Invertebrates axon guidance

This review draws together information about axon guidance at the cellular level in different invertebrates, most of which has come from studies of embryonic systems, to consider the following issues What strategies do growing axons... 

In contrast to the paucity of evidence for diffusible, chemotropic factors in invertebrates, a large number of observations support a role for substrate-associated factors in axon guidance. Such factors appear to be located on a variety of cellular and extracellular structures. Precise identification of these structures is indispensable for characterising the molecular nature of the axon guidance factors and elucidating their function. The various classes of cellular structures implicated in axon guidance in invertebrates are considered in turn, below. 

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. 

Axon guidance factors in invertebrate development, neuroglian... 

Many of the proteins which have been implicated in axon guidance in both vertebrates and invertebrates (including all of the fasciclin proteins discussed above) possess carbohydrate moieties. What is the evidence that this carbohydrate component is important in axon guidance ... 

The cellular studies outlined in Section 3.5 point to a role for factors in the extracellular matrix, and particularly the basal lamina, in axon guidance. A number of studies in invertebrate species have begun to characterize molecules that may subserve axon guidance on such substrates. 

Work on identifying the molecules responsible for axon guidance in invertebrate nervous systems has been underway for only a few years, yet already a number of candidate recognition molecules have been characterized. We can confidently expect this list to grow rapidly as an arsenal of powerful genetic and molecular techniques is brought to bear on the problem. 

In summary, the possibility of chemotropic guidance of axons in invertebrates is intriguing but little direct evidence exists to support it. However, it would be foolish at this stage to regard chemo-tropism as an exclusively vertebrate phenomenon, particularly as the netrins, strong candidates for a chemotropic factor in the vertebrates (Serafini et al., 1994 Kennedy et al., 1994), show a high degree of sequence similarity to the nematode UNC-6 protein (see Section 5.2.1). 

If, indeed, axon repulsion is an important mechanism for growth cone guidance in vertebrates, it is highly likely that it also plays a role in invertebrates. To date, however, invertebrate systems have provided little direct evidence for axon repulsion in vivo. 

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