• Vision scientists discover new angle on

    From ScienceDaily@1:317/3 to All on Wed Mar 2 21:30:48 2022
    Vision scientists discover new angle on path of light through
    photoreceptors
    NIH study in ground squirrels suggests dual function for mitochondria in photoreceptor cells

    Date:
    March 2, 2022
    Source:
    NIH/National Eye Institute
    Summary:
    Researchers have discovered that power-producing organelles in
    the eye's photoreceptor cells, called mitochondria, function as
    microlenses that help channel light to these cells' outer segments
    where it's converted into nerve signals. The discovery in ground
    squirrels provides a more precise picture of the retina's optical
    properties and could help detect eye disease earlier. The findings
    also shed light on the evolution of vision.



    FULL STORY ========================================================================== Researchers at the National Eye Institute (NEI) have discovered that
    power- producing organelles in the eye's photoreceptor cells, called mitochondria, function as microlenses that help channel light to these
    cells' outer segments where it's converted into nerve signals. The
    discovery in ground squirrels provides a more precise picture of the
    retina's optical properties and could help detect eye disease earlier. The findings,published today in Science Advances, also shed light on the
    evolution of vision.NEI is part of the National Institutes of Health.


    ==========================================================================
    "We were surprised by this fascinating phenomenon that mitochondria
    appear to have a dual purpose: their well-established metabolic role
    producing energy, as well as this optical effect," said the study's
    lead investigator, Wei Li, Ph.D./B.M., who leads the NEI Retinal Neurophysiology Section.

    The findings also address a long-standing mystery about the mammalian
    retina.

    Despite evolutionary pressure for light to be translated into signals
    and pass instantly from the retina to the brain, the trip is hardly
    direct. Once light reaches the retina, it must pass through multiple
    neural layers before reaching the outer segment of photoreceptors, where phototransduction (the conversion of light's physical energy into cellular signals) occurs. Photoreceptors are long, tube-like structures divided
    into inner and outer segments. The last obstacle a photon must traverse
    before moving from the inner to the outer segment is an unusually dense
    bundle of mitochondria.

    Those bundles of mitochondria would seem to work against the process of
    vision either by scattering light or absorbing it. So, Li's team set
    out to investigate their purpose by studying cone photoreceptors from
    the 13-lined ground squirrel.

    Unlike other animal models used for vision research, the 13-lined ground squirrel's retina comprises mostly cones, which see color, as opposed
    to rods that enable night vision. Li's team studies the 13-lined ground squirrel to better understand the causes of human eye diseases that
    primarily affect cone photoreceptors.

    The researchers used a modified confocal microscope to observe the
    optical properties of living cone mitochondria exposed to light. Far from scattering light, the tightly packed mitochondria concentrated light along
    a thin, pencil- like trajectory onto the outer segment. Computational
    modeling using high- resolution mitochondrial reconstructions corroborated
    the live-imaging findings.



    ==========================================================================
    "The lens-like function of mitochondria also may explain the phenomenon
    known as the Stiles Crawford effect," said first author of the paper, John Ball, Ph.D., a staff scientist in the Retinal Neurophysiology Section.

    Scientists measuring retinal responses to light have long observed
    that when light enters the eye near the center of the pupil, it appears brighter compared to light of equal intensity entering the eye near the
    edge of the pupil.

    In this study, Li found that the lens-like effect of mitochondria followed
    a similar directional light intensity profile. That is, depending on
    light source location, the mitochondria focused light into the outer
    segment of the cell along trajectories that mirrored those observed from
    the Stiles-Crawford effect.

    Linking mitochondria's lens-like function to the Stiles-Crawford effect
    has potential clinical implications. The long-observed effect may now
    be used as the basis for non-invasively detecting retinal diseases,
    many of which are thought to involve mitochondrial dysfunction at
    their origin. For example, patients with retinitis pigmentosa have been reported to have abnormal Stiles- Crawford effect even when they have
    good visual acuity. More research is needed to explore the structural
    and functional changes in cone mitochondria and their manifestations in detectable optic features.

    Finally, the findings provide new insights into how our eyes may have
    evolved.

    Like the mitochondria in Li's study, within the photoreceptors of birds
    and reptiles, tiny oil droplets are located in the portion of the inner
    segment closest to the outer segment, and they are thought to serve an
    optical role.

    Furthermore, the mitochondrial "microlens" in mammalian cone
    photoreceptors confers a functionality reminiscent of that achieved by
    the compound eye of arthropods like flies and bumblebees.

    "This insight conceptually bridges compound eyes in arthropods with
    the camera eyes of vertebrates, two independently evolved image-forming systems, demonstrating the power of convergent evolution," Li said.

    The study was funded by the NEI Intramural Research Program.

    Video: https://youtu.be/BPWXpam_mcU special promotion
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    Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. John M. Ball, Shan Chen, Wei Li. Mitochondria in cone photoreceptors
    act
    as microlenses to enhance photon delivery and confer directional
    sensitivity to light. Science Advances, 2022; 8 (9) DOI: 10.1126/
    sciadv.abn2070 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/03/220302150348.htm

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