Also, browsing museum.eyewire.org one can find many intriguing examples of mouse retinal ganglion cell types that may be patterned similarly. Note in this image how each cell body is positioned at the edge of its neighbor's dendrite territory.

Intriguingly, there are other retinal cell types that show homotypic dendrite-soma exclusion. For example, Dacey & Brace (1992) showed, in primate retina, that parasol retinal ganglion cells are positioned at the edges of their neighbors’ dendritic arbor territories.

We conclude that newborn starburst neurons make space for themselves in the developing mosaic by contacting neighboring starburst cell bodies using their dendrites (see image for illustration). And that this is the primary mechanism for starburst mosaic patterning.

That’s exactly what we found: Megf10 mutant starbursts could no longer exclude homotypic neighbors from their dendrite territories.

But how do we know that dendrite-soma exclusion is involved in mosaic patterning? Back when I was a postdoc, we studied a cell surface molecule, MEGF10, that is required for starburst exclusion zone formation. Without MEGF10, starbursts are randomly positioned.

It almost looked like the baby starburst neurons were reaching out to grab their neighbors and hold them at arm’s length.

When we labeled dendrites of baby starburst neurons, we saw a striking coordination between the dendritic arbor and neighboring starburst cell bodies. Somata were lined up around the edge of the dendritic arbor, suggesting they were physically excluded from the arbor territory.

To address this problem we focused on mouse starburst amacrine cells, which have long been a model for studying how mosaics form.

Importantly, exclusion zones suffice to impose regular spacing on the entire neuronal array. So the issue of mosaic patterning really boils down to this question: How are exclusion zones established? In other words, what are homotypic neurons doing to repel each other?

It’s important to answer this question because mosaics are a key aspect of visual system organization. Each type of retinal neuron has a specific role in visual processing. Mosaics ensure each cell type is represented at each point in visual space, without blind spots or oversampling.

Many types of retinal neurons are arranged in strikingly regular patterns known as mosaics. The term "mosaic" means that neurons of a given type are spaced evenly across the retinal surface. How do mosaics form during development?
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