If you remember back from when I was at the Society for Neuroscience, I saw a talk by Bruce Appel where he showed videos of oligodendrocytes migrating and myelinating in the zebrafish.
Oligodendrocytes are the myelin forming cell in the central nervous system of vertebrates -- the cells that coat axons in a sheet of fat called myelin that helps the axons conduct action potentials more quickly. At a point in oligodendrocyte development the oligodendrocyte precursor cells (OPCs) have to migrate out from the ventral part of the spine to cover the axons in the spinal cord. However, this process had never been watched inside an animal, so we didn't really understand how they did it.
As a model organism, zebrafish are a handy animal in this case because they are vertebrates -- meaning that they actually have oligodendrocytes, invertebrates don't -- but their embryos are clear so you can watch them move. Appel and his lab added a fluorescent tracer and imaged the oligodendrocytes as they moved to myelinate axons in the spine of the embyros.
At the time the video wasn't available online, but it has made it out onto YouTube. The roundish looking structures are the oligodendrocyte precursor cells (OPCs). The move out to grab axons (the axons can't be seen) and then send processes up and down the axon. You can see that occassionally two OPCs meet on an axon, and they do this dance to spread themselves out.
Here is another one of his videos. In this one they used the same technique, but they used a laser to ablate some of the OPCs. Watch the OPCs go back and repopulate the area of ablation.
All of this stuff is published in a Nature paper here.
Thanks Neurophilosopher for keeping an eye out for this paper and posting the videos.
I uploaded those movies to YouTube!
Sorry, I've just noticed that you said that at the end of the post!