Axon guidance. Neuronal growth cones respond to both contact mediated and chemotropic guidance cues. Cues can be either attractive or repulsive. Cues can act over a short or long range, and may differentially affect particular types of neurons. Only the growth cone is motile.
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Commissural neurons extend ventrally
and then toward floor plate, if within 250mm
from the floor plate
unc5 = repulsion
Eph receptors and ephrins are a major class of
in vitro theycan trigger growth-cone collapse and inhibit
In vivo they can channel axons along specific routes by
preventing growth cones from migrating across their
Different situations in which Eph receptors and ephrins restrict cell
or growth-cone migration by repulsion at boundaries.
a Bidirectional activation of Eph receptor and ephrin-B ligand
restricts mixing between adjacent cell populations by mediating mutual repulsion.
b Boundaries of ephrin expression repel migrating cells expressing
Eph receptor, so restricting them to adjacent territories.
c Boundaries of ephrin expression restrict growth cones of neurons
expressing Eph receptor.
d Boundaries of Eph receptor restrict growth cones of neurons
expressing ephrin-B protein.
Directional activation of Eph receptors and ephrins indicated by
blue and red arrows.
a In retinotectal topographic map, axons project from temporal retina to anterior tectum, and from nasal retina to posterior tectum.
b A graded sensitivity of retinal axons to ephrin-A2 and ephrin-A5, repellents expressed in overlapping gradients in the tectum. The graded sensitivity of axons is due to graded expression of EphA3 or EphA5,
and to overlapping expression of uniform EphA4 (blue line) and
graded ephrin-A5 (red line) expression, which underlies graded
sensitivity of EphA4 along the temporal/nasal axis (black dotted line).
c Model for topographic mapping: arrest of growth-cone movement occurs when attractive forces (green arrows) are counterbalanced by repulsion mediated by Eph receptor activation (red arrows). This level of repulsion occurs in temporal axons (high Eph
receptor) in anterior tectum (low ephrin) and in nasal axons (low Eph receptor) in posterior tectum (high ephrin).
d The results of ephrin-A5 and ephrin-A2 KOs argue against this model. Ephrin-A5-/-mutants, some temporal axons overshoot owing to decreased levels of ephrin repellent, and some nasal axons undershoot, but many axons project to their normal location in the tectum, and entire tectum is occupied by retinal axons, even in the double ephrin mutant.