Once upon a time, way back when I entered graduate school, the first big project I was involved in was essentially a morphological mapping of the circuitry of the larval zebrafish. We did lots of backfills of neurons with horseradish peroxidase, and later the fluorescent dye DiI, and then with injected lineage tracers like rhodamine dextran. I guess technology has greatly advanced, because we never got anything as pretty as this set of fluorescently labeled neurons in the brain and spinal cord of a larval zebrafish.
This image was made using brainbow fluorescent microscopy. Transgenic fish carry an assortment of fluorescent protein genes that are randomly flipped on in the cells to produce these multicolored views of a subset of the neurons. It’s like the good old Golgi silver stain, only in technicolor.