A nice study by Hildebrand et al. was published earlier this week, looking at the connectome of zebrafish larvae. As a reminder is what zebrafish larvae look like under the scanning microscope (this is one of my favorite images ever):
In this study, they did brute force serial sectioning of postnatal day 5 zebrafish larvae which they fed into a silicon wafer machine:
They then were able to use the serial EM images to reconstruct myelinated axons and create some beautiful images:
They found that the paired myelinated axons across hemispheres were more symmetrical than expected.
This means that their positions are likely pre-specified by the zebrafish genome/epigenome, rather than shifting due to developmental/electrical activity, as is thought to occur in the development of most mammalian axons.
While that is an interesting finding, clearly the main advance of this article is a technical one: being able to generate serial EM data sets like this on a faster and more routine basis may soon help to revolutionize the study of neuroscience.