Neurites and growth cones in the chick embryo. Enhanced tissue preservation and visualization of HRP-labeled subpopulations in serial 25-microns plastic sections cut on a rotary microtome.

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Abstract
Study of axonal guidance in developing vertebrates has been hindered by an inability to readily visualize individual growth cones, determine the neuronal population from which they originate, trace their trajectories, and discern their interactions with their embryonic environment. We report a method that combines plastic embedding and serial sectioning with horseradish peroxidase labeling of subpopulations of neurons in the chick embryo. This method labels individual neurites from the soma to the tip of the growth cones, allowing their trajectory to be inferred and their identity to be determined by the position of the somata. As sections are up to 25 micron thick, entire growth cones can often be visualized without laborious reconstruction. Tissue preservation is much better than with similar material embedded in paraffin. Sections are cut relatively quickly using a steel knife on a standard rotary microtome and are suitable for subsequent electron microscopy.