Avian Development Facility:
A Successful First Flight
A new facility for incubating quail eggs in space had an "egg"-ceptionally
successful first flight aboard the space shuttle, giving scientists new data for
puzzling out the problems of bone loss.
August 2002: One of the primary missions of NASA's life sciences researchers
is to develop an understanding of how to safely sustain human life in space. Over
the decades since the first spaceflight, researchers have discovered that a low-gravity
environment has multiple and variable effects on the human body. Because relatively
few humans are exposed to a microgravity environment, researchers often use animal
models to obtain useful data that can be applied to human problems. One such animal
model is the Japanese quail — specifically, researchers study embryonic development
in the species.
"The quails have a short incubation time of 15 days, which makes them an
ideal species to be used on shuttle missions," says Principal Investigator
(PI) J. David Dickman, of the Central Institute for the Deaf in St. Louis, Missouri,
where he studies the effects of microgravity on the development of the vestibular
system. This system is the part of the inner ear that provides humans, as well
as other animals, with a sense of balance. Dickman's most recent experiment flew
on STS-108, which launched on December 5, 2001. Dickman further explains why quail
eggs are a good model for study in microgravity: "The quails' development
happens very quickly, and during embryogenesis [development of the embryo], we
want as much of the exposure to occur in the microgravity environment as possible."
PI Stephen Doty, of the Hospital for Special Surgery in New York, New York,
studies the effects of microgravity on quail skeletal development and also flew
an experiment on STS-108. Doty notes that quails are good models for his work
because the data obtained from studies of Japanese quail skeletal development
are relevant to humans and other organisms: "When looking at the cellular
activity that controls mineralization [of bones], these cellular mechanisms are
all the same, regardless of whether we are looking at a quail, a rat, or a human."
SHOT Into Space
To study the development of quail embryos in microgravity, researchers needed
a means of getting the quail embryos into orbit. This is where Space Hardware
Optimization Technology (SHOT) Inc., in Greenville, Indiana, came in. John Vellinger,
one of SHOT's cofounders, had as a high school student conceived of a project
to study the effects of microgravity on embryogenesis in chicken eggs. In 1983,
Vellinger's research proposal became a national winner in the Shuttle Student
Involvement Program, which was sponsored by NASA and the National Science Teachers
Association. With a grant from Kentucky Fried Chicken, Vellinger paired with engineer
Mark Deuser (also a SHOT cofounder) to develop an incubator for use in a space
environment. Their first-generation hardware flew on STS-29 in March 1989 as the
CHIX in Space experiment.
From this prototype was developed the Avian Development Facility (ADF), used
by Dickman and Doty on STS-108. The ADF is an incubator designed to house 36 Japanese
quail eggs and to fit in a space shuttle middeck locker.
Super Incubator
Kristina Lagel, a Lockheed Martin contractor at Ames Research Center at Moffett
Field, California, was the project scientist for Dickman's and Doty's experiments
on STS-108, as well as for the validation of the hardware. She describes her role
as being a liaison between the PIs and NASA, helping the PIs to meet as many of
their objectives as possible. Lagel expects that the ADF will be readily accepted
by researchers because Vellinger sought input from the avian research community
at the earliest stages of design and development and then tried to meet as many
of their requirements as he could in constructing the ADF.
Although the ADF is similar to incubators found on Earth, it has some special
features that are important for obtaining useful data for the PIs. The ADF features
an onboard centrifuge that maintains half of the eggs under a 1-g force to simulate
gravity while in orbit, thus providing a normal-gravity control that experiences
all the same effects of spaceflight as the experimental animals, except for the
microgravity conditions. Lagel notes, "An on-orbit control is what almost
all science people would love to have." That sentiment is echoed by Dickman,
who says, "The onboard centrifuge is the number one reason the ADF is a fantastic
piece of hardware. For the first time we were able to expose all eggs [experimental
and control] to exactly the same environment."
Lagel adds that another important feature of the ADF is its environmental controls.
Temperature, humidity, oxygen, and carbon dioxide are all controlled by the ADF
and maintained at optimal levels for embryo development. Eggs can be chilled to
forestall the onset of development until the facility is in space, at which time
a crewmember can enter a sequence on the ADF's push-button panel to bring the
facility up to incubation temperature.
Dickman comments that, second to the capability to run onboard control experiments,
the best feature of the ADF is the injection system that can be programmed to
inject a fixative solution into specific eggs at specific times. "That gave
us snapshots of development. Again, it was a first, and something that the astronauts
didn't have to do," he adds.
Dickman and Doty chose to have embryos fixed at days four and seven of development,
as well as day 12, when the eggs had returned to Earth. Because they were looking
at different physiological systems, the researchers were able to share the eggs
to obtain a maximum amount of data from a limited number of samples.
A Matter of Balance
The vestibular system in birds has six different receptor populations, or organs,
within the inner ear (mammals have five). Three of these receptor populations
in birds contain small "stones," or otoliths. The otoliths are sensitive
to the movement of the head relative to gravity and move in response to linear
acceleration (such as is experienced during a space shuttle launch or when accelerating
a car). The other three receptor organs are sensitive to rotational movements
of the head and do not respond to gravity or linear acceleration. These properties
of the vestibular system are what give animals their sense of balance, and disturbances
in the vestibular system can give rise to problems for humans such as motion sickness
and a malady known as benign positional vertigo (BPV).
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Left: The vestibular system in birds contains six different
receptor populations or organs. Three of these organs contain small "stones"
or otoliths (top), which are sensitive to head movement relative to gravity and
move in response to linear acceleration. Vestibular receptor cells (bottom) are
normally organized in a fan-shaped pattern, which allows the cells to respond
to directional movement.
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Dickman's experiment in the ADF looked at four different aspects of the vestibular
system and how their development was affected by a microgravity environment. The
first was the formation of the otolith particles, or otoconia, which the researchers
hypothesized would be larger and more numerous than those grown under the influence
of gravity. The researchers also wanted to see how the neurons that carry signals
from the receptor cells of the vestibular system to the brain formed. In normal
cases, the vestibular receptor cells are organized into a specific fan-shaped
pattern that allows the cells to respond to directional movement in space. Says
Dickman, "We thought that this organizational pattern would be purely genetic,
and we wanted to see if microgravity actually influenced the development of that
pattern." The fourth area of interest was the formation of synapses, small
vesicles filled with neurotransmitter chemicals that carry information from the
receptor cell to the nerve fiber. Again, the researchers were interested in whether
embryogenesis in a microgravity environment would affect the number of synapses
that developed.
As yet, it's too early for Dickman to make any real conclusions from this study;
however, one trend that the researchers have noted is that the otoconia in the
embryos that developed in microgravity do appear to be larger than those of their
control counterparts. Dickman was surprised to see that there are indications
that the fan-shaped arrangement of receptor cells may also be altered under the
influence of microgravity.
How will this research ultimately benefit astronauts or people on Earth? Dickman
uses the example of people affected by BPV, in which the otoconia become dislodged
from the otolith receptor and stuck in the rotational sensor parts of the vestibular
system. Says Dickman, "If we understand the process of how the otoconia are
made in microgravity and hypergravity, and then begin to look at the genetics
that actually control otoconia growth, we might be able to apply that knowledge
directly to BPV here on Earth."
Astronauts also develop vestibular system deficits during long-duration space
missions. "We do think they adapt back [upon return to gravity]," notes
Dickman, but "we think that gravity is necessary for the proper development
of the vestibular system." For the future, if humans expect to colonize space,
then the question of normal development of the vestibular system in the absence
of normal gravity will take on even greater significance.
What About Bones?
Under microgravity conditions, bones demineralize, resulting in osteoporotic
conditions. Doty sees the study of embryos that develop in a microgravity environment
as an important piece of the osteoporosis puzzle: "If you try to study osteoporosis
in a normal model, it might take many months or years to reach an osteoporotic
state. In these spaceflight conditions, we are developing osteoporosis in these
models almost immediately."
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Left: Electron micrographs of quail limb bones that
formed under the influence of microgravity show decreased mineralization compared
to bones formed in normal gravity. The letters "B" and "C"
indicate bone and cartilage sides of the sample, respectively, with the arrows
marking the junction between bone and cartilage cells. The asterisks indicate
where mineralization begins. The bone that developed during spaceflight (top)
shows less mineral compared to the control sample (bottom); the control sample
clearly shows mineral deposits (dark spots) that are absent in the flight sample.
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One of the things that Doty and his team are looking at is development of the
limbs in quail embryos. Having conducted studies on the Russian Space Station
Mir, Doty can say that when an embryo is allowed to develop fully, the development
of the limbs does not appear to change much in microgravity. During the early
periods of development, from days seven to nine, development does appear to slow,
only to catch up later.
"At the cellular level, however," notes Doty, "it looks like
the mineralization process may very well be affected. Even though the limb development
is normal with respect to size, the degree of mineralization may actually be less."
Using the Japanese quail model in the ADF is important to Doty's research because
all of the quail skeletal development occurs during spaceflight. "One of
the problems we've had with studying rodents and humans is that they already have
a significant amount of bone formed prior to exposure to microgravity," says
Doty. That makes it more difficult to determine which changes, if any, are the
result of exposure to microgravity.
Like Dickman, Doty says it's too early to make any definitive statements about
his results from the STS-108 experiments. He was able to make one observation:
more of the embryos that were exposed to 1 g during the experiment appeared to
have reached their "true" developmental state of 12 days at experiment
end than did the embryos exposed to microgravity, which showed a greater variation
in their "staging" (the age they appeared to be).
Doty would like to fly additional experiments in the ADF to study bone development
from generation to generation to see how bone would respond in the absence of
gravity from embryogenesis to adulthood and on to the next generations.
A Future for ADF
Both Dickman and Doty are eager to perform additional experiments using the
ADF. Says Dickman, "It's a beautiful piece of hardware, it performed flawlessly,
and we got back some tremendous animals with which to collect our data."
Doty concurs. "We would be ready to do another experiment tomorrow,"
he says.
In the meantime, both researchers will continue to analyze the data they obtained
from their first ADF experiments and plan for future research when the ADF is
once again available.
Valuable Resources
Avian
Development Facility -- Information about ADF in the Research
On Station Section
Fundametal
Biology Overview -- OBPR Fundamental Biology Program Overview
Fundamental
Biology -- NASA's Fundamental Biology Program Web site
Author: Julie K. Poudrier
Editor: Space
Research Editorial Board
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