Open-book Preparations from Chick Embryos and DiI Labeling of Commissural Axons

Abstract

Successful neural circuit formation relies on the accurate navigation of axons towards their targets during development. Axons are guided by a combination of short-range and long-range, attractive and repulsive cues. The commissural axons of the developing spinal cord have provided an informative in vivo model for the identification of multiple axon guidance molecules and mechanisms. These axons extend ventrally from the dorsal spinal cord and cross the midline at the floor plate, before making a sharp rostral turn towards the head. This simple trajectory has facilitated the identification of many axon guidance molecules, because perturbation of the stereotypical guidance decisions as a result of genetic manipulations can be easily identified. The open-book assay is a method to assess the trajectory of spinal commissural axons. The spinal cord is dissected out, opened at the roof plate and pinned flat. Punctate injections of the lipophilic fluorescent dye, DiI, are used to trace commissural axon trajectories prior to microscopy and analysis.