After a decade of collaboration involving researchers from over 127 institutions, scientists have unveiled the first-ever complete map of the adult fruit fly brain. This achievement marks a monumental step in neuroscience by providing unprecedented insight into the brain’s structural connectivity and function.
Above: Map of the 50 largest neurons in the adult fruit fly brain. Image courtesy of Science and Dorkenwald et al. (2024).
Connectomes: The Brain’s Road Atlas
The neuron is the fundamental unit of the nervous system, which includes the brain. Neurons are connected at junctions called synapses, which together form circuits through which neurons communicate via electrical and chemical signals. How the brain functions ultimately depends on which neurons are connected to which and the strength of their synaptic connections. Therefore, a major step in fully understanding the nervous system is creating a layout of all the brain’s circuits down to every neuron and synapse: a connectome.
Scientists have previously mapped connectomes in smaller organisms, such as C. elegans, which has just 302 neurons. However, until now, scientists had not yet accomplished this in more complex organisms, such as flies, mice, or humans. Fruit flies (Drosophila melanogaster) share 60% of their DNA with humans, and three out of four human genetic diseases have parallels in fruit flies. Thus, understanding the fruit fly brain is a stepping stone to understanding the human brain.
Constructing the Model
The fruit fly brain, though no larger than a poppy seed, houses 139,255 neurons and 54.5 million synaptic connections. A group of researchers from 127 institutions formed the FlyWire consortium to map the adult fruit fly brain for the first time. To construct the model, researchers began by slicing an adult female fruit fly brain into 7,000 thin sections with a diamond knife and capturing 21 million high-resolution images of these slices using transmission electron microscopy. Researchers digitally stacked and aligned these 2D images to create a continuous representation of the brain’s neural connections, which served as the foundation for a 3D model. They then used artificial intelligence tools to segment the neurons and identify synapses by pinpointing presynaptic and postsynaptic sites across the slices. Finally, they traced neurons through the layers to reconstruct their full structures, integrating them into a 3D connectome.
The consortium describes the final model as a “Google Maps” for the brain, complete with labeled structures and pathways. Much like Google Maps guides travelers to their destinations by labeling landmarks and destinations, this brain atlas provides researchers with a clear guide for navigating and understanding neural circuits.
Above: A) Visualization of all reconstructed neurons in the central brain and optic lobes. B) Overview of the available FlyWire resources. C) Various avenues for researchers to explore the data provided by FlyWire. D) The fruit fly brain can be divided into distinct regions called neutrophils. E) Each presynaptic button can connect to multiple postsynaptic partners. F) Electron microscopy images of various neuronal and nonneuronal cell types. Image courtesy of Dorkenwald et al. (2024).
Applying the Model and Future Directions
Though the fruit fly brain model was released just last month, scientists have already begun using it to construct computational models of how information flows through the brain. As computational neuroscientist Sebastian Seung explained, “You start with the connections between neurons, and you use that to help you build a simulation of a network. It’s a totally obvious approach, but you couldn’t do it if you didn’t have the connectome.” For example, one study transformed the connectome into a spiking network model, allowing researchers to simulate how perturbations to neural circuits might affect behavior. This application highlights the potential of connectomes to answer fundamental questions, such as whether brain connectivity changes over time, varies between individuals, or predicts behavior. The detailed map could also help pinpoint which neural circuits underlie specific behaviors, providing a foundation for tailoring treatments for brain diseases. As the first complete connectome of its kind, this model represents a monumental step forward in neuroscience, offering an unprecedented tool for addressing questions that were previously out of reach.
Try the Connectome for Yourself
The FlyWire team has made their comprehensive brain map accessible through Codex (Connectome Data Explorer), a user-friendly online platform. Codex allows anyone with internet access to explore neurons and synaptic pathways in the brain map without needing to download large datasets or use specialized analysis tools. The platform is free to use and has already attracted over 10,000 users worldwide. Codex allows users to explore the brain connectome by visualizing entire regions, circuits, or individual neurons in 3D, with features like searching for specific neurons by name, browsing cell types, viewing annotations and details, examining brain regions (aka neuropils), analyzing network graphs and pathways, and even exporting data. Try it for yourself here!
Above: Codex: FlyWire used to visualize the fruit fly optic lobe. Image courtesy of Codex:FlyWire.
