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 co ne motility during axon navigation in the intact brain. This approach prompted us to reanalyze the hypothesis, originally derived from obser vations of fixed tissue, that growth cone form is position-specific. T he behaviors of Dil-labeled retinal axon growth cones were tracked fro m retina through the optic tract in mouse brain at embryonic day (E) 1 5-17, and these behaviors were matched with different growth cone form s. Patterns of behavior were then analyzed in the different locales fr om the retina through the optic tract. Throughout the pathway, episode s of advance were punctuated by pauses in extension. Irrespective of l ocale, elongated streamlined growth cones mediated advance and complex forms developed during pauses. The rate of advance and the duration o f 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, i n 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 p articular loci, growth cone form appears to be position-specific in st atic preparations, due to the fraction of time spent in a given state in different locales.