The anatomy of the foveola reinvestigated
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Abstract
Objective. In 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. Methods. Serial 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. Results. In 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 degrees causes a very bright spot after passing through this area. However, when the angle of the light beam is changed to 10 degrees, 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 degrees 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. Conclusion. We 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 foveola cones of monkeys are not straight.
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2018. The anatomy of the foveola reinvestigated. PeerJ Preprints 6:e26518v1 https://doi.org/10.7287/peerj.preprints.26518v1note This preprint is not peer-reviewed. You may wish to reference the subsequent peer-reviewed version of this article.
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Supplemental Information
3D model shows the parts of the central Müller cells
A 3D model shows the parts of the central Müller cells of a monkey foveolar between the plateau zone and the outer limiting membrane. The central Muller cells are bigger and brighter in contrast to the other foveolar cells and the peripheral Müller cells. We used the Amira threshold tool to distinguish the central Müller cells from other cells.
3D model reconstructed from serial sections from the central foveolar cones of a monkey retina
A 3D model reconstructed from serial sections from the central foveolar cones of a monkey retina is shown. The 3D model of the central foveolar cones shows that outer segments do not run parallel to the incident light as reported earlier but are curved or even coiled and proceed collaterally to the retinal pigment epithelium which here is indicated in red. The outer limiting membrane is marked blue.
3D model of an individual Müller cell
An individual Müller cell is shown as a 3D model (Fig. 3b) in the retinal environment of the human foveolar center. Here the Amira Volren view was used and the threshold was adjusted.
A stack of the central retinal section from a monkey fovea
A stack of the central retinal section from a monkey fovea is shown with focused ion beam/scanning electron microscopytomographyafter using the Volren tool of Amira software. Müller cells adapt to the shape of the cones including the part containing the nuclei and therefore have a wavy shape. The end of the Müller cells close to the outer limiting membrane is below.
A stack of the central retinal section from a monkey fovea
A stack of the central retinal section from a monkey fovea is shown with focused ion beam/scanning electron microscopy tomography. One view through the stack from the inner segments of cones to the direction of the vitreous (bottom left) and from the front (bottom right) or from the top (top right). Müller cells appear electron lucent whereas cones are electron opaque. Remarkable is that Müller cells often have the shape of a triangle (bottom left).
Transmission of light under the light microscope at different angles
A human fovea from an isolated retina shows the transmission of light under the light microscope at different angles. The yellow color represents the macula pigment. Remnants of retinal pigment cells cause the black dots. The optical equipment is shown in Fig. 3c. When the light enters the fovea at 0 degrees there is a bright spot in the center of the foveola. This area corresponds exactly to the area with prominent Müller cells. However, when the angle of the light beam is changed to approximately 10 degrees (after 9 seconds), the bright foveolar center becomes dark and the SCE-like drop of light intensity becomes visible. Then the angle is slowly reversed and reaches 0 degrees after 16 seconds. The angles in the video are an approximation.
Stack of sections through the human foveolar center
A stack of sections through the human foveolar center is shown. Each line represents an individual section. This stack of sections can be looked through from the top (top left), from the side (top right) or from the front (bottom left). The Müller cells appear as bright cells.
Additional Information
Competing Interests
The authors declare that they have no competing interests.
Author Contributions
Alexander V. Tschulakow conceived and designed the experiments, performed the experiments, analyzed the data, contributed reagents/materials/analysis tools, prepared figures and/or tables, authored or reviewed drafts of the paper, approved the final draft.
Theo Oltrup performed the experiments, analyzed the data, contributed reagents/materials/analysis tools, prepared figures and/or tables, authored or reviewed drafts of the paper, approved the final draft.
Thomas Bende analyzed the data, authored or reviewed drafts of the paper, approved the final draft.
Sebastian Schmelzle contributed reagents/materials/analysis tools, prepared figures and/or tables, authored or reviewed drafts of the paper, approved the final draft.
Ulrich Schraermeyer conceived and designed the experiments, performed the experiments, analyzed the data, contributed reagents/materials/analysis tools, prepared figures and/or tables, authored or reviewed drafts of the paper, approved the final draft.
Human Ethics
The following information was supplied relating to ethical approvals (i.e., approving body and any reference numbers):
The human eyes were gifts from the Clinical Anatomy of the University of Tübingen (ethical number for scientific issues 237/2007B01).
Animal Ethics
The following information was supplied relating to ethical approvals (i.e., approving body and any reference numbers):
Monkeys were kept at Covance Laboratories GmbH (Münster, Germany study numbers 0382055, 8260977, 8274007) or SILABE-ADUEIS (Niederhausbergen, France). The Covance Laboratories GmbH test facility is fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care (AAALAC). This study was approved by the local Institutional Animal Care and Use Committee (IACUC), headed by Dr. Jörg Luft and the work was carried out in accordance with the Code of Ethics of the World Medical Association (Declaration of Helsinki). The monkeys from SILABE-ADUEIS were euthanized due to veterinarian reasons. Since they had not been included in a study before, they do not have a study number.
Data Deposition
The following information was supplied regarding data availability:
Figshare
https://figshare.com/s/8cce33bcc530aa9d5861
https://figshare.com/s/856ac002bf9239c309a9
https://figshare.com/s/facc1fc2b8ea046f52e5
https://figshare.com/s/ac0e1efb7512ba6977e5
https://figshare.com/s/3549145cddee51d324b7
https://figshare.com/s/6424bea929caac6d87bc
https://figshare.com/s/662d00c2dca3eb36622c
Funding
The authors received no funding for this work.
