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
Explore the latest scientific research on sleep and dreams
in this free online course from New Scientist -- Sign_up_now_>>> ========================================================================== Story Source: Materials provided by NIH/National_Eye_Institute. Note:
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

--- up 2 days, 10 hours, 51 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)