Growth cone form is behavior-specific and, consequently, position-specific along the retinal axon pathway

Video time-lapse microscopy has made it possible to document growth cone motility during axon navigation in the intact brain. This approach prompted us to reanalyze the hypothesis, originally derived from observations of fixed tissue, that growth cone form is position-specific. The behaviors of Dil-labeled retinal axon growth cones were tracked from retina through the optic tract in mouse brain at embryonic day (E) 15-17, and these behaviors were matched with different growth cone forms. Patterns of behavior were then analyzed in the different locales from the retina through the optic tract. Throughout the pathway, episodes of advance were punctuated by pauses in extension. Irrespective of locale, elongated streamlined growth cones mediated advance and complex forms developed during pauses. The rate of advance and the duration of pauses were surprisingly similar in different parts of the pathway. In contrast, the duration of periods of advance was more brief in the chiasm compared to those in the optic nerve and tract. Consequently, in the chiasm, growth cones spent relatively more time pausing and less time advancing than in the optic nerve or tract. Thus, because growth cone form is behavior-specific and certain behaviors predominate in particular loci, growth cone form appears to be position-specific in static preparations, due to the fraction of time spent in a given state in different locales.