Earth, Mars, and outer space all differ in their gravitational characteristics. Of course, we are all familiar with the force of gravity here on Earth, and our bodies are well adapted to moving around in this kind of environment. Even though humans have never set foot on Mars (although we hear periodic stories about Martians who live there), we know from remote experiments that the gravitational force there is equivalent to about 38% of Earth's gravity. The gravitational force on Mars would require that we support our bodies to stand, walk, and run. But, of course, our weight would be lower on Mars and this means that our musculoskeletal system would not experience the same loading characteristics that it does on Earth (Figure 10). Presumably, then, our bones and muscles would become weaker on Mars if we did the same kind of things there everday that we do here on Earth.
In the microgravity of outer space, the musculoskeletal system is used even less intensively and in a different way than it is on either Mars or Earth. The absence of gravitational force results in changes to load-bearing tissues, causing a reduction of bone and muscle. We know this from previous studies that have been done on long-duration missions such as Skylab (28-, 59-, and 84-day missions) and from research carried out on very long missions aboard the various Soviet/Russian space stations (missions lasting up to a year). Astronauts in space "float" from place to place instead of using their legs to walk around. In addition, the metabolic state of the musculoskeletal system may be altered by changes in dietary intake and exercise levels, and also by the space motion sickness that many people who go into space experience at the beginning of their trip. The physiological changes that occur within the astronauts while in space, however, are appropriate for space flight. Remember, the body responds to the environment that it finds itself in, so in space, the body establishes a space-normal condition. Any problems that occur only appear when the astronauts return to Earth.
The muscle and bone losses, primarily from weight-bearing tissues, contribute to a reduced fitness of astronauts when they return to Earth. And as you know by now, changes in other body systems have occurred as well. In previous chapters, we have examined how the cardiovascular system, blood, kidney and endocrine, and muscle systems have been affected by space flight. In this chapter, we will focus on the effects of space flight on the skeletal system.
Once the astronaut returns home, the body will respond again to the "new" environment of Earth. The body's goal is to reach an Earth-normal condition as soon as possible. Therefore, in order to study how the body has changed while in space, it is essential that experiments be performed as early as possible after the shuttle has landed. In the case of bone, the physical changes that take place begin to recover slowly. However, the chemical changes that have taken place probably begin to reverse themselves immediately upon return to Earth.
There are two main reasons, then, that laboratory rats offer researchers a valuable opportunity to study bone changes that have occurred either here on Earth or in space. First, the bones of the animals are accessible. That is, the bones can be removed from the rats and studied directly. Secondly, the rats are small enough that their bones react much more quickly to environmental changes. Their bones grow much more rapidly than human bones and, therefore, the bones also change more rapidly. This occurs because rats have a much shorter lifespan than humans and all of thier life processes, including bone formation, are accelerated. In particular, this becomes very important if we want to study bone changes that have occurred on short space missions. In contrast, the bones of astronauts are not accessible directly nor will they change as dramatically as the rat bones will. The use of laboratory rats, then, magnifies the view that researchers can obtain about how the bones are affected by changes in external forces.
Another advantage to using laboratory rats, instead of larger animals like monkeys, for the study of bone changes is that they can be housed in different kinds of cages on the same space mission, thus allowing one to see how cage configuration (rather than space flight itself) might affect their bone growth. For instance, Dr. Holton was interested in determining how bone changes in space would be influenced by group housing (five rats to a cage) as compared with individual housing (one rat per cage). The results of such a study could be important in understanding how to provide the best and most natural conditions possible for rats used for research purposes, and in determining that the pure effect of space flight on the animal's bones was being measured.
Dr. Holton carried out her bone study using laboratory rats on two different space missions. Her primary hypotheses that served as the basis for her experimental design included:
Gravity is necessary for normal development of bone structure and decreased gravity or skeletal unloading causes defective skeletal growth.
The response of bone to space flight will be localized and will differ not only from bone to bone, but also at different sites within the same bone.
The type of housing (group vs. individual) will influence the bone response to space flight and the recovery from space flight.
Before we begin our examination of Dr. Holton's space flight results, let's participate in two "Student Investigations" designed to clarify certain important concepts from this chapter. These activities will prepare you to understand more about bone structure and strength, and besides, they should be fun!