Hyperthermia is more important than hypoxia as a cause of disrupted spermatogenesis and abnormal sperm
Introduction
Testes must remain cooler than body temperature for production of normal, fertile sperm in mammals [1,2], with increased testicular temperature having deleterious effects on sperm production, sperm motility and proportion of morphologically normal sperm [3,4]. The long-standing explanation is the testis operates on the brink of hypoxia under physiological conditions (testicular temperature a few degrees cooler than core body temperature), elevated testicular temperature increases metabolism with a concurrent need for more O2, but blood flow does not significantly increase and consequently hypoxia, secondary to increased testicular temperature, is the major cause of heat-induced changes in spermatogenesis [[5], [6], [7]].
Effects of hyperthermia on testicular function and fertility have been studied in laboratory and farm animals, whereas effects of hypoxia have been studied within the contexts of disrupted blood flow and hypobaric hypoxia, as models of testicular torsion and living at high altitudes, respectively [8,9]. Despite an apparent paucity of studies concurrently examining both conditions, there is impetus for doing so. Male mice exposed to an ambient temperature of 36 °C had increases of ∼1 °C in body temperature and ∼5 °C in testicular temperature at 12 h after the onset of exposure [10]. In a subsequent study [11], male mice exposed to 36 °C for two 12-h intervals (on successive days) and mated to females 10 or 14 d after exposure had significantly lower pregnancy rates and litter sizes. Regarding O2 concentrations, atmospheric air is ∼21% O2, whereas 10.8 and 16.0% O2 in inspired air constituted hypoxia and mild hypoxia, respectively, in rats [12]. Mice breathing air with 12.5, 15.0, 21.0, and 100% O2 had testicular 02 concentrations of 16, 24, 36, and 102 μmol/L [13]. Similarly, breathing 100% O2 reportedly doubled O2 saturation in rat testes [14]. Based on those data, there is an apparent association between O2 content of inspired air and O2 content of the testes, enabling testicular O2 content to be varied from approximately 50 to ≥200% of physiologic concentrations.
The ability to independently alter testicular temperature and testicular O2 content provided a novel opportunity to critically test effects of hyperthermia and hypoxia on spermatogenesis. In a recent ram study [15], insulating the scrotum (testicular hyperthermia) caused expected decreases in motile and morphologically normal sperm. However, it was noteworthy that these effects were neither replicated by hypoxia nor prevented by hyperoxia. To our knowledge, a similar study has apparently never been conducted in rodents. The objective was to determine relative effects of hypoxia versus hyperthermia on sperm quality and production. We tested the hypothesis that hypoxia replicates effects of hyperthermia on reducing number and quality of sperm produced, whereas hyperoxia mitigates effects of hyperthermia.
Section snippets
Experimental design
There were six treatment groups in a 3 × 2 complete factorial design; the factors were O2 concentration (13, 21 and 95% O2) and temperature (20 versus 36 °C). Note that 21% O2 and 20 °C represent atmospheric air and room temperature, respectively, whereas 13% O2 and 36 °C represent hypoxia and hyperthermia. Since inspired air containing 100% O2 is somewhat toxic for mammals [16], CO2 was added to reduce O2 concentration to ∼95%.
Mice and exposure conditions
Male mice (CD-1, ∼50 d of age; n=48) were used. To minimize
Results
There were effects of temperature on most end points involving testes and sperm, including reductions in weights of testis (P < 0.0001) and cauda epididymis (tendency) and in total and progressive motility, percentages of morphologically normal sperm and those with head or tail defects, epididymal sperm reserves (total and per gram), and daily sperm production (Table 1). Only a few histological end points were significantly affected (Fig. 1, Fig. 2). Seminiferous tubule diameter was lower at 13
Discussion
Our hypothesis was not supported; sperm quality and production were not consistently disrupted by hypoxia, nor were hyperthermia-induced disruptions prevented by hyperoxia. Similarly, in a recent ram study [15], insulating the scrotum (testicular hyperthermia) caused expected decreases in motile and morphologically normal sperm. However, as in the present study, in general, these effects were neither prevented by hyperoxia nor were they broadly replicated by hypoxia.
In the present study, mice
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgements
We thank the animal caretakers at the Lethbridge Research Centre vivarium for their care of the mice and other assistance.
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2020, TheriogenologyCitation Excerpt :However, we have substantial evidence challenging this paradigm. In rams [8] and mice [9], whereas hyperthermia (testicular or systemic, respectively) decreased percentages of morphologically normal and motile sperm, breathing hyperoxic air did not prevent effects of hyperthermia, nor did breathing hypoxic air replicate these effects. In Angus bulls, as ambient temperatures increased from 5 to 35 °C, there were increases in testicular temperature (mean ± SEM, 31.8 vs 34.9 °C; P < 0.01) and blood flow (2.45 vs 4.23 mL/min/100 g testis, P < 0.05; [10]).