Cone mitochondria act as microlenses to enhance light delivery and confer Stiles-Crawford-like direction sensitivity

Abstract

Evolution endeavors to maximize the function of biological structures in organisms, and the vertebrate eye is no exception. Cone photoreceptors in the retina are among the most energy-demanding cells in our body, necessitating numerous mitochondria. Intriguingly, these mitochondria adopt a peculiar spatial aggregation immediately beneath the cone outer segment (OS) that houses light-sensitive opsin molecules. Here we demonstrate, via direct live imaging and computational modeling of ground squirrel cones, that such mitochondria bundles concentrate light to enter the OS for detection. This "microlens"-like feature of cone mitochondria produces an angular dependence of light intensity quantitively consistent with the Stiles-Crawford effect, a psychophysical phenomenon believed to improve visual resolution. Thus, in addition to their function as a necessary powerhouse, cone mitochondria play a critical optical role.