The space environment induces tissue degeneration
Astronauts in space are exposed to several different gravitational environments that affect basic biological processes. The majority of human spaceflights have been conducted in low Earth orbit (LEO), which is approximately 160–2,000 km above the Earth's surface , still protected from charged particle space radiation. Normal terrestrial gravity (1 g) forces objects to accelerate toward the center of the Earth. The Earth’s surface resists the downward acceleration of gravity, and it is this force that shapes the nature of our musculoskeletal system and how it supports our body. Therefore, on Earth, organisms are constantly subjected to contact forces that provide an array of mechanical stimulation essential for the function of many physiological systems. The influence of these mechanical contact forces on the human body is especially evident in the effects of physical exercise loads on the weight-bearing skeleton. Increased load during weight-bearing exercise causes increased musculoskeletal growth to enable the body to withstand these increased forces. For orbital flight around the Earth, the force of the Earth's gravity keeps the spacecraft moving in an orbital path. Although zero gravity is not experienced in LEO, near-zero contact forces are exerted by the spacecraft on its inhabitants, or in other words, the astronauts and the contents of the spacecraft are in a state of free fall in orbit around the Earth , resulting in gravitational force of approximately 1 × 10−6 g, microgravity. In space, this lack of normal gravity and resulting loss of mechanical stimulation of cells and tissues are responsible for many of the physiological problems that astronauts experience—from space motion sickness and otolith dysfunction, to cardiovascular, bone, and muscle degeneration. Specifically, as astronauts have journeyed in microgravity, the spaceflight gravitational environment revealed many immediate tissue degenerative consequences for life. These include bone loss [2–9], muscle loss [2,10–15], loss of cardiovascular capacity [16–19], possible defects in wound [20–22] and bone fracture healing [23–25], and impaired immune function [6,26–31]. The majority of space biological experimentation was conducted on the Space Shuttle, which supported short-term (1–2 weeks) experiments on spaceflight effects. In particular, research concentrated on the more noticeable changes that occur as an adaptation to moving from loaded conditions at 1 g to unloaded conditions in microgravity. In the long-term, however, microgravity may also affect normal ongoing tissue regenerative growth and repair, a process dependent on the proliferation and differentiation of tissue-specific adult stem cells that are progenitors of mature terminally differentiated cells in most tissues. New investigations on ISS are now focusing on longer-term experiments that are becoming possible with ISS completion and ISS-resident experimental biology hardware.