation, migration and aggregation, axon growth and synapse formation, neuron death and synapse rearrangement
What are the three requirements for a functioning organism
- Cells must differentiate
- Cells must migrate to appropriate areas and align properly
- Cells must establish the proper relationships between them
Define totipotent cells
Can differentiate into ANY cell type
Define pluripotent cells
Can differentiate into most cell types
Define multipotent cells
Can differentiate into cell types within a certain class (eg, neurons)
Define unipotent cells
Can only develop into one type of cell
When does the neural plate become recognizable
three weeks after conception
What is the neural plate made of
Ectodermal tissue
Where is the neural plate located?
The dorsal surface of the embryo
Three layers of embryonic cells, in order from outermost to innermost
Ecto, meso, endoderm
What induces the development of the neural plate
What experiment proved this?
Chemical signals from the mesodermic layer - referred to as the organizer
1. taking mesodermic cells from an embryo and implanting them in a second results in the development of a second neural plate
What do we often call neural plate cells?
Stem cells
what are the two unique properties of stem cells
- near unlimited capacity for replication if maintained in an appropriate cell culture
- Are either tote, multi or pluripotent
What happens to cells as the neural plate develops into the neural tube?
they become more and more specialized
What was the original view of the developmental relationship of glia and neurons, and what is the updated view?
Old - Glia and neurons were thought to develop independently and never into one another
Updated - Many neurons can develop from glial cells
Why do stem cell cultures not last for ever (why are they in practice not capable of dividing infinitely, despite their being in theory capable?)
errors accumulate overtime, disrupting the division process
what are the three stages of neural tube development? what is the specific process?
neural plate - neural groove - neural tube
- plate folds to form the groove
- lips of the formed groove fuse to form the tube
what does the inside of the neural tube develop into in adult organisms?
the cerebral ventricles and the spinal canal
what do the three swellings on the anterior end of the neural tube develop into?
The fore, mid and hindbrain.
when does neural proliferation commence?
once the lips of the neural groove fuse
does neural proliferation occur equally throughout the neural tube? If not, where does the majority occur?
No, most cell division occurs in the ventricular zone, the region adjacent to the ventricle
What controls proliferation? Which structures emit these?
Chemical signals, emitted by the floor and roof plates
Where is the floor plate of the neural tube
It runs alone the midline of the ventral surface
Where is the roof plate of the neural tube
It runs along the midline of the dorsal surface of the tube
When does the process of migration occur?
Once neural proliferation has created cells
Are the cells migrating during migration fully mature?
No, they are still immature, lacking axons, dendrites and other structures characteristic of fully formed cells
What are the major factors governing migration in the neural tube? For what reason?
time and location; neuron subtypes arise in specific places in a very predictable schedule and migrate together to their target location
Name and describe the two types of cell migration in the neural tube.
Are these types of migration mutually exclusive?
- Radial Migration - neurons move from the ventricular zone in a straight line towards the outside of the tube
- Tangential Migration - neurons travel parallel to a tubes walls (either through the tube or around its centre)
No, many cells engage in both.
What are the two mechanisms of cell migration in the neural tube? Explain
- Somal Translocation - similar to Schwann cells, neurons grow extensions which probe the immediate environment from chemical signals. The cell then moves along the extension (like a train), with the passed extensions being retracted.
- Glia-mediated migration - once proliferation is under way (neural walls thicken), a network of glial cells called radial glial cells paper in the tube. Many cells engaging in radial migration move along this network (more like a train)
Is the promotion of radial migration the only purpose of radial glial cells?
No, although it once was believed to be. They also develop into neurons, some being pluripotent, and others being unipotent
What is the must studied area of the brain (in terms of migration in the neural tube?)
What is the pattern of this migration?
The cortex, which develops in an inside out pattern (that is, the innermost structures are formed first, then outward).
What are the justifications for the statement ‘cortical migration patterns are more complex than we thought’? (2)
- many cells engage in very long tangential migrations
2. patterns of proliferation and migration are different for different cortical areas
What is the neural crest? (where is it located and how is it formed)
Located just dorsally of the neural tube, and is formed from cells that break off the tube during formation
what is the purpose of the neural crest? what does this mean for migration?
Its cells develop into the neurons and glia of the PNS, and other cell types. As a result, migration for these cells is extremely long.
How do neural cells know where to migrate?
Through attraction towards specific chemical cells, and repulsion from others
What is neural aggregation?
Once migration has occurred, cells must align themselves properly with other neurons in the same region to form structures.
What molecules are thought to mediate both migration and aggregation? Where are these located?
Cell-adhesion molecules (CAMs), located on the surface of neurons and other cells
what are the two unique function of CAMs?
- tf?
They can recognize and them adhere to them.
What is the most common form of adjacent cell connections during development?
What is the new evidence suggesting about these molecules?
Gap junctions (narrow connexin bridges that allow cells to exchange cytoplasm) They are thought to participate in mig/agg, as well as other processes of nd.
when do axons and dendrites begin to form on neurons?
Once they have completed migration and aggregation
What are growth cones?
Amoebalike structures that extend/retract fingerlike cytoplasmic extensions called filopodia, to search for the ‘correct route’
Describe the Frog study which demonstrated how accurate growth cones are in forming synapses
Sperry severed the their optic nerve and rotated the eye 180. Once the Retinal ganglion cells (o nerve) regenerated, it was found that each axon had grown back to the same point on the optic tectum (superior colliculus in humans) that It was normally connected to. This was discovered through the behavioural evidence (the frogs visual fields were inverted 180)
What did his famous Frog study cause Sperry to hypothesize? explain
The chemoaffinity hypothesis of axonal development, which states that each postsynaptic surface releases a specific chemical label that specific axons are attracted to during development and regeneration. We have discovered many of these chemicals
What are the newer additions to the chemoaffinity hypothesis, and why were they introduced?
- It fails to account for the discovery that some growing axons follow identical paths in all members of a species, which led to the belief that growth cones are influenced by many chemical and physical markers along their journey. (can be attracted to or repelled from, like in migration)
- some signals come from adjacent growing axons as well.
What are pioneer growth cones.
The first growth cones which travel along a particular route, which must listen to chemical and physical markers to find their path.
How do the subsequent (after pioneer) growth cones find their appropriate destinations? what is this process called?
They follow the path laid out by the pioneer growth cones. This process is termed fasciculation.
What is the topographic gradient hypothesis?
Explains the accurate axonal growth involving topographic mapping in the developing brain.
states that axons growing from one surface to another (topographically) are guided to targets that are arranged on the terminal surface identically to how the axons cell bodies are arranged on the og surface
What guides axons according to the topographic gradient hypothesis? Are these the only mechanisms? If not, what are examples of the others?
Anterior-posterior and medial-lateral gradients.
No, others have been discovered, such as spontaneous neural activity and Neuron-astrocyte interactions.
What is the history of the topographic gradient hypothesis?
- Much axonal nervous system development occurs in topographic patterns. (ie, retina is topographic to the optic tectum in frogs)
- At first, we thought that the topographic integrity was preserved by 1;1 chemoaffinity, but the mechanism must be more complex, bc synaptic connections in retina and optic nerve are often established before the structures reach full size. as they grow at different rates, synapses reorganize to preserve topographic integrity.
What are the reasons we had to reject the chemiaffinity hypothesis? (2)
- different growth rates of retina/optic nerve, while preserving topographic integrity.
- Regeneration studies of retinal-tectum projections
the CH would predict that axons would grow back directly onto their old points of connection, but they instead grew back to fill the available space in an ordered fashion.
What is synaptogenesis
the formation of new synapses, according to the coordinated activity of two neurons
What is ‘the most exciting new discovery about synaptogenesis’? what are the 2 pieces of evidence for this discovery?
It depends of glial cells (particularly astrocytes)
- retinal ganglion cells are 7x more likely to synapse if astrocytes are present, and many synapses were lost when they are removed.
What is the old assumption, and new addition, to the explanation of why astrocytes are so important to synaptogenesis?
- (old) - thought to be due to their nutritional role (cholesterol)
- (new) - they process transfer and store information supplied by neurons.
What is recent research about synaptogenesis focussed on? why is this difficult?
- Elucidating the chemical signals that must be exchanged between pre and post synaptic neurons for a synapse to form.
- The promiscuity of developing neurons during synaptogensis - almost any type of neuron will synapse with any other, although these are lost over time if not used.