Many of us only think about fruit flies when the air above a neglected bunch of bananas becomes suddenly thick with activity, but a team of researchers at Johns Hopkins and Cambridge University have been thinking about them an awful lot — and have also been demonstrating exactly how they, the flies, think about everything. In an unprecedented feat of science, the team has successfully created a synapse-by-synapse map of the brain of a larval fruit fly (Drosophila).
Prior to this achievement, only some portions of insect brains had been reconstructed, researcher Marta Zlatic of the University of Cambridge’s Department of Zoology and MRC Laboratory of Molecular Biology told Hyperallergic.
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“Now, for the first time we have the comprehensive view of the circuit motifs in an entire larval insect brain as well as interactions between the two brain hemispheres and between the brain and nerve cord,” Zlatic said. The brain of the fruit fly is smaller than a poppy seed, and the new map shows all 3,016 neurons and 548,000 synapses (the gaps where communication between neurons occurs).
Due to limits of technology, using electron microscopy to image entire brains and reconstruct their circuits — known as connectomes — has been an uphill battle. Complete synaptic-resolution connectomes have until now only been mapped for much more basic organisms with several hundred brain neurons. The complex brain organization of the young fruit fly has made it an order of magnitude more complicated for the team, led by Michael Winding of Cambridge University, to recreate.
“The larva is a freely-behaving child that has to crawl around and dig to find food, escape from predators, cooperate with other larvae,” Zlatic continued. “They have many modes of locomotion, crawling, digging, rolling (equivalent of our walking, swimming, running). They are capable of complex forms of learning, and have short- and long-term memory, they have to decide what to do next based on context and prior memories.”
In order to map such a complex mind, the fruit fly brain was first sectioned into thousands of thin sections, and each one was imaged with transmission electron microscopy. The resulting images were assembled using specialized software developed by the study’s co-author Albert Cardona. This created a 3D volume that one can navigate “like a 3D google map, and manually trace neurons and annotate synapses,” according to Zlatic. The entire process took 12 years to complete.
The leap from brain-mapping larval fruit flies to brain-mapping a human is still a massive one, but Zlatic feels that the work undertaken here is still one giant step for the understanding of neural networks, writ large.
“Human brains are a lot bigger and have a lot more neurons, so it would take a LOT longer,” Zlatic said. “But we think that the basic principles by which brains work and process information and select actions will be common across the animal kingdom. So by understanding smaller brains we can gain a lot of insights about larger brains.”