gms | German Medical Science

Background: The Stiles–Crawford (SC) effect is a property of the cone photoreceptors of the human and monkey eye and was first described 8 decades ago. It was found that foveal cones have a less pronounced directional sensitivity than parafoveal cones. It was speculated that a change in the shape or the orientation of foveal cones was responsible for the SC-effect. Until now, no morphologic evidence for this assumption has been found.

Methods: The eyes from 52 cynomolgus monkeys (Macaca fascicularis) and human eye donors were fixed and embedded for electron microscopy. Semithin serial sections were cut horizontally and vertically through the foveolar centres. The image stacks were aligned and segmented using Amira® 4.0.1 (Visualization Sciences Group, SAS ) resulting in three-dimensional models.

Results: The foveola has several unknown anatomic features. Three-dimensional reconstruction of serial sections from humans and monkeys clearly showed that in the foveola ( about 300 µm in diameter) the orientation of cones towards the optical axis is different compared to those in the parafovea. The outer segments from foveolar cones are twice as long as from parafoveolar cones. Foveolar Müller cells are much more prominent than in the fovea and parafovea. In tangential sections of the foveola, Müller cells occupy about 30 % of the retina whereas in the parafovea they were much less prominent (estimated 3 %). The diameters of individual cross-sectioned Müller cells were larger than 5 µm which is an extraordinary size.

Conclusion: The shape and orientation of foveolar cones and Müller cells are described for the first time by constructing a 3D model from serial sections. Orientation towards the infalling light in the foveolar cones is different to parafoveal cones in both human and monkey eyes. Müller cells are light wave guides. They have a unique, prominent and specific morphology in the foveola which favours light guiding of photons that enter the retina parallel to the optical axis under an angle of 90 degrees. The specific morphology of foveolar cones and Müller cells contributes to the directional dependency of the Stiles-Crawford effect.