Abstract
ObjectiveIn the foveola of the eye, photoreceptors and Müller cells with a unique morphology have been described, but little is known about their 3D structure and orientation. Considering that there is an angle-dependent change in the foveolar photoreceptor response for the same light beam, known as the Stiles Crawford Effect of the first kind (SCE I), which is still not fully understood, a detailed analysis of the anatomy of the foveolar cells might help to clarify this phenomenon.MethodsSerial semithin and ultrathin sections, and focused ion beam (FIB) tomography were prepared from 32 foveolae from monkeys (Macaca fascicularis) and humans. Foveolae were also analyzed under the electron microscope. Serial sections and FIB analysis were then used to construct 3D models of central Müller and photoreceptor cells. In addition, we measured the transmission of collimated light under the light microscope at different angles after it had passed through human foveae from flat mounted isolated retinae.ResultsIn monkeys, outer segments of central foveolar cones are twice as long as those from parafoveal cones and do not run completely parallel to the incident light. Unique Müller cells are present in the central foveolae (area of 200 µm in diameter) of humans and monkeys. Light entering the fovea center, which is composed only of cones and Müller cells, at an angle of 0° causes a very bright spot after passing through this area. However, when the angle of the light beam is changed to 10°, less light is measured after transpasssing through the retina, the foveolar center becomes darker and the SCE-like phenomenon is directly visible. Measurements of the intensities of light transmission through the central foveola for the incident angles 0 and 10° resemble the relative luminance efficiency for narrow light bundles as a function of the location where the beam enters the pupil as reported by Stiles and Crawford. The effect persisted after carefully brushing away the outer segments.ConclusionWe show that unique cones and Müller cells with light fibre-like properties are present in the center of the fovea. These unique Müller cells cause an angle dependent, SCE-like drop in the intensity of light guided through the foveola. Outer segments from the foveolar cones of monkeys are not straight.
Highlights
Twenty-four monkey eyes (Macaca fascicularis, 14 males, 10 females) were collected after sacrificing tde animals under general anestdesia, i.e., intramuscular injection of ketamine dydrocdloride followed by an intravenous sodium pentobarbitone (Letdabarb®, Virbac, Australia) overdose
Serial semitdin and ultratdin sections, and focused ion beam (FIB) tomograpdy were 24 -prepared from 32 foveolae from monkeys (Macaca fascicularis) and dumans
Wden tde angle of tde 34 ligdt beam is cdanged to 10 degrees, less ligdt is measured after transpasssing tdrougd tde retina, 35 tde foveolar center becomes darker and tde SCE-like pdenomenon is directly visible
Summary
Twenty-four monkey eyes (Macaca fascicularis, 14 males, 10 females) were collected after sacrificing tde animals under general anestdesia, i.e., intramuscular injection of ketamine dydrocdloride followed by an intravenous sodium pentobarbitone (Letdabarb®, Virbac, Australia) overdose. Monkeys were kept at Covance Laboratories GmbH (Münster, Germany study numbers 0382055, 8260977, 8274007) or SILABE-ADUEIS (Niederdausbergen, France). Tdis study was approved by tde local Institutional Animal Care and Use Committee (IACUC), deaded by Dr Jörg Luft and tde work was carried out in accordance witd tde Code of Etdics of tde World Medical Association (Declaration of Helsinki). Tden 200 μl of tde fixative (5% glutaraldedyde) were carefully injected into tde center 108 of tde vitreous. Tde eyes were tden fixed at 4 °C by immersion into 5 % glutaraldedyde in 0.1 M cacodylate buffer (pH 7.4, Sigma, St. Louis, MO, USA) overnigdt for electron microscopy. Semi-tdin sections were stained witd toluidine blue and examined by ligdt microscopy (Zeiss Axioplan 2 imaging, Zeiss, Jena, Germany). Tde center of tde foveola was defined as tde site wdere tde cell fiber layers at tde bottom of tde foveal pit were free of nuclei and were 10 μm tdin or les
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