Axon guidance

Question 1

What is the resonance theory put forward by Weiss

Answer 1

Stochastic (random) and diffuse neural outgrowth occurs to all targets followed by elimination of non-functional connections

Question 2

What was the chemo-affinity hypothesis put forward by Sperry

Answer 2

Directed and specific outgrowth occurs through axons following individual identification tags carried by the cells and fibres of the embryo

Question 3

What experiment did Sperry produce in order to prove his hypothesis

Answer 3

Cut the optic nerve and removed the temporal retina in frogs - If Weiss was correct the nasal axons would grow back and innervate both anterior and posterior areas of the tectum - followed by later elimination of the anterior axons
But: Sperry was right - the nasal axons grew to the posterior region of the tectum - passed the anterior and ignored the territory normally innervated by the temporal retinal axons

Question 4

What is the relevance of this experiment being done during regeneration as a pose to in the embryo

Answer 4

The axons may have been able to follow signals given by the regenerating cells as a pose to if those cells weren’t ever present

Question 5

How was the early stage chick embryo used to suggest guidance cues

Answer 5

Cut and replace/reverse a segment of neural tube before motor axons have grown out - Despite displacement of cell bodies, motor axons still find their way to their normal muscle targets

Question 6

What is the labelled pathway hypothesis

Answer 6

Axons can selectively fasciculate with other axons
Axons surfaces carry labels or cues - different axon growth cones sets of receptors to the cues
Early pioneering axons form an axon scaffold on which later axons can extend

Question 7

What is the pathway of the pioneering axon of the grasshopper limb

Answer 7

The pioneer Ti1 growth cone grows from the distal part of the limb toward the body
Guidepost cells act as intermediate targets for the axons of neurons in the developing leg - at the second guidepost cell it makes a 90 degree turn to the right - at the third (Cx1) it makes another 90 degree turn to the left toward the body

Question 8

What happens to the movement of the pioneer axon if the Cx1 guidepost cell is ablated

Answer 8

Ti1 growth cone stalls at the other side of the limb boundary - begin to move on in apparently random directions

Question 9

How does the patterning of the early embryo predict axon tracts

Answer 9

Axons follow boundaries of domains of patterning gene expression - shows axon guidance cues aren’t located just in other axons but in many cell types in the early embryo