Intercellular contact and cargo transfer between Mu ller glia and to microglia precede apoptotic cell clearance in the developing retina

To clarify our understanding of glial phagocytosis in retinal development, we used real-time imaging of larval zebrafish to provide cell-type specific resolution of this process. We show that radial Mu'' ller glia frequently participate in microglial phagocytosis while also completing a subset of phagocytic events. Mu'' ller glia actively engage with dying cells through initial target cell contact and phagocytic cup formation, after which an exchange of the dying cell from Mu'' ller glia to microglia often takes place. In addition, we find evidence that Mu'' ller glia cellular material, possibly from the initial Mu'' ller cell phagocytic cup, is internalized into microglial compartments. Previously undescribed Mu'' ller cell behaviors were seen, including cargo splitting, wrestling for targets and lateral passing of cargo to neighbors. Collectively, our work provides new insight into glial functions and intercellular interactions, which will allow future work to understand these behaviors on a molecular level.