Biology Reference
In-Depth Information
back or by extending branches, and allowing their original ends to die back.
63
Even the
branching is often initially inaccurate and is sharpened up by the destruction of branches
that connect with the wrong places.
64
This iterative approach to correct innervation is prob-
ably a universal feature of complex neural systems, although others have not been studied in
enough detail for this to be clear yet.
The developing visual system, then, depends on a long sequence of guidance cues and on
the ability of migrating growth cones to integrate the effects of several cues simultaneously.
Some known cues have been described here; there are more cues known already, and prob-
ably many more to find. Some cues are 'absolute' (for example, repulsion by the chondroitin
sulphate at the edge of the developing retina), while others are 'relative' (for example, the
repulsive gradients of the tectum). Interference with any of the critical cues inhibits normal
navigation even when the ultimate destination is present, but intermediate cues perform
their tasks normally even in the absence of the final destination (many experiments on guid-
ance within the retina were performed on cultured eye tissue isolated from the brain). The
intermediate cues therefore behave as bona fide waypoints.
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