How glial cells wire up a fly’s eye
The ‘connective tissue’ of the brain plays a surprisingly active role in the development of the insect’s visual system.
Glials cells, or glia, are cells that were long thought to be nothing more than the glue or connective tissue that holds the hard-working neurons together. They are now understood to also provide nutrition and oxygen to the neurons, and also remove pathogens and detritus from the brain.
Now Vilaiwan Fernandes and colleagues have found new role for them: wiring up the fly eye during development, to create the neural circuits that map visual space.
The fly’s visual system is made up of modular circuits that process visual inputs: each of the 800 parts of the fly’s compound eye sends information to a corresponding group of neurons in the optic lobe of the brain. Connecting the photoreceptors with the optic lobe neurons requires precise coordination during development.
The researchers found that cells called wrapping glia, which grow in a sheath around photoreceptor axons that project into the optic lobe, act as relay stations in this coordinated growth.
Stimaulation by a photoreceptor growth factor triggers the glia to produce insulin-like peptides, and these peptides guide optic lobe neurons to develop into connecting terminals for photoreceptors.
In the image above, development of the retina (top) is coordinated with development of the optic lobe region of the brain (sphere below). All neurons are marked by yellow and their axon projections in cyan; magenta in the optic lobe marks the specific region of the brain where the neuronal differentiation is regulated by glia.