THE SPACE FLIGHT INVESTIGATION

When bones develop and grow on Earth in the presence of gravity, they normally increase simultaneously in length, diameter, and mass. These three growth characteristics contribute to the strength of the bone. During space flight, in the absence of gravity, animal studies have shown that certain bones appear to grow in length at about the same rate as on Earth, but that the diameter of the bone is slightly smaller. In addition, the structure or "architecture" of the bone formed in space is different from that of animals left on Earth. Thus, for laboratory rats that have flown in space, strength does not increase proportionally to the increase in bone size as it does on Earth. In this section, we will discuss some of the actual results that Dr. Holton obtained with laboratory rats, primarily through postflight analyses that compared flight animals with control animals that remained on Earth.

Ultimately, we would like to be able to apply the results of such animal studies to humans. after all, one of the main reasons to carry out such animal experiments is to be able to translate and adapt the results enable us to better understand the physical and chemical changes that have been observed in the astronauts.

Bone mineral loss in astronauts has been documented in most early human space flights. Changes in calcium balance, decreased bone density, and inhibition of bone formation have also been reported. Data from Soviet/Russian flights suggests that:

  • dicephyseal bone (bone in the shaft of the bone) formation may stop during weightlessness;
  • the rate of elongation of long bones in the body is not affected by weightlessness, but
  • the rate of circumferential growth (diameter) is decreased. In addition to the direct observations of the physical changes in bone growth, increased urinary calcium excretion has been observed in astronauts in Skylab and other flights.

Figure 16. During space flight, changes in structure and strength have been noted in both the fore limbs and hind limbs of growing male rats.

Dr. Holton's bone experiment was designed to characterizethe effects of microgravity on the structure and strength of certain fore limb and hind limb bones of the rat (Figure 16). For this experiment, specific pathogen-free (free of disease) male rats were used. These animals were raised and certified as disease-free so that they could be studied without any concern that the results were due to the presence of certain viruses or bacteria. Using such pathogen-free animals also safeguards the health of the humans who fly in space with the animals. All animals were given food and water ad libitum, or "anytime they wanted," throughout the experiment.

The rats were subjected to a rhythm of normal day and night light cycles to mimic a "normal" environment and to match the cycles that the control animals would be exposed to on Earth. In fact, they were exposed to 12 hours of light, during times that generally matched the light cycle of Florida (9:00 a.m. to 9:00 p.m. Eastern Standard Time), their location while being prepared for launch at Kennedy Space Center. During the mission, the control animals were also kept on the same schedule. Keeping their "body rhythms" stable in this fashion was important to assure that the results of Dr. Holton's study would not reflect the stresses of changing body rhythms.

Figure 17. The Animal Enclosure Module.
The animals were either housed in a group fashion with five rats to a cage in a facility called the Animal Enclosure Module (AEM), (Figure 17), or individually with one rat per cage in a facility called the Research Animal Holding Facility (RAHF), which we have already discussed in the previous chapter. The rats were randomly divided into groups 13 days before launch (L-13). The various groups included:

  • Animal Enclosure Module control animals (AEM-C);
  • Animal Enclosure Module flight animals (AEM-F);
  • singly housed (in a rat cage called a VIVarium) control animals (VIV-C);
  • Research Animal holding Facility Flight animals (RAHF-F);
  • preflight control animals that were sacrificed at the beginning of the mission to establish baseline data for certain measurements.
After a nine-day mission, the AEM rats were removed from the shuttle within two hours of landing and the RAHF animals were removed within four hours. Half the animals in each group were sacrificed by decapitation as soon as they were received after landing (R+0). Their bones were processed within 20 minutes of decapitation and all Right rats were processed within six and a half hours after Right. Rapid processing of the Right animals (and their controls) was important to assure that the rats did not have much chance to begin readapting to the Earth's environment, thereby altering their "space-normal" condition. The remaining half of the animals were allowed to recover for nine days (R+ML, or Recovery + Mission Length) in order to understand how their bones readapt to an "Earth-normal" condition.

We will be examining the results of only a subset of Dr. Holton's various measurement sets that were performed on rats, including:

  • bone length, circumference, mass, and strength measurements; and
  • changes in calcium concentration and mineralization rates of bones.
So let's review some of the actual data from Dr. Holton's experiment to see how rat bones changed during space flight and to see if the type of housing arrangement for the rats might have influenced how their bones developed.

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