Gestational exposure to high heat-humidity conditions impairs mouse embryonic development

Written by: Mobashar Ahmad

Undergraduate student (3rd year), Department of Genetic Engineering And Biotechnology.

University of Rajshahi. Rajshahi, Bangladesh.

Background

Our planet experiences rapid rise in temperature, accompanied by a concerning increase in the frequency of heat waves and extreme humidity (1) (2) (3). Climate change poses an existential threat to species––the Bramble Cay melomys, a species of rodent, is just one tragic example, already driven to extinction by its effects (4). Studies predict that high heat-humidity may elevate the rates of human health concerns including pregnancy issues, birth defects, and late onset disorders (5) (6) (7). To gain a better understanding of the negative effects of heat-humidity on human health and to develop effective strategies to combat these, robust scientific investigation is necessary. The authors of this preprint used mice as their model for investigating the effects of high heat and relative humidity on pregnancy, and whether it recapitulates the adverse outcomes of heat and humidity observed in humans.

Methods in a nutshell

The authors randomly divided pregnant mice into two groups for every experiment. The treatment group was treated with high heat and relative humidity for three hours per day from the beginning of their pregnancy. The control group was kept at room temperature with normal humidity for the same amount of time as the experimental group. Both groups of mice then received standard care (optimum temperature, humidity, food, and water) for the remainder of the day. The investigators maintained the procedures for the entire pregnancy to observe the overall outcomes of the situation on newborn mice. They also explored how the treatment affected embryos at different time points during development. To examine this, they nurtured two groups of pregnant mice until their embryos reached the embryonic stages of 3.5, 8.5, 10.5, 13.5, and 18.5 days. Then they followed standard procedures to sacrifice mice, dissect wombs, and extract embryos for comparing embryonic effects.

Key findings

Impacts on overall growth

Mice, that experienced high heat and relative humidity during their full pregnancy, had significantly fewer babies. Moreover, the researchers found differences in the weights of the newborns between the treatment and control groups. Newborn mice that developed in the womb of the treatment group weighed much less. Also, maternal weight gain differed significantly between the two groups after passing the half-period of pregnancy. The preprint authors concluded that high heat and humidity interferes with the reproduction of mice. 

Impacts on embryonic developments

In the second phase of the study, the researchers compared embryos from two groups of mice (treatment vs. control) at five different stages of development. There were no significant differences in number or appearance between treated and control embryos on days 3.5 and 8.5 of pregnancy. By day 10.5 of pregnancy, some treated mice completely lost signs of pregnancy, while some other mice had dead and severe deformed embryos. The same trend of increasing death and defects continued in this group at embryonic day 13.5, compared to the controls. By embryonic day 18.5, the effects of high heat and humidity were most evident in the treated group. Half the embryos did not survive, and a notable number of the remaining embryos showed severe abnormalities.

This preprint shows the cumulative impact of elevated temperatures and humidity on mouse embryonic development. In the later stages of embryonic development, these factors demonstrably contribute to a significant rise in mortality and birth defects. These findings echo the reproductive challenges observed in human populations living in tropical regions.

Why I chose this preprint

I am interested in how teratogens––agents causing birth defects upon exposure during pregnancy––interfere with normal embryonic growth mechanisms. This question could lead us to better understand human development. I like this preprint because the authors use simple, but effective experiments to show that high temperature and relative humidity could play a role as teratogens, which is particularly relevant in the light of global warming. We really need to understand the specific adverse impacts of rising temperatures and humidity on future generations, both in terms of prevalence and underlying mechanisms. The research presented in this preprint serves as a springboard for further molecular and systemic investigations into the mechanisms underlying the adverse effects of heat and humidity as well as the development of mitigation strategies.

Questions for the authors

1. How did you select the temperature, humidity, and treatment duration parameters for this study? Do you think the choice of temperature regime will limit the applicability of the study to other animals?
2. How would you link your results––obtained in mice bred in a laboratory environment––to human pregnancy? If we were to compare it with human pregnancies, which stages would you predict are most vulnerable to heat and humidity? Is there data in the literature that indicates similar trends?
3. A bit of a technical question: why did you use a decreased size of control and treatment while measuring pup weights (Figure 2F)? How did you select the pups that were measured?

References

1. Lindsey R. & Dahman L. (2024, January 18). Climate Change: Global Temperature | NOAA Climate.gov
2. NOAA (National Oceanic and Atmospheric Administration). 2022. Heat stress datasets and documentation. Provided to EPA by NOAA in February 2022. epa.gov
3. Raymond et al., The emergence of heat and humidity too severe for human tolerance. Sci. Adv. 2020; 6, eaaw1838. https://doi.org/10.1126/sciadv.aaw1838
4. Watson J., Bring climate change back from the future. Nature. 2016; 534, 437. https://doi.org/10.1038/534437a
5. Edwards et al., Effects of heat on embryos and foetuses. Int J Hyperthermia. 2003; 19(3):295–324. https://doi.org/10.1080/0265673021000039628
6. Roos et al., Maternal and newborn health risks of climate change: a call for awareness and global action. Acta Obstet Gynecol Scand. 2021; 100(4):566–70. https://doi.org/10.1111/aogs.14124
7. Bonell et al., An expert review of environmental heat exposure and stillbirth in the face of climate change: Clinical implications and priority issues. BJOG. 2024; 131(5):623–631. https://doi.org/10.1111/1471-0528.17622

 

doi: https://doi.org/10.1242/prelights.37819

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