The retina's neurovascular unit: Mueller glial sheaths and neuronal contacts
The neurovascular unit (NVU), comprising vascular, glial and neural elements, supports the energetic demands of neural computation, but this aspect of the retinas trilaminar vessel network is poorly understood. Only the innermost vessel layer - the superficial vascular plexus (SVP) - is ensheathed by astrocytes, like brain capillaries, whereas glial ensheathment in other layers derives from radial Muller glia. Using serial electron microscopy reconstructions from mouse and primate retina, we find that Muller processes cover capillaries in a tessellating pattern, mirroring the tiled astrocytic endfeet wrapping brain capillaries. However, gaps in the Muller sheath, found mainly in the intermediate vascular plexus (IVP), permit different neuron types to contact pericytes and the endothelial cells directly. Pericyte somata are a favored target, often at spine-like structures with a reduced or absent vascular basement lamina. Focal application of adenosine triphosphate (ATP) to the vitreal surface evoked Ca2+ signals in Muller sheaths in all three vascular layers. Pharmacological experiments confirmed that Muller sheaths express purinergic receptors that, when activated, trigger intracellular Ca2+ signals that are amplified by IP3-controlled intracellular Ca2+ stores. When rod photoreceptors die in a mouse model of retinitis pigmentosa (rd10), Muller sheaths dissociate from the deep vascular plexus (DVP) but are largely unchanged within the IVP or SVP. Thus, Muller glia interact with retinal vessels in a laminar, compartmentalized manner: glial sheathes are virtually complete in the SVP but fenestrated in the IVP, permitting direct neural-to-vascular contacts. In the DVP, the glial sheath is only modestly fenestrated and is vulnerable to photoreceptor degeneration.