I. Measurement of Total Body Mass as well as Bone Length and Bone Mass in the Rat

There are certain kinds of rats that will continue to grow throughout their lives. In fact, unlike humans, the epiphyseal plates (the plates responsible for the lengthening of bones) of rats never disappear. It is possible for laboratory rats to live to be about three to four years old and grow to a size of 500-700 grams. The laboratory rats used in most scientific studies are younger and smaller; the rats that Dr. Holton used were about two months old and weighed just below 300 grams at the time of launch. Dr. Holton used the smaller, younger rats since they formed more bone during the short period of the mission, allowing her to observe and study more bone growth characteristics. The fact that they do continue to grow is one of the factors that makes rats such a good model for the study of bone formation. An interesting feature of Dr. Holton's study was an examination of how different housing arrangements might affect how the rat bones develop in space (and, of course, here on Earth for the control animals).

Before the rats were sacrificed, they were each weighed. Then, following decapitation, the different members of the dissection team worked quickly to process the various bones that will be used for the experiment. Once these bones had been appropriately p reserved, the general physical characteristics of certain individual rat bones were determined. These included length, diameter, and weight. We will review the results of bone length and mass measurements in this section. Changes in bone diameter will be examined in the next section.

The techniques for length and mass measurements are simple, the important factor being accuracy since the animals are very small and very small changes are of interest. In order to measure the length of the limb bones, a special dial caliper was used. This is a measuring instrument with two legs or " jaws" that can be adjusted. This instrument is accurate to 0.1 mm. In order to measure mass, the animals were placed on a very accurate electronic balance. Let's discuss the results of the rat's body mass and bone mass measurements.
Figure 18a.
a) Preflight and postflight rat growth during the space flight experiment.

The rats were 65 days old at launch and weighed an average of 285 ± 16 grams (mean value + standard deviation) two days before launch. The day of launch, the ground controls weighed an average of 295 ± 12 gm. Once the shuffle landed, the rats weighed an average of 331 ± 19 gm and by nine days later, the average weight was 342 ± 20 gm. No significant difference in mean body weights was noted among any of the groups.However, from Figure 18a, you can see that the rate of weight gain immediately began to change when the rats returned to Earth. Figure 18b is an expanded version of the postflight mass changes seen on Figure 18a. In fact, during the postflight period, the VIV-S (control rats housed singly in a rat cage called a vivarium) gained weight faster than the other groups. Also, both control groups gained significantly more weight than both flight groups in the postflight period. In particular, the flight animals exhibited a suppression of growth immediately after flight. However, after the second day back on Earth, all four groups continued their growth in a parallel fashion.

Figure 19.

Dr. Holton also measured the length of certain antigravity bones, the tibia, the femur, and the humerus. These are considered the major long bones in the rat. Figure 19 is a graphical representation of the differences in postflight length measurements among the different bones. The tibia, femur, and humerus exhibited significant growth in length between launch and R+ML (Recovery or landing + mission length or 9 days). However, bone length was similar in all groups throughout this time.

Dr. Holton's team also measured the bone mass of the antigravity femur and two bones that are not used to oppose gravity, the first lumbar vertebra and one of the ribs of the animal. In reality, Dr. Holton processed the bones to dissolve the fat, so the measurements were not really of the bone mass, but instead, were of the fat free weight of the bone (in mg). It is referred to as bone mass, however.

Figure 20 indicates that bone mass in the vertebrea and rib did not appear to be affected by the flight. However, the femur in the individually housed flight rats (RAHF-F) was significantly smaller than its control (VIV-S) after R+ML. On the other hand, group-housed animals showed no differences in bone mass of the femur, whether on Earth or in space. In other words, the AEM-F rat femurs were very similar to the AEM-C rat femurs. What do these results suggest? Let's summarize the findings.

  • The rats grew at a constant rate (linearly) throughout the flight period. This changed, however, during the postflight period when the rate of growth for the flight groups was significantly lower.
  • The length of the rat's long bones continued to increase at a steady rate. The different housing arrangements did not seem to affect bone elongation.
  • The bone mass for the vertebra and the rib was not affected by flight.
However, the RAHF-F femurs were significantly smaller in mass than similarly housed ground controls and were slightly smaller than the group-housed rats. The data suggests that singly-housed rats show a decrease in accumulation of bone mass during flight that becomes significant during the postflight period, compared with similarly housed controls. In terms of the femur results, the length remained the same but the bones weighed less for the animals that flew in space. This might suggest that the bone structure is different. We will see in the next section how the diameter of the bone was affected by space flight and we will examine if the bone continued to accumulate minerals at the same rate.

Finally, we will examine how any changes in the mineralization rate of long bones in the rat might affect the strength of those bones.

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