DYNAMICS OF TARGET RECOGNITION BY INTERSTITIAL AXON BRANCHING ALONG DEVELOPING CORTICAL AXONE

Corticospinal axons innervate their midbrain, hindbrain, and spinal ta rgets by extending collateral branches interstitially along their leng th. To establish that the axon shaft rather than the axonal growth con e is responsible for target recognition in this system, and to charact erize the dynamics of interstitial branch formation, we have studied t his process in an in vivo-like setting using slice cultures from neona tal mice containing the entire pathway of corticospinal axons. Cortico spinal axons labeled with the dye -dioctodecyl-3,3,3',3'-tetramethylin docarbocyanine perchlorate (or Dil) were imaged using time-lapse video microscopy of their pathway overlying the basilar pens, their major h indbrain target, The axon shaft millimeters behind the growth cone exh ibits several dynamic behaviors, including the de novo formation of va ricosities and filopodia-like extensions, and a behavior that we term ''pulsation,'' which is characterized by a variable thickening and thi nning of short segments of the axon, An individual axon can have multi ple sites of branching activity, with many of the branches being trans ient. These dynamic behaviors occur along the portion of the axon shaf t overlying the basilar pens, but not just caudal to it. Once the coll aterals extend into the pontine neuropil, they branch further in the n europil, while the parent axon becomes quiescent, Thus, the branching activity is spatially restricted to specific portions of the axon, as well as temporally restricted to a relatively brief time window. These findings provide definitive evidence that collateral branches form de novo along corticospinal axons and establish that the process of targ et recognition in this system is a property of the axon shaft rather t han the leading growth cone.