Hi mstanley,
Parts that come out darker can be parts of neurons that caught more dye or blood vessels wrapping around them.
A circular one like this could be a myelinated axon. Some nerve cell axons are wrapped in a myelin sheath, made up of various lipids and proteins, as insulation to protect the electrical signal. That might be what you are looking at here.
Cheers,
Aleks
Thanks. I knew about myelin sheaths but hadn’t seen one like this. I’m learning new stuff too. 
…love it.
I think rprentki has said in game chat that tubular stuff like this one in slices are blood vessels when they have that outer darker circles.