Complicating Factors: Issues Relating to Romance and Reproduction During Space Missions

Kira Bacal, MD, PhD, MPH

Disclosures

January 02, 2009

Physiological Issues

Numerous physiological changes have been noted during spaceflight, many of which may affect sex and procreation, although it remains unclear whether such effects are due to gravity changes, radiation, noise, vibration, isolation, disrupted circadian rhythms, stress, or a combination of these factors.[3,13] What is known is that "virtually every organ system functions differently" in space,[23] and "existing data suggest that spaceflight is associated with a constellation of changes in reproductive physiology and function."[24] A great deal of research still needs to be done on the mammalian development cycle, from copulation to conception to birth, and growth through reproduction by the next generation.[25,26]

When the issue of off-Earth reproduction arises, immediate concern must be given to the most obvious difference: the lack of a 1G gravitational field. Ronca writes, "life on Earth, and thus the reproductive and ontogenetic processes of all extant species and their ancestors, evolved under the constant influence of the Earth's 1G gravitational field...[It is thus imperative to determine] how the space environment affects critical phases of mammalian reproduction and development, viz. those events surrounding fertilization, embryogenesis, pregnancy, birth, postnatal maturation, and parental care."[27] If, as postulated, gravity affects the regulation of mammalian gene expression (and therefore all aspects of vertebrate development, including cell structure and function, organ system development, and even behavior),[28,29,30] then there are significant implications for successful procreation in an extraterrestrial environment.

Effects of Space on Fertilization

Reproduction begins with the existence of viable gametes. In a very small sample (n = 4) of male astronauts, significant decreases in testosterone levels and sexual drive during flight were documented.[31]

It is known that spermatogonia are radiosensitive.[3,13,25] However, research data from animal models are mixed. Some studies found evidence that male fertility is not diminished in space,[32] while others using rats and wasps suggested that there are negative impacts,[33,34,35] including reduction in testosterone during flight.[24] Research aboard Cosmos 1887 indicated that rat gonadal function may be compromised, and there was a decrease in rat spermatogonial cells noted in research performed aboard the Space Shuttle (STS-51B).[35] Biosatellite II documented changes in wasps' mutation frequencies and sperm cells, as well as disorientation in male wasp mating behavior post-flight.[35]

In terms of female fertility, it is known that ovarian function can be adversely affected by environmental conditions.[24] Research in rats, however, suggested that females ovulated and cycled normally in space, even though no births resulted from mating.[36] Unfortunately, data on the humanovulatory cycle in space is limited.[13] Many female astronauts have used contraceptive drugs to suppress menstruation on orbit during both short and long-duration missions.[13,24,37] To date, this has remained a reasonable method by which female astronauts have avoided the logistical challenges of menstruating on orbit, but it means that relatively little is known about how fertility cycles may be affected by the space environment, including the impact of flight-associated circadian changes.[3,25]

Research flown on the Cosmos 1129 satellite showed no evidence that rats mated during flight, despite opportunities to do so.[35] This may be due to heightened stress or diminished ability, as well as the logistical difficulties associated with microgravity. Regardless of cause, the data suggest that mating and reproduction in space may be difficult.[7]

Assuming that functional sperm are introduced to a viable ovum, the next question is whether fertilization will occur normally in the space environment. There is some evidence that sperm (and other flagellates) swim faster in microgravity and are otherwise sensitive to small changes in gravitational forces, presumably due to microgravity-associated biochemical changes in the axonemal proteins.[38,39] Some research suggests that fertilization may be unaffected,[40,41] but other studies indicate that the space environment has deleterious effects.[39] For example, although Aimar and colleagues found that fertilization in a microgravity environment did occur in their animal model, they also noted that "several characteristics of the fertilization process" were different in space.[40] Mammalian studies of fertilization in simulated microgravity showed no statistically significant differences in vitro, but in vivo studies showed significant decreases in embryo survival to the morula and blastocyst stages, suggesting that spaceflight may cause a higher rate of embryo lethality.[42]

If fertilization is successful, will implantation occur? If so, where? Is the zygote more likely to implant in an inappropriate location without the influence of gravity?[3] Should the in-flight medical system be designed to treat an ectopic pregnancy? If implantation is successful, will the placenta develop properly? There is limited evidence that the development, growth, and function of the placenta in rats may not be affected by a brief exposure to the space environment during gestation.[43]

Effects on Embryogenesis

Embryogenesis is another concern. There is good evidence that the space environment is teratogenic.[25,41,44,45,46] Concerns include exposure to toxins in the "closed loop" cabin environment, risk of decompression, and the largely unexplored effects of micro- or partial-gravity on embryogenesis. Quail eggs flown aboard Mir 18, Mir 19, and STS-76/77 demonstrated increased rates of developmental abnormalities and mortality.[35] Although some of the observed problems may have been due to equipment difficulties on specific flights, there was an absence of normal angiogenesis. More recent studies performed in a simulated microgravity environment demonstrated embryonic failure between days 0-5 in avian eggs,[13,47] as well as other changes to the normal development process.[47] Wasp studies on Biosatellite II documented an excess of deaths in offspring born to flown females, suggesting that there are lasting, lethal changes associated with the space environment.[35]

Unfortunately, much of the data on the impact of the space environment on embryo development is mixed or inconclusive. The NIH.R2-1 study showed that brief exposure to microgravity did not appear to affect certain aspects of bone development in rats,[43] but other research documented several differences, including decreased length of calcified long bone regions.[44]

Other organ systems are also affected by changes in gravitational forces, including the vestibular and neuro-endocrine systems.[43] Bruce and Fritzch agree that "gravity appears to be a critical factor in the normal development of the vestibular system" of rat embryos, where "synaptogenesis in the medial vestibular nucleus is retarded" by exposure to microgravity during development.[48]

In other animal models, abnormalities were found in the development of Aurelia jellyfish and frogs.[43] Gualandris-Parisot also reported abnormalities in salamander embryos,[49] although these changes were noted to be reversible upon returning to 1G. However, the fact that the space environment had effects on embryo formation, especially during cleavage and neurulation, suggests that changes may occur in many animals, not all of whom may tolerate the differences as well as the salamander appears to do.

Shimada et al discovered a developmental period during which time the lens in zebrafish was susceptible to gravitational changes, suggesting that gravity does affect gene expression and differentiation in the lens during specific developmental periods.[29] Additional work documented similar changes in other systems,[28,50,51] leading the authors to conclude that "exposure to microgravity can cause changes in gene expression in a variety of developing organ systems in live embryos and that there are periods of maximum susceptibility to the effects."[28]

Astronauts and rats exposed to microgravity have experienced endocrine imbalances, such as hypothyroidism.[52] Such abnormalities in a pregnant woman can have significant effects on the fetus. For example, cerebellar development is regulated by thyroid hormones, and deficiencies in the neonatal brain can lead to cretinism and other abnormalities.[52] By contrast, other studies have suggested that spaceflight initiated during the post-implantation phase of pregnancy does not affect ovarian-hypophyseal function in rats.[53]

Of particular concern are findings that exposure to spaceflight at certain times and for certain durations in young, postnatal rats led to long-term problems in their motor function.[54,55] This is consistent with well-established notions of "critical developmental periods," during which specific events are required for the normal development of certain systems.[50,55] If these events (such as exposure to a 1G field) do not occur, irreversible, long-term deficits may result. This suggests that even after birth, an infant's normal growth and development can be profoundly affected by the space environment. Similarly, other research suggests that interactions between rat mothers and their neonatal offspring are altered in the space environment,[27] which may also lead to suboptimal development. Unfortunately, there has been very little research on development after birth in microgravity,[25] and a great deal more will be required.[56]

Space radiation, including galactic cosmic rays, solar particles, and geomagnetically trapped particles, is another important factor in embryogenesis.[57] All astronauts, including those in low Earth orbit, are classified as radiation workers, and their exposure increases as exploration missions move beyond the protective Van Allen belts. For example, the crew on a Mars mission will likely exceed a radiation dose of 50 rem/year,[37] while crew members on the International Space Station reportedly receive about 25-40 rem/year[37] or 1 mSv/day, roughly 150 times the value absorbed on Earth.[57]

Guidelines recommend that pregnant women be exposed to no more than 0.2-0.5 rem, due to evidence that exposure to greater than 10 rem can lead to fetal defects, such as microcephaly and retardation.[37,58] Radiation is known to cause DNA damage, cell death, and chromosomal abnormalities, as well as developmental deformities.[57] Some studies suggest that a synergistic effect may exist between radiation and microgravity, leading to greater than expected abnormalities from either factor alone.[57] One theory posits that the microgravity environment impairs cellular repair mechanisms that would otherwise be better able to address the radiation-induced DNA damage. However, other studies challenge these findings, leading to difficulties in developing reliable guidelines.

Even if the fetus can develop normally in the space environment, many potential dangers remain. How will the physiological adaptations to pregnancy interact with the physiological adaptation to microgravity?[3,23,25] Are they synergistic, additive, or oppositional? Rat studies suggest that pup mass at birth was significantly decreased by spaceflight,[59] leading to concerns of low birth weight or growth-retarded babies. Hyperemesis gravidarum and space motion sickness could certainly combine to threaten a pregnancy, as could the cardiovascular changes, bone demineralization, and alterations to red cell mass.

Other studies mated post-flight male rats with non-flown females. The offspring displayed both growth retardation and higher rates of abnormalities, such as hemorrhage, hydrocephaly, and renal malformations.[24] Male progeny were also noted to have reduced epididymis weight at 30 days of age, though not at 100 days.[24] These effects were apparently transient, because when the flown rats were mated approximately 3 months after spaceflight, no such abnormalities in offspring were seen.[24]

Although both male and female human astronauts have produced healthy offspring after spaceflight, studies suggest that the space environment may have transient post-flight effects on reproductive health. Further research on this topic would be of benefit to astronauts and their families, particularly because these issues could persist if crews leave microgravity for a partial gravity environment, such as the moon or Mars, rather than returning to Earth.

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