Heat stress during in vitro fertilization decreases fertilization success by disrupting anti-polyspermy systems of the oocytes

Impact of Heat Stress on Oocyte Developmental Competence and Pre-Implantation Embryo Viability in Cattle

Rectal and vaginal temperatures are utilised in both in vivo and in vitro models to study the effects of heat stress on oocyte competence and embryo viability in cattle. However, uterine temperature increases by only 0.5 °C in heat-stressed cows, significantly lower than simulated increases in in vitro models. Temperature variations within oviducts and ovarian follicles during heat stress are poorly understood or unavailable, and evidence is lacking that oocytes and pre-implantation embryos experience mild (40 °C) or severe (41 °C) heat stress inside the ovarian follicle and the oviduct and uterus, respectively. Gathering detailed temperature data from the reproductive tract and follicles is crucial to accurately assess oocyte competence and embryo viability under realistic heat stress conditions. Potential harm from heat stress on oocytes and embryos may result from reduced nutrient availability (e.g., diminished blood flow to the reproductive tract) or other unidentified mechanisms affecting tissue function rather than direct thermal effects. Refining in vivo stress models in cattle is essential to accurately identify animals truly experiencing heat stress, rather than assuming heat stress exposure as done in most studies. This will improve model reliability and aid in the selection of heat-tolerant animals.

The abundance and localization of heat shock proteins (HSP)-60, -70, and -90 in the oviductal ampulla of hamadryas baboon (Papio hamadryas) during the menstrual cycle

The presence of HSPs in female reproductive and their relationship with the steroid hormone fluctuation have been reported in several mammals but not in non-human primates. The present research dealt with the oviductal expression and localization of the more studied HSPs (60, 70, and 90) as well as the morphological changes in the Hamadryas baboon (Papio hamadryas) during the follicular, preovulatory, and luteal phases of the menstrual cycle. Therefore, western blots, histomorphological, and immunohistochemical analyses were carried out. The results of western blot analysis displayed the lowest HSP expression in the luteal phase. The histomorphology showed that the mucosal epithelium consisted of undifferentiated cuboidal cells in follicular and luteal phases and well-distinguishable columnar ciliated and non-ciliated cells during the preovulatory phase. Immunohistochemistry evidenced that the mucosal epithelium contained cytoplasmic and nuclear HSP60, 70, and 90 immunostaining in the follicular and luteal phases. During the preovulatory phase, the non-ciliated cells showed: (i) cytoplasmic HSP60; (ii) nuclear and cytoplasmic HSP90. Ciliated cells showed cytoplasmic and ciliary HSP70 and ciliary HSP90. The stromal cells and myocytes of muscular layer displayed a decreased cytoplasmic HSP60 in the preovulatory phase and nuclear and low cytoplasmic HSP70 throughout the menstrual cycle. Nuclear HSP90 decreased in ampulla stromal cells and the follicular phase myocytes. These findings indicate that the expression pattern of HSP60,70, and 90 is related to the morphofunctional features of the baboon oviductal ampulla during the menstrual cycle and could represent a referent point for further studies in the oviduct of Primates.

Equine chorionic gonadotropin treatment and timed artificial insemination for dairy cow production under heat stress

This study investigated the effects of timed artificial insemination (TAI) and equine chorionic gonadotropin (eCG) administration on lactating dairy cows under heat-stress conditions (average temperature-humidity index: 80). Timed artificial insemination was performed on the cows with (n = 57) or without (control, n = 41) supplementation with 500 IU of eCG at the day of PGF2α treatment using the CIDR-Ovsynch protocol. GnRH was administered, and a progesterone device (CIDR) was inserted on Day -10 of the treatment protocol. The CIDR was removed on Day -3, and the cows were treated with PGF2α. Two days later, a 2nd GnRH injection was administered. Subsequently, AI was performed on Day 0 (16-20 h after the 2nd GnRH injection), and pregnancy was diagnosed on Days 32 and 60. Plasma progesterone (P4) concentrations were measured after AI. Results showed that the eCG group had a higher pregnancy per AI (P/AI) than the control group (43.9 vs. 12.2%, P = 0.002), which was also accompanied by elevated P4 levels. Four cows in the eCG group had multiple calves, representing 7.0 and 16.0% of the group and pregnant cows, respectively. In conclusion, 500 IU of eCG combined with CIDR-Ovsynch in lactating dairy cows under severe heat stress conditions successfully improved fertility. However, the protocol may have a slight risk of multiple births.

Exploring the impact of heat stress on oocyte maturation and embryo development in dairy cattle using a culture medium supplemented with vitamins E, C, and coenzyme Q10

Heat stress is a significant factor affecting the fertility of dairy cattle due to the generation of free radicals. In assisted reproductive techniques, the inclusion of protective antioxidants becomes crucial to mitigate potential cellular damage. This study aimed to explore the impact of supplementing vitamins E, C, and coenzyme Q10 into the oocyte culture medium, with the goal of ameliorating the adverse effects of heat stress on oocyte maturation and embryo development in dairy cattle. A group of fifty Holstein dairy cows were synchronized, and their oocytes were harvested using the ovum pick-up method. High-quality oocytes were subjected to in vitro maturation (IVM) and in vitro fertilization (IVF) procedures, utilizing a culture medium containing, no supplements (Group 1), 100 μM of vitamins E (Group 2) and C (Group 3), along with 50 μM of coenzyme Q10 (Group 4). The ensuing zygotes were cultured, and the ensuing embryos were evaluated for blastocyst formation by the seventh day. An analysis of the blastocysts' inner cell mass (ICM) and trophectoderm (TE) cells was also conducted. The findings revealed that the group receiving supplementation of vitamin E and coenzyme Q10 exhibited significantly higher maturation and cleavage rates in comparison to both the control and the vitamin C groups. Furthermore, the count of ICM, TE, and blastocyst cells was notably elevated in the vitamin E supplemented group when compared to the control group. In summary, the effectiveness of vitamin E in enhancing IVM, IVF, and embryo development under conditions of heat stress surpassed that of vitamin C and coenzyme Q10.

Heat Stress as a Barrier to Successful Reproduction and Potential Alleviation Strategies in Cattle

In recent decades, the adverse effects of global warming on all living beings have been unanimously recognized across the world. A high environmental temperature that increases the respiration and rectal temperature of cattle is called heat stress (HS), and it can affect both male and female reproductive functions. For successful reproduction and fertilization, mature and healthy oocytes are crucial; however, HS reduces the developmental competence of oocytes, which compromises reproduction. HS disturbs the hormonal balance that plays a crucial role in successful reproduction, particularly in reducing the luteinizing hormone and progesterone levels, which leads to severe problems such as poor follicle development with a poor-quality oocyte and problems related to maturity, silent estrus, abnormal or weak embryo development, and pregnancy loss, resulting in a declining reproduction rate and losses for the cattle industry. Lactating cattle are particularly susceptible to HS and, hence, their reproduction rate is substantially reduced. Additionally, bulls are also affected by HS; during summer, semen quality and sperm motility decline, leading to compromised reproduction. In summer, the conception rate is reduced by 20-30% worldwide. Although various techniques, such as the provision of water sprinklers, shade, and air conditioning, are used during summer, these methods are insufficient to recover the normal reproduction rate and, therefore, special attention is needed to improve reproductive efficiency and minimize the detrimental effect of HS on cattle during summer. The application of advanced reproductive technologies such as the production of embryos in vitro, cryopreservation during the hot season, embryo transfer, and timed artificial insemination may minimize the detrimental effects of HS on livestock reproduction and recover the losses in the cattle industry.

Ubiquitin C-terminal hydrolase L1 (UCHL1), a double-edged sword in mammalian oocyte maturation and spermatogenesis

BACKGROUND

Recent studies have shown that ubiquitin-mediated cell apoptosis can modulate protein interaction and involve in the progress of oocyte maturation and spermatogenesis. As one of the key regulators involved in ubiquitin signal, ubiquitin C-terminal hydrolase L1 (UCHL1) is considered a molecular marker associated with spermatogonia stem cells. However, the function of UCHL1 was wildly reported to regulate various bioecological processes, such as Parkinson's disease, lung cancer, breast cancer and colon cancer, how UCHL1 affects the mammalian reproductive system remains an open question.

METHODS

We identified papers through electronic searches of PubMed database from inception to July 2022.

RESULTS

Here, we summarize the important function of UCHL1 in controlling mammalian oocyte development, regulating spermatogenesis and inhibiting polyspermy, and we posit the balance of UCHL1 was essential to maintaining reproductive cellular and tissue homeostasis.

CONCLUSION

This study considers the 'double-edged sword' role of UCHL1 during gametogenesis and presents new insights into UCHL1 in germ cells.

The Melatonin Treatment Improves the Ovarian Responses After Superstimulation in Thai-Holstein Crossbreeds Under Heat Stress Conditions

The effect of heat stress with melatonin treatment on the superovulatory responses and embryo characteristics in Thai-Holstein crossbreeds under heat stress conditions was examined. Six non-lactating cows (replication = 4; n = 24) were assigned to one of 2 treatments in double cross-over design. All cows were superstimulated with traditional treatment. Melatonin group (n = 12): cows received intramuscularly injection 18 mg/50 kg. simultaneously with GnRH injection, while those in the control group (n = 12) received none. Bloods samples were taken to determine lipid peroxidation (MDA) and the activity of the antioxidant enzymes (superoxide dismutase; SOD). The experiment was conducted from April to September, which determined severe heat stress (the mean temperature-humidity index above 77). The results revealed that numbers of large follicles and corpora lutea were higher in the melatonin group than in the control group (p < 0.01). Numbers of recovered ova/embryos, fertilized ova, and transferable embryos were higher in the melatonin group (p < 0.01); meanwhile, more degenerated embryos were found in the control group (p < 0.01). Increased activity of the antioxidant enzymes SOD after melatonin administration decreased MDA levels (p < 0.05). In summary, melatonin administration benefited the ovarian response and embryo quality in superstimulated Thai-Holstein crossbreed affected by heat stress.

«The role of reproductive biology in SDGs» Global warming and cattle reproduction: Will increase in cattle numbers progress to global warming?

The livestock industry produces a large amount of greenhouse gases (GHG) that cause global warming. A high percentage of GHG emissions are derived from cattle and has been suggested to be a factor in global warming. With the global increase in the consumption of livestock products, the number of farm animals has increased. In addition, the reduction in productivity and reproductive capacity of cattle has resulted in accelerated GHG emissions. In a high-temperature environment, the pregnancy rate decreases, leading to an increase in animals that do not contribute to production. Consequently, GHG emission per unit product increases, thereby accelerating global warming. To reduce this environmental impact, it is important to improve the breeding efficiency of cattle by the use of reproductive technology and, thus, reduce the number of non-productive animals. Thus, reproductive biology plays a major role in mitigating global warming related to the livestock industry.

Relationship of the Temperature-Humidity Index (THI) with Ovarian Responses and Embryo Production in Superovulated Thai-Holstein Crossbreds under Tropical Climate Conditions

Heat stress strongly negatively affects reproductive traits in dairy cattle. The purpose of the present study was to investigate the effect of heat stress in superstimulated Thai-Holstein crossbreds under tropical climate conditions. Data included 75 records from 12 superovulated Thai-Holstein crossbreds between 2018 and 2020. Cows were superstimulated with conventional treatment. The mean temperature-humidity index (THI) was evaluated for three data collection periods: during the 9, 21 and 42-day periods before the insemination day to determine the period in which THI mostly affected superstimulation responses. The THI levels/thresholds were determined and interpreted together with the superovulatory response. A significantly negative correlation was obtained for the THI during the period 9 days before insemination. Negative effects on the number of large follicles and corpora lutea began at a THI of 72 and were apparently severe after a THI of 77, similar to the ovulation rate, fertilized ova and transferable embryos (p < 0.05). Meanwhile, more degenerated embryos were found with THI values (p < 0.05). The superovulatory response in Thai-Holstein crossbreds under tropical climate conditions is highly affected by heat stress starting at a THI of 72 and becomes more severe at a THI higher than 77.

Heat stress on cattle embryo: gene regulation and adaptation

Global warming has been affecting animal husbandry and farming production worldwide via changes in organisms and their habitats. In the tropics, these conditions are adverse for agriculture and animal production in some areas, due to high temperatures and relative humidity, affecting competitiveness related to economic activities. These environments have deteriorated livestock production, due to periods of drought, reduction in forage quality and heat stress, eliciting negative effects on reproduction, weight gain, and reduced meat and milk production. However, the use of animals adapted to tropics such as breeds derived from subspecies Bos primigenius indicus and native breeds from tropical countries or their crossings, is an alternative to improve production under high-temperature conditions. Therefore, physiological adaptation including gene expression induced by heat stress have been studied to understand the response of animals and to improve cross-breeding between cattle breeds to maintain high productivity in adverse weather conditions. Heat stress has been associated with lower reproductive performance in cows, due to the impact on blastocyst production, decreased implantation and increased embryonic death. Thus, for decades, in vitro fertilization and embryo transfer techniques have focused on studying the optimal conditions for production of high-quality embryos to transfer. The aim of this review is to discuss the effects of heat stress in bovine embryos, and their physiological and genetic modulation, focusing on the genes that are related with major adaptability to heat stress conditions and their relationship with different embryonic stages.

Heat stress on oocyte or zygote compromises embryo development, impairs interferon tau production and increases reactive oxygen species and oxidative stress in bovine embryos produced in vitro

Interferon tau (IFNT) is the cytokine responsible for the maternal recognition of pregnancy in ruminants and plays a role modulating embryo-maternal communication in the oviduct inducing a local response from immune cells. We aimed to investigate IFNT production, reactive oxygen species, and oxidative stress under the influence of heat stress (HS) during different stages of bovine in vitro embryo production. HS was established when the temperature was gradually raised from 38.5°C to 40.5°C in laboratory incubator, sustained for 6 hr, and decreased back to 38.5°C. To address the HS effects on IFNT production, reactive oxygen species, and oxidative stress, ovaries from a slaughterhouse were used according to treatments: control group (38.5°C); oocytes matured under HS; oocytes fertilized under HS; zygotes cultured in the first day under HS; and cells submitted to HS at oocyte maturation, fertilization, and the first day of zygote culture. The HS negatively affected cleavage and blastocyst rates, in all HS groups. On Day 7, all HS-treated embryos showed decrease IFNT gene and protein expressions, whereas reactive oxygen species were increased in comparison to the control. In conclusion, the compromised early embryo development due to higher temperatures during in vitro oocyte maturation, fertilization, and/or zygote stage have diminished IFNT expression and increased reactive oxygen species in bovine.

Thermoprotective molecules to improve oocyte competence under elevated temperature

Heat stress is an environmental factor that challenges livestock by disturbing animal homeostasis. Despite the broad detrimental effects of heat stress on reproductive function, the germline and the early preimplantation embryo are particularly prone. There is extensive evidence that elevated temperature reduces oocyte developmental competence through a series of cellular and molecular damages. Further research revealed that the oocyte respond to stress by activating cellular mechanisms such as heat shock response, unfolded protein response and autophagy to improve survival under heat shock. Such knowledge paved the way for the identification of thermoprotective molecules that alleviate heat-induced oocyte oxidative stress, organelle damage, and apoptosis. Therefore, this review depicts the deleterious effects of heat shock on oocyte developmental competence, heat-induced cellular and molecular changes, outlines pro-survival cellular mechanisms and explores thermoprotective molecules to improve oocyte competence.

Review: Challenges for dairy cow production systems arising from climate changes

The so-called global change refers to changes on a planetary scale. The term encompasses various issues like resource use, energy development, population growth, land use and land cover, carbon and nitrogen cycle, pollution and health, and climate change. The paper deals with challenges for dairy cattle production systems in Europe arising from climate change as one part of global changes. Global warming is increasing, and therefore ecosystems, plant and animal biodiversity, and food security and safety are at risk. It is already accepted knowledge that the direct and indirect effects of global warming in combination with an increasing frequency of weather extremes are a serious issue for livestock production, even in moderate climate zones like Central Europe. The potential and already-measurable effects of climate change (including increase in temperature, frequency of hot days and heat waves), in particular the challenges on grassland production, fodder quality, nutrition in general, cow welfare, health as well as performance of dairy production, will be reviewed. Indirect and direct effects on animals are correlated with their performance. There are clear indications that with selection for high-yielding animals the sensitivity to climate changes increases. Cumulative effects (e.g. higher temperature plus increased pathogen and their vectors loads) do strengthen these impacts. To cope with the consequences several possible adaptation and mitigation strategies must be established on different levels. This includes changes in the production systems (e.g. management, barn, feeding), breeding strategies and health management.

Cellular and Molecular Adaptation of Bovine Granulosa Cells and Oocytes under Heat Stress

Simple Summary

Heat stress can have large effects on most aspects of reproductive function in dairy cows. A hot environment can increase blood, rectal, and uterine temperatures, alter ovarian folliculogenesis, suppress fertility, oogenesis, and embryogenesis and ultimately reduce conception and pregnancy rates. Among the components of the female reproductive tract, the ovarian pool of follicles and their enclosed granulosa cells and oocytes are highly sensitive to hyperthermia. Many effects of elevated temperature on granulosa cells and developing oocytes involve increased production of reactive oxygen species, subsequently induce cellular apoptosis, and decrease the developmental ability of oocytes to be fertilized. Furthermore, heat stress-associated reproductive disorders are associated with altered progesterone and reduced estradiol production by ovarian follicles. The review mainly focuses on the follicle-enclosed granulosa cells and oocytes, provides new insights into the cellular and molecular adaptations of granulosa cells and oocyte under heat stress, depicts the role of the follicle microenvironment, and discusses some mechanisms that might underlie oocyte impairment. This study provides a possible way for the genetic adaptation to heat stress both for the regulation of body temperature and cellular resistance to elevated temperature.

Abstract

Heat stress has long been recognized as a challenging issue that severely influences the reproductive functions of dairy cattle, disrupting oocyte development during fetal growth. These detrimental effects of heat stress are the result of either the hyperthermia associated with heat stress or the physiological adjustments made by the heat-stressed animal to regulate body temperature. In addition, elevated temperatures have been implicated in increasing the production of reactive oxygen species. Thus, understanding the impact of heat stress on reproductive functions, from a cellular to molecular level, might help in selecting heat-resilient dairy cattle and developing heat stress mitigation strategies. In the present paper, we have attempted to describe the changes in the reproductive system and function of dairy cattle in response to heat stress by reviewing the latest literature in this area. The review provides useful knowledge on the cellular and genetic basis of oocyte and granulosa cells in heat-stressed dairy cattle, which could be helpful for future research in this area.

Dairy cow reproduction under the influence of heat stress

Dairy farming is vulnerable to global warming and climate change. Improving and maintaining conception rates (CRs) have a paramount importance for the profitability of any dairy enterprise. There is an antagonistic relationship between fertility and milk yield, and intensive selection for milk yield has severely deteriorated reproductive efficiency. Irrespective of geography and husbandry, modern dairy cows experience heat stress (HS) effects leading to fertility declines, but it worsens in tropical climates. The threshold of HS experience among modern dairy cow has lowered, leading to decreased thermal comfort zone. Studies show that this threshold is lower for fertility than for lactation. HS abatement and robustness response to lactation yield lead to negative energy balance, and cow's reproductive requirements remain unfulfilled. The adverse effects of HS commence from developing oocyte throughout later stages and its fertilization competence; the oestrus cycle and oestrus behaviour; the embryo development and implantation; on uterine environment; and even extend towards foetal calf. Even cows can become acyclic under the influence of HS. These harmful effects of HS arise due to hyperthermia, oxidative stress and physiological modifications in the body of dairy cows. Proper assessment of HS and efficient cooling of dairy animals irrespective of their stage of life at farm is the immediate strategy to reduce fertility declines. Other long- and short-term mitigation strategies to reduce fertility declines during HS include feeding care, reducing disease and mastitis rates, using semen from cooled bulls, timed artificial inseminations (AI), allied hormonal interventions and use of embryo transfer technology. Ultimate long-term solution should be well-planned breeding for fertility improvement and HS tolerance.

Role of endoplasmic reticulum stress on developmental competency and cryo-tolerance in bovine embryos

Endoplasmic reticulum (ER) stress, a dysfunction in protein folding capacity of the ER, is involved in many physiological responses including mammalian reproductive systems. Studies have shown that ER stress interferes with the developmental process of in vitro oocyte maturation and embryo development; however, little is known about its effects on bovine preimplantation embryonic development. In this study, we examined the effects of ER stress during IVC on developmental competency and cryo-tolerance in bovine embryos. IVF-derived zygotes were cultured in CR1aa medium supplemented with tauroursodeoxycholic acid (TUDCA) and/or tunicamycin (TM), which are ER stress-inhibitory and stress-inducing agents, respectively, for 8 days. TM treatment decreased the blastocyst developmental rate and increased the percentage of apoptotic cells compared to that in the control group (10.2 ± 2.3% vs. 39.75 ± 1.3% and 17.8 ± 1.2% vs. 3.6 ± 1.1%, respectively; P < 0.01). However, the blastocyst developmental rate was increased and the percentage of apoptotic cells was decreased by addition of TUDCA in IVC medium compared to that in the control group (50.9 ± 0.9% vs. 39.75 ± 1.3% and 1.13 ± 1.0% vs. 3.6 ± 1.1%, respectively; P < 0.01). Importantly, in the group treated with TM plus TUDCA, the developmental rate and the percentage of apoptotic cells in blastocysts were similar to that in the control group, indicating that TUDCA ameliorates the adverse effects of TM alone on embryo development. In addition, TUDCA treatment significantly reduced the reactive oxygen species, expression of ER stress (GRP78, ATF4, ATF6, IER1, and sXBP1) and pro-apoptotic (CHOP and BAX) genes, while it increased anti-apoptotic BCL2 gene expression and glutathione levels. Moreover, TUDCA improved blastocyst cryo-tolerance as marked by a significantly increased hatching rate and decreased the number of apoptotic cells recorded at 48 h after a post-warming. Therefore, in concordance with a previous report in mice or pig, we showed that TUDCA supplementation during IVC increases the developmental competency of bovine in vitro-derived embryos. Additionally, we found that the presence of TUDCA in IVC medium improves the cryo-tolerance of bovine embryos. These results suggest that modulation of ER stress during IVC contributes to the production of high-quality bovine embryos in terms of cryo-tolerance.

Heat exposure affected the reproductive performance of pregnant mice: Enhancement of autophagy and alteration of subcellular structure in the corpus luteum

To investigate whether autophagy and subcellular changes are involved in the corpus luteum after heat exposure, a total of 30 early pregnant mice were divided equally into heat stress (HS) and non-HS (NHS) groups (n = 15). Mice in the HS group were exposed to 40.5 ± 0.2 ℃ for 7 consecutive days. Ovaries were collected for immunohistochemistry (IHC), western blot (WB) analysis and transmission electron microscopy (TEM). Serum was collected to determine progesterone by RIA and uteri were collected to count the implantation sites. Results showed that heat exposure increased rectal temperature, decreased body weight and number of implantation sites. WB analysis revealed that ovarian expression of LC3B and Atg7 was up-regulated, while p62 was down-regulated in the HS group. IHC results demonstrated that ovarian staining intensity of LC3B was more intense in the HS group than that of the NHS group. LC3B was mainly localized in the granulosa cells, oocytes and luteal steroidogenic cells of the HS group. TEM results revealed double-layered separated membranes indicative of autophagosomes in the luteal steroidogenic cells of the HS group. Moreover, TEM showed that the mitochondrial cristae became dearth, structure-less, swollen after HS. Additionally, the nucleus expanded and accumulation of lipid droplets increased after HS. Results also showed that heat exposure decreased serum progesterone level and ovarian P450scc expression. These results indicate that HS enhanced autophagy and altered the subcellular structure of luteal steroidogenic cells, which may contribute to interfering with the maintenance of luteal function in early pregnant mice.

Time-dependent effects of heat shock on the zona pellucida ultrastructure and in vitro developmental competence of bovine oocytes

The aim of this study was to determine the effects of different exposure lenght to heat shock (HS) during in vitro maturation (IVM) on zona pellucida (ZP) ultrastructure and developmental competence of bovine oocytes. Cumulus-oocyte complexes (COCs) were matured in vitro (IVM) at 38.5 °C for 24 h (control group, CG), or incubated at 41 °C (HS) for 6 h (HS-6h), 12 h (HS-12h), 18 h (HS-18h), and 22h (HS-22h) followed by incubation at 38.5 °C to complete a full 24-h period of maturation. After IVM, oocytes were subjected to scanning electron microscopy (SEM) or in vitro fertilization and culture until the blastocyst stage. For heat-shocked oocytes, with exception of those in the HS-6h group, SEM examinations revealed that ZP surfaces were rough and characterized by a presence of spongy network. Oocytes from the HS-22h group displayed an increase in the number of pores, as well as a higher proportion of oocytes with amorphous ZPs. The proportion of oocytes that reached metaphase II (MII) stage decreased in all HS groups, regardless of the duration of exposure to 41 °C. These results provide evidence that HS during IVM for 12-22 h reduces the developmental competence of bovine oocytes, increasing the percentage of oocytes with abnormal chromosomal organization, and reducing fertilization and blastocysts formation rate. The effects of HS were more pronounced for the 22-h exposure group. The damage induced by HS on oocyte function clearly increased upon exposure to elevated temperature.

Heat stress, a serious threat to reproductive function in animals and humans

Global warming represents a major stressful environmental condition that compromises the reproductive efficiency of animals and humans via a rise of body temperature above its physiological homeothermic point (heat stress [HS]). The injuries caused by HS on reproductive function involves both male and female components, fertilization mechanisms as well as the early and late stages of embryo-fetal development. This occurrence causes great economic damage in livestock, and, in wild animals creates selective pressure towards the advantages of better-adapted genotypes to the detriment of others. Humans undergo several types of stress, including heat, and these represent putative causes of ongoing progressive decay in procreation; an increasing number of remedies in the form of antioxidant preparations are now being proposed to counteract the effects of stress. This review aims to describe the results of the most recent studies that aimed to highlight these effects and to draw information on the mechanisms acting as the basis of this problem from a comparative analysis.

Heat-shock-induced cathepsin B activity during IVF and culture compromises the developmental competence of bovine embryos

Heat stress can cause significant reproductive dysfunction in mammals and previous studies report that expression and activity of cathepsin B (CTSB), a lysosomal cysteine protease, is negatively correlated with the developmental competence of bovine oocytes and embryos. However, the relationship between heat shock (HS) and CTSB remains largely unknown. Here, we investigated the effects of HS during IVF and early embryonic stages of IVC on CTSB activity and developmental competence in bovine embryos. HS (40 °C for 6 h during IVF and 20 h during IVC) caused a significant increase in CTSB activity irrespective of the developmental stage or duration of HS. The developmental rate to the blastocyst stage was also significantly decreased by HS. Additionally, HS during IVC significantly increased the number of apoptotic cells in blastocysts. Notably, these HS-induced changes in blastocyst development and quality were significantly improved by inhibition of CTSB activity, indicating a key role for CTSB. These results showed that CTSB activity plays an essential role in HS-induced dysfunction in bovine embryo development, and that inhibition of this activity could enhance the developmental competence of heat-shocked embryos.

Review: Sperm-oocyte interactions and their implications for bull fertility, with emphasis on the ubiquitin-proteasome system

Fertilization is an intricate cascade of events that irreversibly alter the participating male and female gamete and ultimately lead to the union of paternal and maternal genomes in the zygote. Fertilization starts with sperm capacitation within the oviductal sperm reservoir, followed by gamete recognition, sperm-zona pellucida interactions and sperm-oolemma adhesion and fusion, followed by sperm incorporation, oocyte activation, pronuclear development and embryo cleavage. At fertilization, bull spermatozoon loses its acrosome and plasma membrane components and contributes chromosomes, centriole, perinuclear theca proteins and regulatory RNAs to the zygote. While also incorporated in oocyte cytoplasm, structures of the sperm tail, including mitochondrial sheath, axoneme, fibrous sheath and outer dense fibers are degraded and recycled. The ability of some of these sperm contributed components to give rise to functional zygotic structures and properly induce embryonic development may vary between bulls, bearing on their reproductive performance, and on the fitness, health, fertility and production traits of their offspring. Proper functioning, recycling and remodeling of gamete structures at fertilization is aided by the ubiquitin-proteasome system (UPS), the universal substrate-specific protein recycling pathway present in bovine and other mammalian oocytes and spermatozoa. This review is focused on the aspects of UPS relevant to bovine fertilization and bull fertility.

Effects of heat stress on bovine preimplantation embryos produced in vitro

Summer heat stress decreases the pregnancy rate in cattle and has been thought to be associated with the early embryonic death caused by the elevation of maternal body temperature. In vitro cultures have been widely used for the evaluation of effects of heat stress on oocytes, fertilization, preimplantation, and embryonic development. Susceptibility to heat stress is present in developmental stages from oocytes to cleavage-stage (before embryonic gene activation, EGA) embryos, leading to a consequent decrease in developmental competence. On the other hand, advanced-stage embryos such as morula or blastocysts have acquired thermotolerance. The mechanism for the developmental stage-dependent change in thermotolerance is considered to be the accumulation of antioxidants in embryos in response to heat-inducible production of reactive oxygen species. The supplementation of antioxidants to the culture media has been known to neutralize the detrimental effects of heat stress. Besides, EGA could be involved in acquisition of thermotolerance in later stages of embryos. Morulae or blastocysts can repair heat-induced unfolded proteins or prevent DNA damage occurring in processes such as apoptosis. Therefore, embryo transfer (ET) that can bypass the heat-sensitive stage could be a good solution to improve the pregnancy rate under heat stress. However, frozen-thawed ET could not improve the pregnancy rate as expected. Frozen-thawed blastocysts were more sensitive to heat stress and showed less proliferation upon heat exposure, compared to fresh blastocysts. Therefore, further research is required to improve the reduction in pregnancy rates due to summer heat stress.

Effect of Heat Stress on Reproduction in Dairy Cows: Insights into the Cellular and Molecular Responses of the Oocyte

Among the components of the female reproductive tract, the ovarian pool of follicles and their enclosed oocytes are highly sensitive to hyperthermia. Heat-induced alterations in small antral follicles can be expressed later as compromised maturation and developmental capacity of the ovulating oocyte. This review summarizes the most up-to-date information on the effects of heat stress on the oocyte with an emphasis on unclear points and open questions, some of which might involve new research directions, for instance, whether preantral follicles are heat resistant. The review focuses on the follicle-enclosed oocytes, provides new insights into the cellular and molecular responses of the oocyte to elevated temperature, points out the role of the follicle microenvironment, and discusses some mechanisms that might underlie oocyte impairment. Mechanisms include nuclear and cytoplasmic maturation, mitochondrial function, apoptotic pathways, and oxidative stress. Understanding the mechanism by which heat stress compromises fertility might enable development of new strategies to mitigate its effects.