Direct and indirect effects of whole body heat exposure on germ cells and spermatozoa

Abstract

Exposure to extreme temperature conditions such as occurs in certain occupations is known to induce male infertility. In humans and most of the eutherian mammals, it has been shown that whole body heat stress decreases fertility and produces defective embryos. However, the mechanisms producing fertility defects post exposure to whole body heat are not yet fully determined. Hence, the present study aimed at gaining some insight into the mechanisms producing fertility defects after whole body heat stress. Laboratory mice were exposed to constant temperatures higher than their core body temperature of 37-38°C for 8h on three consecutive days. A decrease in testis weight occurred as early as 7 days post exposure to heat with weights comparable to controls by 21 days post heat exposure. Histology of the testicular tissue showed germ cell apoptosis affecting the pachytene spermatocytes and the spermatids. Germ cell apoptosis was investigated by TUNEL 16h, 7, 14 and 21 days following the heat treatment. Apoptosis was found to be stage-specific affecting early and late stages of the seminiferous epithelium cycle on day 7 and 14 post exposure to heat stress with involvement of all stages of seminiferous epithelium 16h following heat stress. Reduction in germ cell apoptosis was evident 21 days post exposure. To determine the mechanism of germ cell apoptosis, caspase-3 proteins were detected in apoptotic germ cells following heat stress which showed strong positivity. Hence, we concluded that whole body heat stress results in caspase-3 mediated germ cell apoptosis. Furthermore, changes in cauda epididymal spermatozoa were investigated to determine sperm apoptosis following heat stress. Sperm apoptosis was detected by the exteriorization of phosphotidylserine (PS) on the outer layer of the plasma membrane. The results showed an early and late apoptosis in caudal epididymal spermatozoa 16hr following heat treatment. A second experiment was conducted to determine the time of appearance of changes in spermatozoa following whole body heat stress for which the duration of exposure to heat of 37-38°C was reduced to one-day period. This showed more spermatozoa in the early phase of apoptosis with fewer dead spermatozoa, which is suggestive of a time and temperature dependant pattern of sperm apoptosis. Arid-adapted Notomys alexis (hopping mice) were also investigated to determine whether differences in response to whole body heat have evolved in extreme climate conditions. Therefore hopping mice were exposed to similar temperatures as laboratory mice to investigate heat influences on germ cells and spermatozoa. Like the laboratory mice, germ cells and spermatozoa of hopping mice also showed apoptosis. However, unlike in laboratory mice, stage specificity in apoptosis could not be determined in hopping mice because of the presence of more than one cell association within the cross sections of the seminiferous tubules. Similar to laboratory mice, germ cell apoptosis in hopping mice was caspase-3 mediated. The vasculature of hopping mice was also determined to look for any variations in the cooling mechanisms, which could have resulted in germ cell apoptosis. We found the absence of a coiled testicular artery suggestive of a lack of such a cooling mechanism in hopping mice that could have resulted in germ cell and sperm apoptosis. We also investigated changes in germ cells following experimental cryptorchidism, which showed tubular degeneration with low numbers of germ cells lining the seminiferous epithelium. Thus the findings indicated that whole body heat had a detrimental effect on developing germ cells and spermatozoa. It is also known that heat stress changes other bodily functions to maintain body homeostasis. Heat stress is associated with an activation of the hypothalamic-pituitary axis, which is followed by an increase in blood cortisol levels in the circulation as a result of increased synthesis of corticosteroids from the adrenocortical cells. Hence the current study also determined changes, if any, in the adrenal glands, especially the adrenocortical cells of laboratory and hopping mice following whole body heat exposure. Heat stress resulted in the formation of vacuoles, dilated capillaries and interstitial fibrosis in cortical areas of the adrenal glands in both the species. In addition, large syncytial bodies were evident in hopping mice adrenal cortical cells. Thus it is evident from the study that germ cell and sperm apoptosis are a result of either a direct effect of heat on germ cells and/ or due to changes in the body hormonal milieu following whole body heat stress. Thus this study showed an activation of caspases in producing germ cell apoptosis and externalization of PS in inducing sperm apoptosis following whole body heat stress with changes in the adrenocortical cells following heat stress. This study demonstrated that an aridadapted species Notomys alexis, although evolved in extreme environmental conditions, also is affected by high temperatures. Hence, this study gives some insight in the reasons for reduced fertility following whole body heat stress.