Exposure to a Physiologically Relevant Elevated Temperature Hastens In Vitro Maturation in Bovine Oocytes

Impact of preovulatory follicle maturity on oocyte metabolism and embryo development

Improved oocyte competence for embryo development and pregnancy was observed following ovulation of preovulatory follicles with greater physiological maturity, as indicated by estradiol production, prior to the gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) surge. It was hypothesized that follicular fluid from preovulatory follicles of greater maturity better supports the maturing oocyte's metabolic requirements and improves embryo development. The objective was to determine if differences in preovulatory follicular fluid due to follicle maturity influence oocyte metabolism during in vitro maturation (IVM) and affect embryo development. Bovine preovulatory follicular fluid was collected 18 h after a GnRH-induced LH surge. Serum estradiol concentration at GnRH administration categorized follicles as greater or lesser maturity. Immature bovine oocytes were submitted to 24 h IVM in medium supplemented with 20% follicular fluid from preovulatory follicles of greater or lesser maturity. Embryo development was recorded. Oocyte maturation media and media conditioned by developing embryos were submitted for metabolomics. A randomized block design was utilized to determine differences in embryo development and media metabolites (P ≤ 0.05). Blastocysts from oocytes matured in greater vs. lesser maturity follicular fluid had a more moderate rate of development (P = 0.01). At the conclusion of 24 h IVM, abundance of 66 metabolites differed between greater and lesser follicle maturity treatments. Nine metabolites differed in media conditioned by developing embryos. Metabolome results suggest improved amino acid, purine, and glucose metabolism, followed by a more efficient rate of embryo development, in oocytes matured in greater vs lesser maturity follicular fluid.

Heat shock interferes with the amino acid metabolism of bovine cumulus-oocyte complexes in vitro: a multistep analysis

By affecting the ovarian pool of follicles and their enclosed oocytes, heat stress has an impact on dairy cow fertility. This study aimed to determine how heat shock (HS) during in vitro maturation affected the ability of the bovine cumulus-oocyte complexes (COCs) to develop, as well as their metabolism of amino acids (AAs). In this study, COCs were in vitro matured for 23 h at 38.5 °C (control; n = 322), 39.5 °C (mild HS (MHS); n = 290), or 40.5 °C (severe HS (SHS); n = 245). In comparison to the control group, the MHS and SHS groups significantly decreased the percentage of metaphase-II oocytes, as well as cumulus cell expansion and viability. The SHS decreased the rates of cleavage and blastocyst formation in comparison to the control and MHS. Compared to the control and MHS-COCs, the SHS-COCs produced significantly more phenylalanine, threonine, valine, arginine, alanine, glutamic acid, and citrulline while depleting less leucine, glutamine, and serine. Data showed that SHS-COCs had the highest appearance and turnover of all AAs and essential AAs. Heat shock was positively correlated with the appearance of glutamic acid, glutamine, isoleucine, alanine, serine, valine, phenylalanine, and asparagine. Network analysis identified the relationship between HS and alanine or glutamic acid, as well as the relationship between blastocyst and cleavage rates and ornithine. The findings imply that SHS may have an impact on the quality and metabolism of AAs in COCs. Moreover, the use of a multistep analysis could simply identify the AAs most closely linked to HS and the developmental competence of bovine COCs.

Magnitude and persistence of higher estrus-associated temperatures in beef heifers and suckled cows

Higher estrus-associated temperatures (HEAT) are a hallmark feature in sexually active females. The overarching aim of this study was to characterize the variability, magnitude, and persistence of HEAT in heifers and suckled beef cows as well as identify associated factors when occurring during thermoneutral conditions at the onset of the spring breeding season. In both heifers and cows, estrus was induced using a 7-d controlled internal drug release (CIDR)-PGF2α protocol. Vaginal temperature after prostaglandin F2α administration was recorded every 5 min using a Thermochron iButton affixed to a blank CIDR (containing no progesterone). Estrus was defined as when a heifer first stood to be mounted or when a cow had an Estrotect patch score of 3 or 4. Level of HEAT varied among individual animals. When comparing common HEAT variables using a mixed model with date nested within a year, maximum HEAT (39.9 ± 0.1 and 40.0 ± 0.1 °C) and duration (15.5 ± 0.8 and 15.4 ± 0.7) were similar in heifers and cows, respectively. However, the magnitude and persistence of HEAT differed. Total area under the HEAT curve was 117.1 ± 13.5 and 158.7 ± 12.3 for heifers vs cows, respectively (P = 0.0571). Further, 42.9% of heifers and 49% of cows had maximum HEAT ≥ 40 °C which persisted up to 6.5 and 10 h, respectively. When ambient conditions were predominantly thermoneutral, temperature humidity index had minimal impact on HEAT (mixed model, repeated measures over time). Toward identifying associated factors with different aspects of HEAT using best fit hierarchical linear regression models, baseline vaginal temperature and baseline duration were the most highly associated independent variables. Follicle size, estradiol and progesterone levels, and other available animal-related variables (e.g., age, weight, hair coat score) explained only a small amount of variation in HEAT. In summary, level of HEAT varies in estrus females even under thermoneutral conditions. Because HEAT can persist for an extended time, direct effects on fertility important components are unavoidable. Whether HEAT is a good or bad component of the periovulatory microenvironment is the basis of ongoing and future studies.

Ameliorating Effect of Sodium Selenite on Developmental and Molecular Response of Bovine Cumulus-Oocyte Complexes Matured in Vitro Under Heat Stress Condition

Selenium (Se), an essential trace element, plays an important role in the antioxidative defense mechanism, and it has been proven to improve fertility and reproductive efficiency in dairy cattle. The present study evaluated the potential protective action of Se supplement of in vitro maturation (IVM) media on the maturation and subsequent development of bovine cumulus-oocyte complexes (COCs) exposed to heat stress (HS). The treatment with Se improved the viability of cumulus cells (CCs) and oocytes (P < 0.05). The proportion of oocytes reached metaphase II (MII) and those arrested at metaphase I (MI) was greater and lower in treatment than control respectively (P < 0.05). Supplementation with Se increased the percentage of cleaved embryos, total blastocysts, and blastocyst/cleavage ratio (P < 0.05). Moreover, the upregulation of CCND1, SEPP1, GPX-4, SOD, CAT, and downregulation of GRP78, CHOP, and BAX in both Se-treated CCs and oocytes were recorded. The upregulation of NRF2 was detected in Se-treated CCs other than in oocytes, which showed upregulation of IGF2R and SOX-2 as the markers of quality as well. Se supplement in IVM media improved the viability, maturation, and the level of transcripts related to antioxidant defense and quality of heat-treated oocytes, which coincided with greater subsequent development outcomes. Se ameliorated the viability of CCs along with upregulation of antioxidative candidate gene expression and downregulation of apoptosis-related ones to support their protective role on restoring the quality of oocytes against compromising effects of HS.

Influences of Supplementing Selective Members of the Interleukin-6 Cytokine Family on Bovine Oocyte Competency

This work explored whether supplementing selective members of the interleukin-6 (IL6) cytokine family during in vitro bovine oocyte maturation affects maturation success, cumulus-oocyte complex (COC) gene expression, fertilization success, and embryo development potential. Human recombinant proteins for IL6, IL11, and leukemia inhibitory factor (LIF) were supplemented to COCs during the maturation period, then fertilization and embryo culture commenced without further cytokine supplementation. The first study determined that none of these cytokines influenced the rate that oocytes achieved arrest at meiosis II. The second study identified that LIF and IL11 supplementation increases AREG transcript abundance. Supplementation with IL6 supplementation did not affect AREG abundance but reduced HAS2 transcript abundance. Several other transcriptional markers of oocyte competency were not affected by any of the cytokines. The third study determined that supplementing these cytokines during maturation did not influence fertilization success, but either LIF or IL11 supplementation increased blastocyst development. No effect of IL6 supplementation on subsequent blastocyst development was detected. The fourth experiment explored whether each cytokine treatment affects the post-thaw survivability of cryopreserved IVP blastocysts. None of the cytokines supplemented during oocyte maturation produced any positive effects on post-thaw blastocyst re-expansion and hatching. In conclusion, these outcomes implicate IL11 and LIF as potentially useful supplements for improving bovine oocyte competency.

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.

Heat Stress: A Serious Disruptor of the Reproductive Physiology of Dairy Cows

Global warming is a significant threat to the sustainability and profitability of the dairy sector, not only in tropical or subtropical regions but also in temperate zones where extreme summer temperatures have become a new and challenging reality. Prolonged exposure of dairy cows to high temperatures compromises animal welfare, increases morbidity, and suppresses fertility, resulting in devastating economic losses for farmers. To counteract the deleterious effects of heat stress, cattl e employ various adaptive thermoregulatory mechanisms including molecular, endocrine, physiological, and behavioral responses. These adaptations involve the immediate secretion of heat shock proteins and cortisol, followed by a complex network of disrupted secretion of metabolic and reproductive hormones such as prolactin, ghrelin, ovarian steroid, and pituitary gonadotrophins. While the strategic heat stress mitigation measures can restore milk production through modifications of the microclimate and nutritional interventions, the summer fertility records remain at low levels compared to those of the thermoneutral periods of the year. This is because sustainment of high fertility is a multifaceted process that requires appropriate energy balance, undisrupted mode of various hormones secretion to sustain the maturation and fertilizing competence of the oocyte, the normal development of the early embryo and unhampered maternal-embryo crosstalk. In this review, we summarize the major molecular and endocrine responses to elevated temperatures in dairy cows, as well as the impacts on maturing oocytes and early embryos, and discuss the consequences that heat stress brings about in dairy cattle fertility.

Alternative Culture Systems for Bovine Oocyte In Vitro Maturation: Liquid Marbles and Differentially Shaped 96-Well Plates

In vivo-matured oocytes exhibit higher developmental competence than those matured in vitro but mimicking the in vivo environment by in vitro conditions has been challenging. Until now, conventional two-dimensional (2D) systems have been used for in vitro maturation of bovine cumulus-oocytes-complexes (COCs). However, using such systems present certain limitations. Therefore, alternative low-cost methodologies may help to optimize oocyte in vitro maturation. Here, we used two different systems to culture COCs and evaluate their potential influence on embryo development and quality. In the first system, we used treated fumed silica particles to create a 3D microenvironment (liquid marbles; LM) to mature COCs. In the second system, we cultured COCs in 96-well plates with different dimensions (flat, ultra-low attachment round-bottom, and v-shaped 96-well plates). In both systems, the nuclear maturation rate remained similar to the control in 2D, showing that most oocytes reached metaphase II. However, the subsequent blastocyst rate remained lower in the liquid marble system compared with the 96-well plates and control 2D systems. Interestingly, a lower total cell number was found in the resulting embryos from both systems (LM and 96-well plates) compared with the control. In conclusion, oocytes matured in liquid marbles or 96-well plates showed no remarkable change in terms of meiotic resumption. None of the surface geometries influenced embryo development while oocyte maturation in liquid marbles led to reduced embryo development. These findings show that different geometry during maturation did not have a large impact on oocyte and embryo development. Lower embryo production after in vitro maturation in liquid marbles was probably detected because in vitro maturation was performed in serum-free medium, which makes oocytes more sensitive to possible toxic effects from the environment.

Review: Reproductive consequences of whole-body adaptations of dairy cattle to heat stress

Heat stress has far-reaching ramifications for agricultural production and the severity of its impact has increased alongside the growing threats of global warming. Climate change is exacerbating the already-severe consequences of seasonal heat stress and is predicted to cause additional losses in reproductive performance, milk production and overall productivity. Estimated and predicted losses are staggering, and without advancement in production practices during heat stress, these projected losses will threaten the human food supply. This is particularly concerning as the worldwide population and, thus, demand for animal products grows. As such, there is an urgent need for the development of technologies and management strategies capable of improving animal production capacity and efficiency during periods of heat stress. Reproduction is a major component of animal productivity, and subfertility during thermal stress is ultimately the result of both reproductive and whole-body physiological responses to heat stress. Improving reproductive performance during seasonal heat stress requires a thorough understanding of its effects on the reproductive system as well as other physiological systems involved in the whole-body response to elevated ambient temperature. To that end, this review will explore the reproductive repercussions of whole-body consequences of heat stress, including elevated body temperature, altered metabolism and circulating lipopolysaccharide. A comprehensive understanding of the physiological responses to heat stress is a prerequisite for improving fertility, and thus, the overall productivity of dairy cattle experiencing heat stress.

Granulosa cell-derived extracellular vesicles mitigate the detrimental impact of thermal stress on bovine oocytes and embryos

Climate change-induced global warming results in rises in body temperatures above normal physiological levels (hyperthermia) with negative impacts on reproductive function in dairy and beef animals. Extracellular vesicles (EVs), commonly described as nano-sized, lipid-enclosed complexes, harnessed with a plethora of bioactive cargoes (RNAs, proteins, and lipids), are crucial to regulating processes like folliculogenesis and the initiation of different signaling pathways. The beneficial role of follicular fluid-derived EVs in inducing thermotolerance to oocytes during in vitro maturation (IVM) has been evidenced. Here we aimed to determine the capacity of in vitro cultured granulosa cell-derived EVs (GC-EVs) to modulate bovine oocytes’ thermotolerance to heat stress (HS) during IVM. Moreover, this study tested the hypothesis that EVs released from thermally stressed GCs (S-EVs) shuttle protective messages to provide protection against subsequent HS in bovine oocytes. For this, sub-populations of GC-EVs were generated from GCs subjected to 38.5°C (N-EVs) or 42°C (S-EVs) and supplemented to cumulus-oocyte complexes (COCs) matured in vitro at the normal physiological body temperature of the cow (38.5°C) or HS (41°C) conditions. Results indicate that S-EVs improve the survival of oocytes by reducing ROS accumulation, improving mitochondrial function, and suppressing the expression of stress-associated genes thereby reducing the severity of HS on oocytes. Moreover, our findings indicate a carryover impact from the addition of GC-EVs during oocyte maturation in the development to the blastocyst stage with enhanced viability.

Thermoprotective molecules: Effect of insulin-like growth factor type I (IGF-1) in cattle oocytes exposed to high temperatures

The adverse effects of heat stress (HS) on the welfare and productivity of cattle are the result of the associated hyperthermia and the physiological and behavioral mechanisms performed by the animal to regulate body temperature. The negative effects of HS on in vitro oocyte maturation and in vitro bovine embryo production have been reported; being one of the major concerns due to economic and productive losses, and several mechanisms have been implemented to reduce its impact. These mechanisms include supplementation of the medium with hormones, adjuvants, identification of protective genes, among others. This review aims to explore the cellular and molecular mechanisms of insulin-like growth factor-1 (IGF-1) during in vitro and in vivo maturation of bovine oocytes and its thermoprotective effect under HS. Although the supplementation of the culture medium during oocyte maturation with IGF-1 has been implemented during the last years, there are still controversial results, however, supplementation with low concentration showed a positive effect on maturation and thermoprotection of oocytes exposed to higher temperatures. Additionally, IGF-1 is involved in multiple cellular pathways, and it may regulate cell apoptosis in cases of HS and protect oocyte competence under in vitro conditions.

Effects of Heat Stress on Bovine Oocytes and Early Embryonic Development-An Update

Heat stress is a major threat to cattle reproduction today. It has been shown that the effect of high temperature not only has a negative effect on the hormonal balance, but also directly affects the quality of oocytes, disrupting the function of mitochondria, fragmenting their DNA and changing their maternal transcription. Studies suggest that the induction of HSP70 may reduce the apoptosis of granular layer cells caused by heat stress. It has been shown that the changes at the transcriptome level caused by heat stress are consistent with 46.4% of blastocyst development disorders. Cows from calves exposed to thermal stress in utero have a lower milk yield in their lifetime, exhibit immunological disorders, have a lower birth weight and display a shorter lifespan related to the expedited aging. In order to protect cow reproduction, the effects of heat stress at the intracellular and molecular levels should be tracked step by step, and the impacts of the dysregulation of thermal homeostasis (i.e., hyperthermy) should be taken into account.

Positive relationship of rectal temperature at fixed timed artificial insemination on pregnancy outcomes in beef cattle

The overarching aim was to examine the relationship of rectal temperature at fixed time artificial insemination (FTAI) on pregnancy outcomes in a typical breeding season with expected pregnancy rates approaching 50% using Bos indicus and Bos taurus cattle. This represents a continuum of steps to test the hypothesis that elevated body temperature at or around insemination is functionally important to maximize pregnancy outcomes. Rectal temperature of Bos indicus cattle at FTAI ranged from 37.0 to 40.9 °C; 60.6% were hyperthermic. Positive factors impacting pregnancy outcomes were rectal temperature at FTAI, body condition, and estrus patch scores. Rectal temperature at FTAI was positively associated with pregnancy outcomes (P < 0.0001); per each 1 °C increase pregnancy odds increased 1.9 times (95% CI: 1.4 to 2.6). Highest pregnancy outcomes occurred with rectal temperatures exceeding 40 °C (P = 0.0004). Rectal temperature before FTAI in Bos taurus cattle ranged from 37.8 to 41.8 °C; 43.3% were hyperthermic. Factors impacting pregnancy were rectal temperature at FTAI, estrus activity, parity, and ambient conditions on day of FTAI. Rectal temperature of Bos taurus cattle at FTAI was positively associated with pregnancy (P = 0.0286); odds increased 1.45 times (95% CI: 1.0 to 2.0) per each 1 °C increase. Highest pregnancy outcomes occurred with rectal temperatures at FTAI exceeding 40 °C (P = 0.057). Moreover, positive relationship of rectal temperature at FTAI to pregnancy persisted in estrual females (71.25% of total; P = 0.0408; OR 1.5; 95% CI: 1.0 to 2.2). Mindful that 1) elevated temperatures observed in Bos indicus and Bos taurus cattle directly promote meiotic resumption of the oocyte in vitro and that 2) in vivo hyperthermia alters intrafollicular components which others have shown to potentiate ovulation and promote meiotic resumption, it is biologically plausible that an acute elevation in body temperature at or around time of insemination is functionally important to maximize pregnancy outcomes.

Heat-induced increases in body temperature in lactating dairy cows: impact on the cumulus and granulosa cell transcriptome of the periovulatory follicle

Cows acutely heat stressed after a pharmacologically induced luteinizing hormone (LH) surge had periovulatory changes in the follicular fluid proteome that may potentiate ovulation and impact oocyte developmental competence. Because the cellular origins of differentially abundant proteins were not known, we have examined the cumulus and granulosa cell transcriptomes from the periovulatory follicle in cows exhibiting varying levels of hyperthermia when occurring after the LH surge. After pharmacological induction of a dominant follicle, lactating dairy cows were administered gonadotropin releasing hormone (GnRH) and maintained in thermoneutral conditions (~67 temperature-humidity index [THI]) or heat stress conditions where THI was steadily increased for ~12 h (71 to 86 THI) and was sufficient to steadily elevate rectal temperatures. Cumulus-oocyte complexes and mural granulosa cells were recovered by transvaginal aspiration of dominant follicle content ~16 h after GnRH. Rectal temperature was used as a continuous, independent variable to identify differentially expressed genes (DEGs) increased or decreased per each 1 °C change in temperature. Cumulus (n = 9 samples) and granulosa (n = 8 samples) cells differentially expressed (false discovery rate [FDR] < 0.05) 25 and 87 genes, respectively. The majority of DEGs were upregulated by hyperthermia. Steady increases in THI are more like the "turning of a dial" than the "flipping of a switch." The moderate but impactful increases in rectal temperature induced modest fold changes in gene expression (<2-fold per 1 °C change in rectal temperature). Identification of cumulus DEGs involved in cell junctions, plasma membrane rafts, and cell-cycle regulation are consistent with marked changes in the interconnectedness and function of cumulus after the LH surge. Depending on the extent to which impacts may be occurring at the junctional level, cumulus changes may have indirect but impactful consequences on the oocyte as it undergoes meiotic maturation. Two granulosa cell DEGs have been reported by others to promote ovulation. Based on what is known, several other DEGs are suggestive of impacts on collagen formation or angiogenesis. Collectively these and other findings provide important insight regarding the extent to which the transcriptomes of the components of the periovulatory follicle (cumulus and mural granulosa cells) are affected by varying degrees of hyperthermia.

5-Aminolevulinic acid combined with sodium ferrous citrate mitigates effects of heat stress on bovine oocyte developmental competence

High summer temperatures have deleterious effects on oocyte developmental competence. The antioxidant and autophagy-related properties of 5-aminolevulinic acid (5-ALA) gives the compound a broad range of biological activities. This study aimed to evaluate the effects of: 1) a high temperature-humidity index (THI) on the developmental competence of bovine oocytes, and 2) 5-ALA administration in combination with sodium ferrous citrate (SFC) during in vitro maturation (IVM) on bovine oocyte developmental competence evaluated at high THI. Bovine ovaries were collected from a local slaughterhouse at moderate environmental temperature (MT; THI of 56.2) and high environmental temperature (HT; THI of 76.7) periods; cumulus-oocyte complexes (COCs) were aspirated from medium-sized follicles, matured in vitro for 22 h, fertilized, and cultured for 10 days. For COCs collected during the HT period, 0 (control), 0.01, 0.1, 0.5, or 1 µM 5-ALA was added to the maturation medium in combination with SFC at a molar ratio of 1:0.125. The results showed that HT adversely affected blastocyst and hatching rates compared with MT. Adding 5-ALA/SFC (1 µM/0.125 µM) to the maturation medium of oocytes collected during the HT period improved cumulus cell expansion and blastocyst rates compared with the no-addition control. In conclusion, this study showed that high THI can disrupt bovine oocyte developmental competence. Adding 5-ALA to SFC ameliorates this negative effect of heat stress and improves subsequent embryo development.

Effects of Heat Stress on Follicular Physiology in Dairy Cows

Simple Summary

Environmentally induced hyperthermia, also called heat stress (HS), compromises reproductive physiology in mammals. The number of oocytes is fixed after birth and they are stored in the ovary in a quiescent state (at the stage of the first meiotic prophase) in primordial follicles. There is evidence that HS alters the oocyte quality, the dynamics of follicular growth and ovulation. The dairy cow, submitted to the metabolic stress of high milk production, is a good model for studying the effects of HS on ovarian function. The aim of this review is to describe the influence of HS during the stages of follicular development in dairy cattle, from the activation of primordial follicles to ovulation. Some clinical aspects are also considered.

Abstract

Follicular organization starts during mid-to-late fetal life with the formation of primordial follicles. The bilateral interplay between the oocyte and adjoining somatic cells during follicular growth and ovulation may be sensitive to heat stress (HS). Mechanisms giving rise to pre-ovulatory temperature gradients across reproductive tissues are mostly regulated by the pre-ovulatory follicle, and because the cooling of the gonads and genital tract depends on a counter-current transfer system of heat, HS may be considered a major factor impairing ovulation, fertilization and early embryo development. There is evidence of a long-lasting influence of HS on oogenesis and final follicular maturation. Follicular stages that are susceptible to HS have not been precisely determined. Therefore, the aim of this review was to describe the influence of HS during the staged follicular development in dairy cattle, from the activation of primordial follicles to ovulation. Some clinical prospects are also considered.

Evaluation of Seasonal Heat Stress on Transcriptomic Profiles and Global DNA Methylation of Bovine Oocytes

Heat stress affects oocyte developmental competence and is a major cause of reduced fertility in heat stressed cattle. Negative effects of heat stress on the oocyte have been observed at morphological, biochemical and developmental levels. However, the mechanisms by which heat stress affects the oocyte at the transcriptional and epigenetic levels remain to be further elucidated. Here we aimed to investigate the effect of heat stress on oocyte quality, transcriptomic profiles and DNA methylation of oocytes collected through the transition from spring to summer under Louisiana conditions. Summer season resulted in a lower number of high quality oocytes obtained compared to the spring season. There was no difference in in vitro maturation rates of oocytes collected during spring as compared to summer. RNA sequencing analysis showed that a total of 211 and 92 genes were differentially expressed as a result of heat stress in GV and MII oocytes, respectively. Five common genes (E2F8, GATAD2B, BHLHE41, FBXO44, and RAB39B) were significantly affected by heat in both GV and MII oocytes. A number of pathways were also influenced by heat stress including glucocorticoid biosynthesis, apoptosis signaling, and HIPPO signaling in GV oocytes, and Oct4 pluripotency, Wnt/beta-catenin signaling, and melatonin degradation I in MII oocytes. In addition, fluorescent immunocytochemistry analysis showed no difference in global levels of DNA methylation and DNA hydroxymethylation at either the GV or MII stage between spring and summer oocytes. The results of this study contribute to a better understanding of the effect of heat stress on the molecular mechanisms altered in bovine oocytes.

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.

Developmental competence of heat stressed oocytes from Holstein and Limousine cows matured in vitro

The negative effects of heat stress on dairy cattle's fertility have been extensively studied, but the relevant knowledge for beef cattle is rather limited. The aims of this study were to investigate the effects of HS during in vitro maturation on the developmental potential of oocytes derived from Limousine and Holstein cows and to estimate the effect of the differential gene expression of important genes in oocytes, cumulus cells and blastocysts in the growth competence between the breeds. In seven replicates, cumulus oocyte complexes from Holstein and Limousine cows were matured for 24 hr at 39°C (controls C; Hol_39, Lim_39) or at 41°C from hour 2 to hour 8 of IVM (treated T; Hol_41, Lim_41), fertilized, and presumptive zygotes were cultured for 9 days at 39°C. Cleavage and embryo formation rates were evaluated 48 hr post-insemination and on days 7, 8 and 9, respectively. From all groups, subsets of cumulus cells, oocytes and blastocysts were analysed for the relative expression of genes related to metabolism, stress, apoptosis and placentation. No difference was detected in cleavage rate or in blastocyst formation rate among the control groups. In both breeds, heat stress reduced blastocyst yield, but at all days the suppression was higher in Limousines. In Holsteins, altered gene expression was detected in cumulus cells (G6PD, GLUT1) and blastocysts (PLAC8), while in Limousines, differences were found in oocytes (G6PD, HSP90AA1), in cumulus cells (CPT1B, HSP90AA1, SOD2) and blastocysts (DNMT, HSP90AA1, SOD2). It appears that Holstein COCs are more tolerant than Limousine COCs, possibly due to compulsory, production driven selection.

A novel route of follicle-stimulating hormone administration with a split-single ischiorectal fossa in Thai-Holstein crossbred superovulation programs under heat stress conditions

The objectives of this study were to compare the efficiency of a split single injection of follicle-stimulating hormone (FSH) given by either intramuscular (split-single IM) or ischiorectal fossa (split-single IRF) injection to the traditional treatment and to determine the concentrations of FSH. The temperature and humidity index (THI) values were interpreted together with the ovarian responses and embryo characteristics. The ovarian responses in the split-single IRF group were similar to those of the control group (p > .05) but higher compared with the split-single IM group (p < .05). Higher peak levels of plasma FSH in the split-single IRF group did not differ compared with the control group (p > .05) but were lower in split-single IM administration (p < .05). The results showed a significant decrease in the numbers of large follicles and corpora lutea (CLs) in the moderate THI compared with low and high THI (p < .05). The high THI affected ovulation rate as well as the numbers of transferable embryos and degenerated embryos (p < .05). In conclusion, the split-single IRF administration had a comparable superovulatory response to the traditional twice-daily protocol. Moreover, the ovulation rate, ovarian follicle responses, and embryo quality were affected by heat stress.

Heat Shock Protein 70 Improves In Vitro Embryo Yield and Quality from Heat Stressed Bovine Oocytes

Heat shock protein 70 (HSP70) is a chaperon that stabilizes unfolded or partially folded proteins, preventing inappropriate inter- and intramolecular interactions. Here, we examined the developmental competence of in vitro matured oocytes exposed to heat stress with or without HSP70. Bovine oocytes were matured for 24 h at 39 °C without (group C39) or with HSP70 (group H39) and at 41 °C for the first 6 h, followed by 16 h at 39 °C with (group H41) or without HSP70 (group C41). After insemination, zygotes were cultured for 9 days at 39 °C. Cleavage and embryo yield were assessed 48 h post insemination and on days 7, 8, 9, respectively. Gene expression was assessed by RT-PCR in oocytes, cumulus cells and blastocysts. In C41, blastocysts formation rate was lower than in C39 and on day 9 it was lower than in H41. In oocytes, HSP70 enhanced the expression of three HSP genes regardless of incubation temperature. HSP70 at 39 °C led to tight coordination of gene expression in oocytes and blastocysts, but not in cumulus cells. Our results imply that HSP70, by preventing apoptosis, supporting signal transduction, and increasing antioxidant protection of the embryo, protects heat stressed maturing bovine oocyte and restores its developmental competence.

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.

Impact of an acute heat shock during in vitro maturation on interleukin 6 and its associated receptor component transcripts in bovine cumulus-oocyte complexes

An acute heat stress event after the LH surge increased interleukin 6 (IL6) levels in the follicular fluid of the ovulatory follicle in hyperthermic cows. To examine direct consequences of a physiologically-relevant elevated temperature (41.0°C) on the cumulus-oocyte complex (COC), IL6 transcript abundance and related receptor components were evaluated throughout in vitro maturation. Heat-induced increases in IL6 were first noted at 4 hours of in vitro maturation (hIVM); peak levels occurred at 4.67 versus 6.44 hIVM for 41.0 and 38.5°C COCs, respectively (SEM = 0.23; P < 0.001). Peak IL6ST levels occurred at 6.95 versus 8.29 hIVM for 41.0 and 38.5°C, respectively (SEM = 0.23; P < 0.01). Transcript for LIF differed over time (P < 0.0001) but was not affected by 41.0°C exposure. Blastocyst development after performing IVF was not affected by 41.0°C exposure for 4 or 6 h. When limiting analysis to when IL6 was temporally produced, progesterone levels were only impacted by time and temperature (no interaction). Heat-induced shift in the temporal production of IL6 and IL6ST along with its impact on progesterone likely cooperate in heat-induced hastening of meiotic progression described by others.

Heat stress reduces maturation and developmental capacity in bovine oocytes

The ovarian pool of follicles, and their enclosed oocytes, is highly sensitive to hyperthermia. Heat-induced changes in small antral follicles can later manifest as impaired follicle development and compromised competence of the enclosed oocytes to undergo maturation, fertilisation and further development into an embryo. This review describes the main changes documented so far that underlie the oocyte damage. The review discusses some cellular and molecular mechanisms by which heat stress compromises oocyte developmental competence, such as impairment of nuclear and cytoplasmic maturation and mitochondrial function, changes in the expression of both nuclear and mitochondrial transcripts and the induction of apoptosis. The review emphasises that although the oocyte is exposed to heat stress, changes are also evident in the developed embryo. Moreover, the effect of heat stress is not limited to the summer; it carries over to the cold autumn, as manifest by impaired steroid production, low oocyte competence and reduced fertility. The spontaneous recovery of oocytes from the end of the summer through the autumn until the beginning of winter suggests that only subpopulations of follicles, rather than the entire ovarian reserve, are damaged upon heat exposure.

Molecular, functional, and cellular alterations of oocytes and cumulus cells induced by heat stress and shock in animals

Global warming is considered as the main environmental stress affecting ecosystems as well as physiological and biochemical characteristics, and survivability of living organisms. High temperature induces various stresses and causes reduction of fertility through reducing the oocyte developmental competence and alteration in surrounding cells' functions. This causes major economic loss to livestock creating a selective pressure on animals to the advantage of better adapted genotypes and to the detriment of others. In this review, a search in Science Direct, Google Scholar, PubMed, Web of Science, Scopus, and SID databases until 2020 was conducted. Keywords which include heat stress, shock, high temperature, oocyte, cumulus, and animals were investigated. Studies have exhibited that heat stress can disturb the development and function of oocyte and cumulus cells (CCs) concerning reproductive efficiency. Heat stress has deleterious consequences on oocyte maturation and development via reduced number of polar body extrusion, adenosine monophosphate, and guanosine monophosphate synthesis. Heat stress caused the alteration of cytoplasmic and nuclear features as well as trans-zonal projections and gap junctions. In addition, heat stress is accompanied with reduced mitochondrial activity (copy mDNA number, distribution, and membrane potential) in cumulus-oocyte complexes. This review targets the description of results in the most recent studies that aimed to call attention to the influences of heat stress on molecular, functional, and cellular changes in oocytes and CCs in animals to design evidence on the acting mechanisms as the core of this problem from a comparative review.

Influence of long-term thermal stress on the in vitro maturation on embryo development and Heat Shock Protein abundance in zebu cattle

The objective of this study was to investigate the influence of long-term temperature stress during the in vitro maturation (IVM) of oocytes on the in vitro embryo production (IVP) and the abundance of HSP70 and HSP90 in zebu cattle. Viable cumulus-oocyte complexes (COCs) were incubated for 24 h at 37 °C, 38.5 °C, or 40 °C for the low-, physiological, and high-temperature stress treatments, respectively. Thereafter, they were subjected to in vitro fertilization and culture. Temperature did not affect the polar body extrusion. However, IVP was adversely affected when IVM took place at 37 °C and 40 °C. The highest abundance of HSP70 was observed in cumulus cells after maturation of COCs at 40 °C. In contrast, HSP70 was more abundant in oocytes at both 37 °C and 40 °C; however, at 40 °C, the difference to the control group (38.5 °C) was not significant. In contrast, the highest abundance of HSP90 was observed in oocytes and cumulus cells at 37 °C. It appears that HSP70 and HSP90 respond to cold and heat stress in different ways. In conclusion, moderately high (40 °C) and low (37 °C) thermal stress for 24 h during IVM is detrimental to the developmental competence of oocyte and is accompanied by changes in the abundances of HSP70 and HSP90, especially in cumulus cells.

Short term temperature elevation during IVM affects embryo yield and alters gene expression pattern in oocytes, cumulus cells and blastocysts in cattle

Heat stress causes subfertility in cattle by inducing alterations in steroidogenic capacity, follicular function and ovulation defects, which eventually negatively affect oocyte quality and embryo survival. Here, the effects of short, moderate temperature elevation during IVM, on embryo yield, and on the expression of various genes was evaluated. In 8 replicates, cumulus oocyte complexes (COCs) were matured for 24 h at 39 °C (controls n = 605) or at 41 °C from hour 2 to hour 8 of IVM (treated, n = 912), fertilized, and presumptive zygotes were cultured for 9 days at 39 °C. Cleavage and embryo formation rates were evaluated 48 h post insemination and on days 7, 8, 9 respectively. Cumulus cells, oocytes and blastocysts from 5 replicates were snap frozen for the relative expression analysis of genes related to metabolism, thermal and oxidative stress response, apoptosis, and placentation. In treated group, cleavage and embryo formation rates were statistically significantly lower compared with the control (cleavage 86.7% vs 74.2%; blastocysts: day 7, 29.9% vs 19.7%, day 8, 34.2% vs 22.9% and day 9 35.9% vs 24.5%). Relative mRNA abundance of three genes in cumulus cells (HSP90AA1, CPT1B, G6PD) and three genes in blastocysts (DNMT3A, PLAC8, GPX1) indicated significantly different expression between groups (p < 0.05)., The expression of G6PD, SOD2, GXP1 in oocytes and PTGS2 in blastocysts tended to differ among groups (0.05<p < 0.08). Heat stress altered (p < 0.05) the correlation of expression between HSPs and other genes in oocytes (G6PD, GPX1, CCNB1), cumulus cells (LDH, CCNB1) and blastocysts (AKR1B1, PLAC8). These results imply that exposure of oocytes to elevated temperature, even for only 6 h, disrupts the developmental competence of the oocytes, suppresses blastocyst yield and significantly alters the coordinated pattern of gene expressions.

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.

Gamete quality in a multistressor environment

Over the past few decades, accumulated evidence confirms that the global environment conditions are changing rapidly. Urban industrialization, agriculture and globalization have generated water, air and soil pollution, giving rise to an environment with a growing number of stress factors, which has a serious impact on the fitness, reproduction and survival of living organisms. The issue raises considerable concern on biodiversity conservation, which is now at risk: it is estimated that a number of species will be extinct in the near future. Sexual reproduction is the process that allows the formation of a new individual and is underpinned by gamete quality defined as the ability of spermatozoa and oocytes to interact during fertilization leading to the creation and development of a normal embryo. This review aimed to provide the current state of knowledge regarding the impact of a broad spectrum of environmental stressors on diverse parameters used to estimate and evaluate gamete quality in humans and in canonical animal models used for experimental research. Effects of metals, biocides, herbicides, nanoparticles, plastics, temperature rise, ocean acidification, air pollution and lifestyle on the physiological parameters that underlie gamete fertilization competence are described supporting the concept that environmental stressors represent a serious hazard to gamete quality with reproductive disorders and living organism failure. Although clear evidence is still limited, gamete capacity to maintain and/or recover physiological conditions is recently demonstrated providing further clues about the plasticity of organisms and their tolerance to the pressures of pollution that may facilitate the reproduction and the persistence of species within the scenario of global change. Changes in the global environment must be urgently placed at the forefront of public attention, with a massive effort invested in further studies aimed towards implementing current knowledge and identifying new methodologies and markers to predict impairment of gamete quality.

Effect of season on the in-vitro maturation and developmental competence of buffalo oocytes after somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) is a valuable technology tool with various uses in transgenic animals, regenerative medicine, and stem cell research. However, the efficiency of SCNT embryos appears to have poor developmental competency. Environmental issues may adversely affect SCNT embryos in buffalo. Thereafter, the present study aimed to explore the effect of season on the maturation of buffalo oocytes and subsequent developmental capability after parthenogenetic activation and SCNT in buffalo. Buffalo oocytes (n = 6353) were collected from local slaughterhouse at various seasons; spring (March-April), summer (May-August), autumn (September-November), and winter (December-January). A significant increase (p < 0.05) was recorded in the maturation rate (57.07%) at autumn compared with spring, summer, and winter (50.46, 50.93, and 50.66%, respectively). No significant differences were recorded in the fusion and the cleavage rates among all seasons. Blastocyst development rate was higher (p < 0.05) in autumn and winter (16.52 ± 8.45% and 15.98 ± 7.17%, respectively) than in spring and summer (9.47 ± 6.71% and 10.84 ± 6.58%, respectively) seasons. It could be concluded that the season had a significant effect on oocyte development competence which can be used for SCNT in buffalo.

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.

Invited review: Management strategies capable of improving the reproductive performance of heat-stressed dairy cattle

Impaired fertility during periods of heat stress is the culmination of numerous physiological responses to heat stress, ranging from reduced estrus expression and altered follicular function to early embryonic death. Furthermore, heat-stressed dairy cattle exhibit a unique metabolic status that likely contributes to the observed reduction in fertility. An understanding of this unique physiological response can be used as a basis for improving cow management strategies, thereby reducing the negative effects of heat stress on reproduction. Potential opportunities for improving the management of dairy cattle during heat stress vary greatly and include feed additives, targeted cooling, genetic selection, embryo transfer and, potentially, crossbreeding. Previous studies indicate that dietary interventions such as melatonin and chromium supplementation could alleviate some of the detrimental effects of heat stress on fertility, and that factors involved in the methionine cycle would likely do the same. These supplements, particularly chromium, may improve reproductive performance during heat stress by alleviating insulin-mediated damage to the follicle and its enclosed cumulus-oocyte complex. Beyond feed additives, some of the simplest, yet most effective strategies involve altering the timing of feeding and cooling to take advantage of comparatively low nighttime temperatures. Likewise, expansion of cooling systems to include breeding-age heifers and dry cows has significant benefits for dams and their offspring. More complicated but promising strategies involve the calculation of breeding values for thermotolerance, the identification of genomic markers for heat tolerance, and the development of bedding-based conductive cooling systems. Unfortunately, no single approach can completely rescue the fertility of lactating dairy cows during heat stress. That said, region-appropriate combinations of strategies can improve reproductive measures to reasonable levels.

Contrasting effects of heat shock during in vitro maturation on development of in vitro-fertilized and parthenogenetic bovine embryos

This study investigated the influence of heat shock during in vitro maturation on embryo development following in vitro fertilization (IVF) or parthenogenesis (Part). Immature bovine cumulus-oocyte complexes were exposed to heat shock (41.0°C) during the first 12 hr of in vitro maturation (IVM), followed by 12 hr at 38.5°C. Control group consisted of in vitro maturation for 24 hr at 38.5°C. Oocytes were in vitro-fertilized or activated with ionomycin and cultured in vitro for 192 hr post-in vitro insemination or parthenogenetic activation (hpia). There was an interaction (p < .01) between temperature of IVM and method of oocyte activation (IVF or Part) for cleavage at 48 hpia. Heat shock had a negative impact (p < .01) on cleavage of IVF embryos, whereas no (p > .05) effect was found in the Part embryos. Embryo development towards blastocyst stage at 168 and 192 hpia decreased in both IVF and Part embryos derived from heat-shocked oocytes. Heat shock increased (p < .05) the apoptotic index in Part blastocysts, but no effect (p > .05) was found in IVF counterparts. Heat shock also down-regulated the expression of AQP3 (p < .01) and up-regulated the expression of HSP70.1 (p < .01) in Part blastocysts, whereas it down-regulated the expression of ATP1A1 (p < .05) in IVF blastocysts. In conclusion, the effects of heat shock during IVM on early embryo cleavage and blastocyst apoptosis are influenced by the method of oocyte activation and expression of some genes can be disturbed in embryos derived from heat-shocked oocytes.

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.

Oocyte quality and heat shock proteins in oocytes from bovine breeds adapted to the tropics under different conditions of environmental thermal stress

In order to evaluate the influence of thermal stress on physiological parameters, and the oocyte quality of Girolando (n = 12) and adapted Pantaneira (n = 12) cattle, twelve sessions of ultrasound guided follicular aspiration (OPU) were performed, between January and November 2014 (during dry (May-September) and rainy season (October-April) in Brazil). The recovered cumulus-oocyte complexes (COCs) were selected and classified, according to quality, immediately after OPU. The oocytes were then stored in 3% paraformaldehyd before conducting immunofluorescence analysis under confocal microscopy to identify HSP70 and 90 proteins. Before each OPU session, the rectal temperature (RT) and respiratory frequency (RF) of each animal were measured. The black globe humidity index (BGHI) was calculated on the day of the OPUs and 90 days before each OPU session, and related to the thermal stress of the animals. The quality of oocytes from Girolando cattle, but not Pantaneira, showed a negative relationship with BGHI of 90 days before OPU. RT of both breeds did not exceed normal values for cattle below BGHI 95. BGHI variation on the day of OPU did not affect RF of the adapted Pantaneira breed (p = 0.3221). On the other hand, Girolando cattle showed a positive relationship between RF and BGHI (p = 0.0103). With increasing BGHI, the amount of HSP70 increased in Girolando oocytes, however, decreased in the Pantaneira breed. We have not observed a relationship between HSP 90 and BGHI, however Girolando cattle produced a greater amount of this protein in relation to the Pantaneira breed. In conclusion, higher BGHIs, 90 days before OPU session, negatively affect oocyte quality of Girolando cattle and positively affect oocyte quality of the Pantaneira breed. Higher BGHIs on the day of the OPU session negatively affected the respiratory frequency of the Girolando breed, and lead to a higher recruitment of HSP70 to protect oocyte maturation. The opposite pattern was observed for Pantaneira. In addition, Pantaneira cattle produced twice as much as HSP70 as Girolando cattle, suggesting that a natural higher production of this protein could be involved in the mechanisms of adaptation to heat conditions.

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.

Effects of reproductive season on embryo development in the buffalo

Interest in buffalo farming is increasing worldwide due to the critical role played by buffaloes as sources of animal protein in tropical and subtropical environments. However, reproductive seasonality negatively affects the profitability of buffalo farming. Buffaloes tend to be short-day breeders, with seasonality patterns increasing with greater distances from the Equator. Although ovarian cyclic activity may occur throughout the year, seasonal anoestrus and cycles in calving and milk production are recorded. When buffaloes are forced to mate during the unfavourable season, to meet market demand, they may undergo a higher incidence of embryo mortality. This review addresses the effects of the reproductive season on embryo development in the buffalo, analysing the different factors involved in determining embryo mortality during the unfavourable season, such as impaired luteal function, oocyte competence and sperm quality. The review then focuses on strategies to control the photoperiod-dependent annual fluctuations in conception and embryo mortality in the female buffalo.

Heat stress impairs gap junction communication and cumulus function of bovine oocytes

The intimate association of cumulus cells with one another and with the oocyte is important for regulating oocyte meiotic arrest and resumption. The objective of this study was to determine the effects of heat stress on cumulus cell communication and functions that may be related to accelerated oocyte meiosis during early maturation. Bovine cumulus-oocyte complexes underwent in vitro maturation for up to 6 h at thermoneutral control (38.5°C) or elevated (40.0, 41.0 or 42.0°C) temperatures. Gap junction communication between the cumulus cells and the oocyte was assessed using the fluorescent dye calcein after 4 h of in vitro maturation. Dye transfer was reduced in cumulus-oocyte complexes matured at 41.0°C or 42.0°C; transfer at 40.0°C was similar to control (P < 0.0001). Subsequent staining of oocytes with Hoechst revealed that oocytes matured at 41.0 or 42.0°C contained chromatin at more advanced stages of condensation. Maturation of cumulus-oocyte complexes at elevated temperatures reduced levels of active 5’ adenosine monophosphate activated kinase (P = 0.03). Heat stress exposure had no effect on active extracellular-regulated kinase 1/2 in oocytes (P = 0.67), associated cumulus cells (P = 0.60) or intact cumulus-oocyte complexes (P = 0.44). Heat-induced increases in progesterone production by cumulus-oocyte complexes were detected during the first 6 h of maturation (P = 0.001). Heat-induced alterations in gap junction communication and other cumulus-cell functions likely cooperate to accelerate bovine oocyte meiotic progression.

Mitochondrial-related consequences of heat stress exposure during bovine oocyte maturation persist in early embryo development

Hyperthermia during estrus has direct consequences on the maturing oocyte that carries over to the resultant embryo to compromise its ability to continue in development. Because early embryonic development is reliant upon maternal transcripts and other ooplasmic components, we examined impact of heat stress on bovine oocyte transcripts using microarray. Oocytes were matured at 38.5ºC for 24 h or 41.0ºC for the first 12 h of in vitro maturation; 38.5ºC thereafter. Transcriptome profile was performed on total (adenylated + deadenylated) RNA and polyadenylated mRNA populations. Heat stress exposure altered the abundance of several transcripts important for mitochondrial function. The extent to which transcript differences are coincident with functional changes was evaluated by examining reactive oxygen species, ATP content, and glutathione levels. Mitochondrial reactive oxygen species levels were increased by 6 h exposure to 41.0ºC while cytoplasmic levels were reduced compared to controls (P < 0.0001). Exposure to 41.0ºC for 12 h increased total and reduced glutathione levels in oocytes at 12 h but reduced them by 24 h (time × temperature P < 0.001). ATP content was higher in heat-stressed oocytes at 24 h (P < 0.0001). Heat-induced increases in ATP content of matured oocytes persisted in early cleavage-stage embryos (8- to 16-cell embryos; P < 0.05) but were no longer apparent in blastocysts (P > 0.05). Collectively, results indicate that direct exposure of maturing oocytes to heat stress may alter oocyte mitochondrial processes/function, which is inherited by the early embryo after fertilization.

Developmental and molecular responses of buffalo (Bubalus bubalis) cumulus–oocyte complex maturedin vitrounder heat shock conditions

To investigate the effects of physiologically relevant heat shock during oocyte maturation, buffalo cumulus–oocyte complexes (COCs) were cultured at 38.5°C (control) or were exposed to 39.5°C (T1) or 40.5°C (T2) for the first 6 h ofin vitromaturation (IVM), followed by 38.5°C through the next 18 h/IVM and early embryonic development up to the blastocyst stage. Gene expression analysis was performed on selected target genes (HSF-1,HSF-2,HSP-70,HSP-90,BAX,p53,SOD1,COX1,MAPK14) in denuded oocytes and their isolated cumulus cells resulting from control COCs as well as from COCs exposed to a temperature of 39.5°C (T1). The results indicated that heat shock significantly (P< 0.01) decreased the maturation rate in T1 and T2 cells compared with the control. Afterin vitrofertilization (IVF), cleavage rate was lower (P< 0.01) for oocytes exposed to heat stress, and the percentage of oocytes arrested at the 2- or 4-cell stage was higher (P< 0.01) than that of the control. The percentage of oocytes that developed to the 8-cell, 16-cell or blastocyst stage was lower (P< 0.01) in both T1 and T2 groups compared with the control group. mRNA expression levels for the studied genes were decreased (P< 0.05) in treated oocytes (T1) except forHSP-90andHSF-1, which were increased. In cumulus cells isolated from COCs (T1), the expression for the target genes was upregulated except forBAX, which was downregulated. The results of this study demonstrated that exposure of buffalo oocytes to elevated temperatures for 6 h severely compromised their developmental competence and gene expression.

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.

Influence of maternal nutrition and heat stress on bovine oocyte and embryo development

The global population is expected to increase from 7.6 to 9.6 billion people from 2017 to 2050. Increased demand for livestock production and rising global temperatures have made heat stress (HS) a major challenge for the dairy industry. HS been shown to have negative effects on production parameters such as dry matter intake, milk yield, and feed efficiency. In addition to affecting production parameters, HS has also been shown to have negative effects on the reproductive functions of dairy cows. Mitigation of HS effects on dairy cow productivity and fertility necessitate the strategic planning of nutrition, and environmental conditions. The current review will discuss the potential nutriepigenomic strategies to mitigate the effect of HS on bovine embryo.

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.

Microfluidic analysis of oocyte and embryo biomechanical properties to improve outcomes in assisted reproductive technologies

Measurement of oocyte and embryo biomechanical properties has recently emerged as an exciting new approach to obtain a quantitative, objective estimate of developmental potential. However, many traditional methods for probing cell mechanical properties are time consuming, labor intensive and require expensive equipment. Microfluidic technology is currently making its way into many aspects of assisted reproductive technologies (ART), and is particularly well suited to measure embryo biomechanics due to the potential for robust, automated single-cell analysis at a low cost. This review will highlight microfluidic approaches to measure oocyte and embryo mechanics along with their ability to predict developmental potential and find practical application in the clinic. Although these new devices must be extensively validated before they can be integrated into the existing clinical workflow, they could eventually be used to constantly monitor oocyte and embryo developmental progress and enable more optimal decision making in ART.

Causes of declining fertility in dairy cows during the warm season

In the Northern Hemisphere, from June to September and in the Southern Hemisphere from December to March, there are periods of reduced fertility (sub-fertility) in dairy cows that are described as summer infertility. Several factors contribute to sub-fertility during this time, such as ambient temperature, humidity and photoperiod. During the warm season there is a reduction in feed intake that may compromise the energy balance of the cow and/or induce an imbalance in the activity of the hypothalamo-hypophyseal-ovarian axis. These factors reduce the reproductive performance of the cow and compromise the quality of oocytes, embryos and corpora lutea. This paper reviews current knowledge on the metabolic and endocrine mechanisms that induce summer infertility and describe their effects on follicle, oocyte and embryo development in dairy cows.

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.