A Müller glia-retinal pigment epithelium apical network surrounding cone outer segments in fish and mouse

Homeostasis of the vertebrate retina is supported by two non-neuronal cell types, Muller glia and the retinal pigment epithelium (RPE). Their apical domains delineate the subretinal space, where photoreceptor outer segments reside. While the individual interactions between the supporting cells and photoreceptors are well studied, Muller glia and RPE are presumed to remain physically separate in a healthy retina. Here, we describe a revised model of the subretinal space architecture based on detailed FIB-SEM and confocal microscopy imaging of the zebrafish and mouse outer retina. Long apical processes extend from both Muller glia and the RPE, interacting with one another while also encircling cone outer segments. These contact regions, at the interface of the three cell types are highly elaborated in fish and, although sparser, are still present in mouse. The growth and overlap of Muller glial and RPE apical processes occurs subsequent to the emergence of the outer segment in both species, and coincides with the onset of visual function in mouse. Although enriched at cone sites, this specialized interaction does not appear to depend solely on cone localization in either species, pointing to a general hallmark of the vertebrate subretinal space. Together, our work provides major insights into the architecture of the subretinal space and the interactions between Muller glia, RPE and photoreceptors, while demonstrating conservation across species.