25 JULY 2014 • VOL 345 ISSUE 6195 385 SCIENCE
sciencemag.org
line. Instead, the light bounces around—
or diffuses—among the particulates in the
medium, effectively traveling at a much
lower speed than in vacuum. In this diffusive regime, Maxwell’s equations no longer
govern light transport; instead, the much
simpler diffusion equation, known as Fick’s
law, takes precedence, with the diffusivity
D being the sole material parameter that
needs to be controlled, rather than the ε
and µ of Maxwell’s equations.
To investigate the cloaking of light in the
diffusive regime, Schittny et al. immerse
either a metal cylinder or sphere into a
tank of water mixed with paint. The paint
flakes in water form a suspension that, for
light, produces a strongly scattering, diffusive medium—analogous to the smoke
that a magician might use to obscure the
audience’s view. Patterns of light projected
from a computer monitor are used to illuminate the cylinder, which blocks the light
and forms a shadow easily viewed by the
observer. Around the roughly 3-cm diameter of the cylinder, a material coating only
3 mm thick is applied, with a diffusivity
designed to perfectly offset the scattering
from the metal object. As a result, the entire
object is rendered invisible relative to the
medium—that is, the shadow is removed,
and the light pattern from the monitor restored (see the figure).
So, while cloaking an object in empty
space for light waves remains an unrealized goal, Schittny et al. have shown that if
we alter the environment, slowing the light
down and perhaps confusing it a bit, we can
actually render objects completely invisible. It is a remarkable result in terms of its
simplicity and yet completeness. Schittny et
al. have cleverly changed the properties of
the environment to achieve optical cloaking, rather than trying to create the challenging and inherently limited cloak for
free space. Because the goal of invisibility is
often to provide camouflage—where an object might go unnoticed relative to its environment, like a gecko changing its color to
match the background scenery—the diffusive cloak is an intriguing concept and one
that represents a convincing step forward
for invisibility research. ■
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10.1126/science.1256753
to maintain neuronal energy homeostasis
and neural activity (2, 6).
Most newly formed mitochondria are
transported from the cell body down the
length of the axon and often fuse with
resident mitochondria along the way (2).
Mitochondrial fusion and fission together
constitute an important quality-control
mechanism that is believed to help adjust
mitochondrial length and vitality, replenish
supplies of biomolecules, and dilute out defective mitochondrial components (1). Such
maintenance is essential not only for proper
bioenergetic function, but also to prevent
the release of reactive oxygen species or molecules that trigger programmed cell death
(apoptosis) from partially depolarized mitochondria (7, 8).
Ultimately, aged mitochondria are believed to become damaged irreparably, at
which point cells discard them or fission off
sections to be degraded through an autoph-agy-based process (mitophagy). Autophagosomes form around mitochondria, which
then fuse with lysosomes, delivering digestive enzymes and accomplishing their degradation (1, 9). Neuronal autophagosomes
likely mature by fusing with endosomes
and lysosomes during retrograde transport
Astrocytes eyeball axonal
mitochondria
By Thomas C. Burdett and
Marc R. Freeman
Retinal neurons transfer mitochondria to astrocytes for
rapid turnover to meet energy demands
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Sustaining a healthy population of mitochondria in a neuron requires a balance of de novo biogenesis of new mitochondria or mitochondrial com- ponents and removal of damaged mi- tochondrial membrane, proteins, and
DNA (1, 2). Recent evidence suggests that
mitochondria must be actively transported between
the cell body and axon (2).
A new study by Davis et al.
(3) offers a surprising alternative: Axons expel mitochondria to neighboring
astrocytes for degradation.
This unexpected finding
might reshape how we think
of mitochondrial disposal in
the nervous system.
Axons enable long-distance communication
in the nervous system by
propagating signals to distant synapses at extraordinary lengths from the
neuronal cell body. Axons
within the spinal tract of
the blue whale (perhaps
the longest in nature) are
estimated to exceed 30 m
in length, and the longest human axons
extend an average of 1 m from the base
of the spine to the toes. By comparison, a
neuronal cell body is less than 100 µm in
diameter (4). Thus, long axons can represent the majority of a neuron’s volume and
pose a major logistical challenge to ship
nutrients, organelles, and other biomateri-als essential for maintaining axon integrity,
synaptic connectivity, and neuronal function (2, 4).
The active transport of organelles and
vesicles, as well as the maintenance of
membrane potential along the length of the
axon, are energetically demanding tasks
that require a constant supply of adenosine
triphosphate from axonal mitochondria (5).
As such, mitochondria must be both properly distributed and functioning efficiently
Eye on mitochondria. Retinal ganglion cells in the optic nerve head of the
mouse are enwrapped by astrocytes and rapidly turn over mitochondria to
meet high metabolic demands.
