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Does space travel impact the human reproductive system?

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Does space travel impact the human reproductive system?

Recently, scientists have reviewed the prevailing literature to grasp how space travel, space radiation, and microgravity affect human reproductive functions. This review was published in Npj Microgravity.

The relevant information for this review was obtained from many databases, reminiscent of Pubmed, Medline Complete, and Web of Science. A complete of 364 studies were identified within the initial search. Nevertheless, after removing the duplicates, 16 studies fit all eligibility criteria and were included in the present systematic review.

Study: The effect of space travel on human reproductive health: a scientific review. Image Credit: Supamotionstock.com/Shutterstock.com

Why is it vital to grasp the impact of space travel on human reproduction?

It can be crucial to grasp whether participating in multi-year missions in deep space or colonizing other planets has hostile health effects, including human reproduction. Before conducting space travel, it’s imperative to implement all safety measures.

During space travel, astronauts are exposed to many hazardous aspects, reminiscent of ionizing radiation, hypogravity, and hypergravity. Previous studies have indicated that exposure to microgravity damages the endocrine system in males.

As well as, it affects bone and muscle mass, alters electrolyte balance and the cardiovascular system, and enhances glomerular filtration rate in each sexes. 

Ionization radiation is roughly 500 times greater in space than within the Earth. Animal model studies have shown that exposure to ionization radiation causes DNA damage, sperm DNA fragmentation, and apoptosis in ovarian follicles.

Not many studies can be found related to the impact of space travel on the reproductive system. Nevertheless, several studies have been conducted to grasp the effect of space travel on endocrine regulation of sex hormones.

These alterations in hormone levels impacted skeletal muscle protein metabolism and musculoskeletal health.

Understanding the impact of space travel 

a) Females

A previous study based on female mouse models has shown that microgravity adversely impacts the expansion and differentiation of embryonic stem cells. This results in the impairment of the decidualization of the endometrium, an important factor for implantation and normal pregnancy. 

It was estimated that exposure to space radiation during a typical Mars mission could decrease ovarian reserve by 50% by damaging primordial follicles.

Moreover, exposure to space radiation could also decrease the reproductive capability of girls astronauts by reducing the time interval for menopause onset. Exposure to fifteen Gy radiation causes lack of ovarian function in humans.

Animal models using female rats revealed that the luteinizing hormone was reduced in conditions just like thirty-seven-day space flights. Nevertheless, no changes within the estrous cycle stages were found.

Microgravity caused a 12% and 15% reduction in luteinizing hormone and progesterone, respectively. This may very well be because of dysfunction of the corpus luteum.

No change in menstrual cycle length was related to microgravity. It have to be noted that although abnormal uterine bleeding is a standard problem of reproductive-aged women, research has not analyzed whether this problem is enhanced during space travel.

Typically, during space flight training or actual space flight, women astronauts undergo medically induced amenorrhea using combined oral contraceptives containing progestin and estrogen. 

The aforementioned combination of oral contraceptives has been related to higher concentrations of transferrin, lower circulating concentrations of albumin, and enhanced levels of inflammation markers. These conditions enhance the potential for venous thromboembolism during space flight.

b) Male

Male rats exposed to microgravity exposure exhibited a decrease in total sperm count, testosterone concentrations, and testes weight in comparison with the healthy or control male rats. 

Echinogammarus marinus models also revealed that exposure to ionizing radiation causes DNA fragmentation that affects the male reproductive system.

In humans, therapeutic radiation higher than 1 Gy may lead to azoospermia. This level of radiation also enhances the danger of hereditary disorders.

When men with rectal cancer are treated with radiotherapy, their testes are sometimes exposed to direct or scattered radiation, which in turn ends in reduced serum testosterone levels.

Microgravity was found to affect the sperm motility negatively. Nevertheless, in the course of the transportation of male human gametes into space, it was observed that sperm integrity was protected by cryopreservation.

Future outlook 

Within the case of female astronauts, pilots, cabin crews, and frequent flyers, the radiation may exceed the really useful level. Hence, it’s important to grasp the impact of excess radiation exposure on their overall health.

Moreover, studying the impact of multi-year duration space flights or colonization on pregnancy, particularly fetal growth, is imperative. 

The fate of the offspring conceived and developed in space have to be analyzed in future research.

Understanding whether microgravity and space radiation impacts the pro-oxidant/antioxidant balance while pregnant is vital. An altered balance could increase the danger of miscarriage, pre-term birth, and inappropriate fetal growth.

In the long run, space radiation’s effect on fresh and frozen semen samples have to be studied. This study could provide insights into the potential for developing a human sperm bank outside the Earth.

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