Cone-to-Müller cell ratio in the mammalian retina: A survey of seven mammals with different lifestyle

Mammalian retinal glial (Muller) cells are known to guide light through the inner retina to photoreceptors (Franze et al., 2007; Proc Natl Acad Sci U S A 104:8287-8292). It was shown that Muller cells transmit predominantly red-green and less violet-blue light (Labin et al., 2014; Nat Commun 5:4319). It is not known whether this optical function is reflected in the cone-to-Muller cell ratio. To determine this ratio in the retinas of mammals with different lifestyle, we evaluated the local densities of cones and Muller cells in the retinas of guinea pigs, rabbits, sheep, red deer, roe deer, domestic pigs, and wild boars. Retinal wholemounts were labeled with peanut agglutinin to mark cones and anti-vimentin antibodies to identify Muller cells. Wholemounts of guinea pig and rabbit retinas were also labeled with anti-S-opsin-antibodies. With the exceptions of guinea pig and pig retinas that had cone-to-Muller cell ratios of above one, the local densities of cones and Muller cells in the retinas of the species investigated were roughly equal. Because the proportion of S-cones is usually low (for example, 5.3% of all cones in the dorsal guinea pig retina expressed S-opsin), it is suggested that Muller cells are mainly coupled to M-cones. Exceptions are the ventral peripheries of guinea pig and rabbit retinas which are specialized areas with high S-cone densities. Here, up to 50% of Muller cells may be coupled to S-cones, and 40% of S-cones may be not coupled to Muller cells. Among the species investigated, the density of Muller cells in the central retina was inversely correlated with the axial length of the eyes. It is suggested that (with the exception of specialized S-cone areas) Muller cells support high acuity vision by predominant guidance of red-green light to M-opsin expressing cones.