Abstract

The nervous system consists of neurons and glial cells. It is commonly thought that glial cells provide mechanical support for neurons. In contrast, we found that glial cells are in fact significantly softer than all other surrounding cellular structures and rather act as shock absorbers. We also found that in vitro neurons actively probe their mechanical environment. They retract their processes and re-extend them randomly when mechanical stresses exceeding ~300 Pa oppose their leading edge. Interestingly, glial cell processes are softer than 300 Pa and neurons attach to them during migration to precisely follow the glia processes, even when they are considerably bent. This suggests that the mechanical properties of glial cells may facilitate and direct neuronal migration in the developing brain. Other glial cells in the CNS are also observed to perform highly specialized functions, such as Müller cells in vertebrate retinae. While cells are mostly transparent, they are phase objects that differ in shape and refractive index. Any image that is projected through layers of cells will normally be distorted by refraction, reflection, and scattering. Counter-intuitively, the retina of the vertebrate eye is inverted and light must pass through several tissue layers before reaching the light-detecting photoreceptor cells. We investigated the optical properties of retinal tissue and individual Müller cells, which are glial cells spanning the entire thickness of the retina. We found that these cells amazingly act as optical fibers and guide light from the retinal surface to the photoreceptor cells. Their parallel arrangement in the retina is highly reminiscent of fiber-optic plates used for low-distortion image transfer. Thus, Müller cells seem to transfer images through the vertebrate retina with minimal distortion and low loss. This finding explains a fundamental feature of the inverted retina as an optical system, and it ascribes another completely new kind of function to glial cells.