The zebrafish brain's complex network of blood vessels develops not only through vessel growth, but also as a result of pruning, which is driven by changes in blood flow.
Jiu-lin Du at the Chinese Academy of Sciences in Shanghai and his team followed the development of transgenic zebrafish larvae in which cells of the blood vessels, blood and brain carry fluorescent tags. Over the course of six days after fertilization, the researchers imaged the creatures' developing midbrain. They found that, as the network of blood vessels expands, pruning occurs — mainly at loop-shaped segments — when cells lining the vessels migrate towards adjacent unpruned segments, resulting in decreased vessel-network complexity. Pruned segments have a lower and more variable blood flow than unpruned ones, and experimental blockade of blood flow triggers the pruning process.