In their new report, the authors direct axonal growth by focusing a beam of light just ahead of the leading edge of a lamellipodium. Forces of light lower than those used for optical tweezers accelerated growth at the leading edge. Shining the laser on one side of the growth cone caused filopodia to accumulate on that side, thereby initiating as much as a 90° turn of the growth cones.
Although the mechanism behind the effect of light is yet unproven, Ehrlicher describes the group's favorite explanation as “biased monomer diffusion.” According to the theory, the electrical field gradient created by the beam, which polarizes the nearby pool of actin monomers, draws the monomers toward the beam's focus, thus favoring polymerization. “In effect, we trick the actin polymerization process,” says Ehrlicher.
Whatever the mechanism, Ehrlicher is excited by the possibilities that light control over cell motility may offer. One immediate use will be to construct more precise and longer-lasting in vitro neural networks, in which a small number of neurons are connected in a specific fashion to study neuronal interactions. A long-term goal is to use lasers in vivo to guide the regrowth of severed peripheral nerves in accident victims. ▪