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Frontiers
Snails, Fish and Gravity Sensors: On a Mission to Find Answers
May/June 1998
This April, dozens of snails and fish went where only
a few men and women have gone before: outer space. Traveling on the Space
Shuttle Columbia, the snails and fish were part of a research project
funded by NSF to study the development of gravity sensors in space by animals
in the early stages of life.
The snails and fish flew aboard Neurolab, a shuttle research mission dedicated
to the study of the life sciences. Neurolab focused on the most complex and
least understood part of the human body--the nervous system, which faces major
challenges in space.
Gravity-sensing systems have the same basic structure in all vertebrates,
whether fish or humans. The gravity-detecting organ is lined with sensory cells
that send signals to the brain when they are triggered; the "triggers" are
small, rock-like particles of calcium carbonate, referred to as statoliths
in snails and otoliths in fish (and in humans). In humans, this system is a
component of the inner ear. A simpler system exists in snails, one that is far easier to analyze and develops faster, say scientists.
"Data from the aquatic experiments aboard Neurolab are helping us understand
the basic mechanisms that maintain postural and spatial orientation," explains
Christopher Platt, program manager in NSF's Division of Integrative Biology
and Neuroscience, which funded the experiments. "Gravity is always present
on Earth, so it's been hard to explore its role in development and in controlling
movement. Neurolab allowed unique tests to take place, ones that will shed
light on how gravitational sensors work by showing the changes in sensory structure
or nerve function that appear in its absence. These studies may tell us how
long exposure to lack of gravity may lead to abnormalities in the otolith organs,
which is relevant to long-term spaceflight and to certain kinds of posture
and balance problems in people on Earth."
Other benefits of the aquatic studies aboard Neurolab include development
of an electrode that offers potential use as a connection to the nervous system
in people with deafness caused by hair cell damage. The electrode might also
someday be used as an interface to signal motor prostheses how and when to
move.
Tracking the progress of the snails and fish flying aboard Columbia were scientists
on "The Aquatic Team," as they were known to shuttle crewmembers. Researcher
Michael Wiederhold of the University of Texas Health Science Center at San
Antonio monitored freshwater snails and swordtail fish in the beginning stages
of their development into adults.
Wiederhold is trying to answer such questions as what physiological changes
occur in the components of the gravity sensors of animals in space, whether
signals sent from the inner ear to the brain are altered, and, if alterations
do occur, whether behavior of the animal changes. "Looking at changes that
occur in organisms in space is very practical," says Wiederhold. "Long-term
space missions and space exploration point to a time when it will be necessary
to begin generating life--including human beings--away from the Earth. Studies
such as those on Neurolab are, therefore, vital."
Upon return from their spaceflight in Neurolab, the freshwater snails and
swordtail fish were compared to a control group on Earth to determine whether
the size of their statoliths and otoliths increased while they were in "microgravity." On
Earth, the pull of gravity eventually signals developing statoliths and otoliths
to stop growing. "In space, however," says Wiederhold, "without this signal,
they should develop to a larger size than they do on Earth. And if indeed they
increase in size, how will that affect these animals? That's what we hope to
discover through these experiments."
Scientist Steven Highstein of the Washington
University School of Medicine in St. Louis, Missouri, is also studying aspects
of the inner ear, but his research involves the inner ears of astronauts flying
aboard Columbia, as well as those of oyster toadfish aboard Neurolab. Says
Highstein, "The otolith organs of these fish experienced the removal of gravity
at the same time as the otolith organs of the humans on board." Since this
system is very similar in fish and humans, by recording data from the nerves
of the otoliths in the oyster toadfish, researchers will soon be better able
to understand changes in the same signals sent by the astronauts' inner ears
as both humans and fish adapt to microgravity.
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