Cells and cell-interactions that guide motor axons in the developing chick embryo.

A considerable challenge confronts any developing neuron. Before it can establish a functional and specific connection, it must extend an axon over tens and sometimes hundreds of microns through a complex and mutable environment to reach one out of many possible destinations. The field of axonal guidance concerns the control of this navigation process. To satisfactorily identify the cell interactions and molecular mechanisms that mediate axonal guidance, it is essential to first identify the pertinent cell populations. Embryonic surgeries have provided solid information on which tissues are critical and which are irrelevant to the navigation of motor axons within the chick embryo. The gross anatomical nerve pattern is established as axons respond to both positive (path) and negative (barrier) tissue environments. Analysis of the interactions of motoneurons with these tissues reveals that several cellular interactions--chemotaxis, substratum preference, and perhaps contact paralysis--are important to the common patterns of motor axon advance. Axons simultaneously interact with population-specific cues that have begun to be identified on the tissue level.