Axon guidance cues

Therefore, it appears that while guidepost neurons may be used as cues to guide the growth of later appearing axons, such neurons are not always necessary. Other axon guidance cues must be available in these situations. 

No single cellular axon guidance cue appears to be unique to a particular invertebrate group or class of neuron and hence one cannot a priori exclude any specific cellular structure as the site of axon guidance molecules. Therefore, the cellular identity of guidance factors needs to be established anew every time one begins to work with a new system. 

Directed axon growth involves active recognition of guidance cues... 

A variety of observations of axon growth both during normal development and in response to surgical transplantation of neuronal targets (reviewed in Goodman and Shatz, 1993) also provide indirect support for chemotropic guidance cues in vertebrates. 

The conclusion from this study was that a variety of peripheral neurons normally provide guidance information for outgrowing nerves, but that this role is only a necessary one in situations where alternative cues are out of filopodial reach. The implication is that there is no intrinsic difference between peripheral neurons that can be experimentally shown to be necessary for axon guidance and those that are not. 

Halpem et al. (1991) and Sink and Whitington (1991a) have also reported a close association between the axons of embryonic motoneurons and muscles in the Drosophila embryo. As the RP motor axons advance to their target muscles, they traverse three sheets of muscle fibres. Each motor axon branches extensively over these muscles, including non-target muscles. It thus appears as if the muscles in this sheet contain a guidance cue or cues, which can be recognised by all of the RP... 

An electron-microscopic study showed that growth cones and filopodia of the Til neurons in the embryonic grasshopper limb bud contact the basal lamina, rather than the ectoderm of the limb bud (Anderson and Tucker, 1988). These authors concluded that guidance cues for these axons reside in the basal lamina, rather than the ectoderm. They also found that there is a proximal-distal gradient in basal lamina structure, with thinner basal lamina at the tip and thicker at the base (Anderson and Tucker, 1989). This correlates with the proximal-distal orientation of the pioneering Trl axons and it was suggested that this correlation may be a causative one for guidance of the Trl axons. 

UNC-6/netrin and SLT-l/slit guidance cues orient axon outgrowth mediated by MlG-10/RlAM/lamellipodin. Curr. Biol. 16, 845-853. 

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