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Ulrich Schraermeyer, Sebastian Schmelzle, Sigrid Schultheiss, Sylvie Julien; Foveolar cones of monkeys and humans have a unique, still unknown morphology with impact for understanding the Stiles-Crawford effect. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5960.
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© ARVO (1962-2015); The Authors (2016-present)
The Stiles-Crawford (SC) effect is a property of the cone photoreceptors of the human 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.
The eyes from 52 cynomolgus monkeys (Macaca fascicularis Raffles) and 2 human eye donors were fixed and embedded for electron microscopy. Semithin sections were cut from 22 foveae. Serial sections were made from individual foveae. The image stack was aligned and segmented using Amira® 4.0.1 (Visualization Sciences Group, SAS ) resulting in a three-dimensional model. Focused ion beam electron microscopy was also performed.
Three-dimensional reconstruction of serial sections from humans and monkeys clearly showed that in the foveola (200 - 300 µm in diameter) the inner segments of the cones were curved whereas in the fovea and parafovea they were oriented along the optical axis. Inner and outer segments together were S shaped in the foveola. The orientation of inner and outer segments from cones in the foveola in well-fixed specimens was extremely regular and nearly crystalline and not orientated parallel to the optical axis.
The shape and orientation of the foveolar cones is different to parafoveal cones in both human and monkey eyes. This orientation contributes to the directional sensitivity of the Stiles-Crawford effect.
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