A study examined the effects of microgravity (µG) environment on human sperm motility. As The Space age progresses, they need to determine whether humans can pursue a normal life in space, including reproductive function. They have been studying the effects of the environmental factors in space that are different from those on earth, such as gravity, solar rays, ultraviolet rays, electromagnetic waves, temperature, and radioisotopes.

They have been concerned with the effect of a long?term stay in space on the human body, and have examined the effect of a µG environment on human sperm motility. The parachute method (drop from a tower or balloon) and parabolic flight have been used to simulate a µG environment on earth. By these methods, a gravity of 1 × 10−3 G can be produced. The clinostat, water submersion, and suspension methods have been used to simulate a µG environment where the G level on the individual is unchanged.

THE EFFECT OF an outer space environment on reproduction must be considered from some viewpoints. They can anticipate some effects of a vacuum environment on fetal growth, the effects of temperature and humidity on the fetal growth and reproduction, the possibility of genetic damage by solar, cosmic rays and electromagnetic fields, and the impact of stress on fetal growth and hormone balances.

There are also major concerns related to the inhibition of spermatogenesis as a result of decreased testicular blood flow caused by the movement of body fluid to the head in males, as well as decreased testosterone levels under µG and their effects on fertilization and embryogenesis. As we enter a cosmic age, the effect of µG on reproduction is a major topic. In the present study, they examined human sperm motility in a µG environment.

In clinostat experiments, the effect of a µG environment on sperm motility tended to decrease, although the decrease was not significant. There was also no significant difference in sperm motility between the stationary control and rotation control at various r.p.m. speeds. There was no signification difference between the stationary control and clinostat rotation. They used frozen sperm to examine sperm motility using a sperm motility analyzer under µG and normal gravity.

They reported that the motility of the more rapid sperm increased significantly under µG. The observed increase in sperm motility was thought to have possibly been caused by hyperactivation is seen immediately before fertilization as a result of problems caused by the solvent and the rotation stimulus.

Furthermore, as motility tended to be reduced in parabolic flight experiments, they consider human sperm motility to be lowered by microgravity. One reason for the decline in sperm motility might be a chemical change in the intracellular environment. 

Further study is needed, not only into the effect of gravity on the various processes of reproduction, for example, fertilization, early embryogenesis, gestation, and delivery, but also the problems of cosmic rays and stress in space. Our findings suggest that sperm motility is reduced under µG.