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Bone Physiology Introduction

It is now 39 years since the first manned orbital flight by the Russian cosmonaut Yuri Gagarin confirmed that it was possible for humans to survive under weightless conditions. During this period the duration of exposure to microgravity conditions has lengthened, and because of this we have gained greater insight into the deleterious effects microgravity conditions have on the musculoskeletal system. The evolution of the skeletal system is based on the minute-to-minute interaction of gravity acting through the mechanical strain exerted by muscle on bone, and the reaction of the bone cellular remodeling system to constantly changing levels of mechanical strain. Changing patterns of skeletal remodeling permit growth and development of our supporting structures from infancy through old age. Bone remodeling refers to cellular processes determining both bone formation and bone resorption. Bone mass thus reflects the sum of these two opposing processes. It is through bone formation and bone resorption that bones grow and change shape during our lives.

This teaching module will examine the skeletal responses of humans and animals to exposure to gravito-inertial forces less than those on Earth under which maintenance of the skeletal system normally occurs. Data derived from both earth-bound and space flight experiments will be referenced. The operational target for this presentation is an understanding of the potential hazards to astronaut health and safety during exposure of the skeletal system to microgravity conditions for extended time periods as might be encountered during extra-terrestrial space travel. Although the focus is on the extreme environment of space flight, it is important to recognize that conditions similar to weightlessness occur on Earth. Individuals with medical disorders leading to immobility include the aged, who, when confined to bed rest, lack the mechanical strain on bone, which leads to progressive bone loss and increased risk of fracture. Thus, lessons learned from the study of microgravity conditions will lead to improved medical care for many of the chronically disabled on Earth.

Space flight leads to loss of bone mass by altering the normal process of bone remodeling. Decreased bone mass is associated with an increased risk of fracture, which imperils both astronaut health and the ability to carry out essential activities during flight and during habitation on another planet. Bone mass in the vertebral bodies of the spine or in the limbs can be measured by several methods. Standard x-rays of the spine or limbs are not sufficiently sensitive to detect small but significant changes in bone mass. The methods most commonly used today are: 1) dual energy x-ray absorptiometry or DEXA, and 2) ultrasound methods, which have been the object of recent development. The use of DEXA measurements in flight crews before and after varying periods of exposure to weightlessness indicates that average bone loss approximates 1-2% per month. However, there is great variation from person to person in the extent of loss both for total body bone mass and for loss at specific skeletal sites such as the lower (lumbar) spine and proximal femur (hip). It is essential that we understand the basic mechanisms governing bone formation and resorption at Earth's gravity and in microgravity conditions. It is only in this manner that effective countermeasures to bone loss can be developed for the protection of flight crew.

Teaching Goals:

The objective of this module is to promote an understanding of the mechanisms by which microgravity impairs normal bone structure and bone cell function, the methods used to study these processes, and the countermeasures that can be employed to counteract the negative effects of microgravity on skeletal tissues. Specific topics for study include:

This module should orient the student to basic bone physiology and earth bound models of current use for studying the changes in bone during microgravity. The relationship of calcium metabolism to microgravity-related changes in bone will be explained.

The student should appreciate that the topics listed above are targeted to microgravity, but, in principal, extend beyond microgravity to common earth-bound situations. These include individuals immobilized due to injury, subjects with spinal cord injury and loss or arm and leg function, and the elderly at chronic bed rest. It is likely that similar mechanisms leading to bone loss operate in each of these situations, and that an understanding of mechanisms in one will assist in limiting the hazards related to bone loss in the other.

Discipline Lead

Jay R. Shapiro, M.D