Figure 1. Retinal microglia tile the vitreal surface and synaptic layers.

Neural retinas showing Iba1 immunostaining (white) and nuclear counterstaining (blue). A) Radial section through the mid-peripheral retina from 1 month-old (mo) D2 mice. Microglia cells (Iba1⁺) position their somata at the NFL (cell #1), GCL (#2), inner (#3) or outer IPL (#4), and within the innermost surface of the OPL (#5). Cells localized to the NFL and GCL extend tangential processes along the vitreal surface, IPL microglia project complex arbors throughout the IPL, while microglia at the OPL send sturdy processes across the INL reaching the IPL or thinner and branched processes that line the INL outer surface. The absence of microglia in the ONL is prevalent, although their somata within the subretinal space sometimes project processes across the ONL (not shown). B-D) The whole mount view of a similar field lateral to the optic disc (asterisk) depict the parallel mosaics of retinal microglia. Same scale bar. B) At the depths corresponding to the NFL and GCL, microglia are aligned with blood vessels (space shown with dashed line), adopting a radial orientation towards the optic disc, while parenchymal microglia array more regularly. C) At the IPL, regularly tiled microglia show small somata and complex, wispy processes. D) Deeper, the OPL microglia exhibit very discrete non-overlapping territories like IPL microglia, but with higher Iba1 expression levels. B′-D′) High magnification views of parenchymal microglia corresponding to the dotted frames in B-D. Observe that these pairs of cells tile their territories and show diverse soma size and shape, number and branching complexity of primary processes, and Iba1 expression levels. These elements suggest different degrees of cell activation. Same scale bar.