Braking News

Growing axons are guided by cues in the environment: local or released from a distance, positive and negative (Goodman 1996). The motile growth cone at the tip of the axon senses and interprets these cues, changing its behavior to alter the direction or speed of growth. Many cues have been identified and their in vivo importance has been verified, but how their binding is transduced into changes in growth cone behavior remains poorly understood. Given the plethora of cues, it would simplify things if common elements of transduction pathways emerged: for example, if stopping of a growth cone at a choice point were always (or, at least, often) caused by elevation of messenger X within the growth cone, whatever the external cue. Recent work points to such key transduction molecules. Moreover, a new paper in Nature literally and figuratively illuminates the inner workings of the growth cone by fluorescently imaging Ca2+ in neurons growing within the developing spinal cord, providing evidence for its importance in vivo in regulating axonal elongation (Gomez and Spitzer 1999). Considering that the response of growth cones to cues is, for technical reasons, typically studied in the reductionist world of the culture dish, this is a noteworthy accomplishment.