Presence and significance of temperature gradients among different ovarian tissues

Clinical pregnancy rates after blastocyst culture at a stable temperature of 36.6°C versus 37.1°C: a prospective randomized controlled trial

STUDY QUESTION

Is there a difference in clinical pregnancy rates (CPRs) in good prognosis patients after single embryo transfer (SET) on Day 5, in case of stable culture at 36.6°C or 37.1°C?

SUMMARY ANSWER

CPR (with heartbeat at 7 weeks) after blastocyst transfer do not differ after culturing at 36.6°C or 37.1°C.

WHAT IS KNOWN ALREADY

Since the beginning of IVF, embryo culture has been performed at 37.0°C; however, the optimal culture temperature remains unknown. Changes in incubator types have led to significant improvements in temperature control. Stable temperature control, i.e. with temperature differences of max. 0.1°C between chambers, is possible in some incubators. A previous prospective pilot study showed that embryo development on Day 5/6 was not affected when embryos were cultured at a stable temperature of 36.6°C or 37.1°C, but culture at 37.1°C resulted in an increased CPR when compared to culture at 36.6°C (74.2% vs 46.4%).

STUDY DESIGN, SIZE, DURATION

A prospective randomized controlled trial was performed in a tertiary fertility centre between February 2017 and November 26, 2022. A sample size of 89/89 patients with fresh single embryo transfer (SET) was required to achieve 80% power to detect a difference of 0.22 between group proportions (0.43-0.65) at a significance level of 0.05 using a two-sided z-test with continuity correction.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Patients were recruited on the day of oocyte retrieval based on inclusion criteria with final randomization after denudation once six mature oocytes were present. The primary endpoint was CPR (heartbeat at 7 weeks); secondary endpoints were fertilization rate, blastocyst development, biochemical pregnancy rate, live birth rate (LBR), and cumulative live birth rate (CLBR).

MAIN RESULTS AND THE ROLE OF CHANCE

A total of 304 patients were eligible for the study; of these 268 signed the consent, 234 (intention-to-treat) were randomized and 181 (per-protocol) received a SET on Day 5: 90 received culture at 36.6°C and 91 at 37.1°C. Patients were on average 32.4 ± 3.5 versus 32.5 ± 4.2 years old, respectively. No differences were observed in embryological outcomes per cycle between culture at 36.6°C versus 37.1°C: 12.0 ± 3.8 vs 12.1 ± 3.8 COCs retrieved (P = 0.88), 10.0 ± 3.1 versus 9.9 ± 2.9 mature oocytes inseminated (P = 0.68), with a maturation rate of 84.2% (901/1083) versus 83.5% (898/1104) (P = 0.87); and 8.0 ± 3.1 versus 7.9 ± 2.7 normally fertilized oocytes with a fertilization rate of 79.7% (720/901) vs 80.5% (718/898) (P = 0.96), respectively. On average 1.5 ± 1.7 versus 1.4 ± 1.9 (P = 0.25) and 1.1 ± 1.1 versus 0.9 ± 1.0 (P = 0.45) supernumerary blastocysts were vitrified on Day 5 and Day 6, respectively. The utilization rate per fertilized oocyte was 46.1% vs 41.5% (P = 0.14). A SET was performed for 181 patients, leading to a biochemical pregnancy rate of 72.2% (65/90) versus 62.7% (57/91) (P = 0.17), respectively. The CPR per fresh transfer cycle was 51.1% (46/90) versus 48.4% (44/91) [OR (95% CI) 1.11 (0.59-2.08), P = 0.710]. To date, a CLBR of 73.3% (66/90) versus 67.0% (61/91) (P = 0.354) has been observed, respectively. In each group, seven patients without live birth have remaining blastocysts frozen. The CPR for the intention-to-treat groups were 38.3% vs 38.6% [OR (95% CI) 0.98 (0.56-1.73), P = 0.967], respectively, for culture at 36.6°C versus 37.1°C.

LIMITATIONS, REASONS FOR CAUTION

Only selected patients with expected good prognosis were eligible for the study.

WIDER IMPLICATIONS OF THE FINDINGS

Embryos tend to tolerate small changes in temperature deviations during culture to the blastocyst stage, as demonstrated by their similar implantation potential at two slightly different temperatures.

STUDY FUNDING/COMPETING INTEREST(S)

There is no funding or conflicts of interest to declare.

TRIAL REGISTRATION NUMBER

NCT03548532.

TRIAL REGISTRATION DATE

23 October 2017.

DATE OF FIRST PATIENT’S ENROLMENT

10 November 2017.

Cervix-Deep Rectal Temperature Differential on the Day of Ovulation is Correlated With Embryo Recovery Results in Mares

Variations in temperature throughout the reproductive tract have been noted in many species. A recent study found the cervix-rectum temperature differential (CR-TD) in cattle was related to fertility. The present study aimed to assess the CR-TD in mares around the time of ovulation and relate it to embryo recover. Over 52 cycles, mares were inseminated with a fertile stallion and embryo recovery was undertaken on Day 7 post ovulation. Further 10 control mares were not inseminated. Rectal and cervical temperatures were measured using a precision thermometer on four or five occasions: the day of deslorelin administration and insemination, the day before ovulation, the day of ovulation (Day 0), the day after ovulation and, for inseminated mares, before embryo recovery on Day 7. One-way ANOVA showed that the CR-TD was significantly lower on the day of ovulation in the 36 positive cycles, in which an embryo was recovered, versus the 16 in which the embryo flush was negative (0.21 ± 0.17 vs. 0.40 ± 0.09°C; p < 0.001). Control cycles showed equivalent CR-TD to positive (0.13 ± 0.22 vs. 0.21 ± 0.17°C; p = 0.196) but not negative cycles (0.13 ± 0.22 vs. 0.40 ± 0.09°C; p < 0.001). A positive embryo recovery was associated with lower CR-TDs from the time of insemination and deslorelin to the day after ovulation compared to the day of embryo flushing (RM ANOVA; p < 0.001; Pairwise comparisons; p ≤ 0.01, in all cases). Rectal or cervical temperatures per se showed no significant differences between positive, negative or control cycles at any time point. In conclusion, a thermoregulatory process occurs close to ovulation which results in a lower CR-TD in cycles that produced an embryo versus those which did not. Further characterisation of TDs within the reproductive tract of the mare would increase our understanding of the conditions required for optimum fertility.

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.

Temperature-sensitive migration dynamics in neutrophil-differentiated HL-60 cells

Cell migration plays an essential role in wound healing and inflammatory processes inside the human body. Peripheral blood neutrophils, a type of polymorphonuclear leukocyte (PMN), are the first cells to be activated during inflammation and subsequently migrate toward an injured tissue or infection site. This response is dependent on both biochemical signaling and the extracellular environment, one aspect of which includes increased temperature in the tissues surrounding the inflammation site. In our study, we analyzed temperature-dependent neutrophil migration using differentiated HL-60 cells. The migration speed of differentiated HL-60 cells was found to correlate positively with temperature from 30 to 42 °C, with higher temperatures inducing a concomitant increase in cell detachment. The migration persistence time of differentiated HL-60 cells was higher at lower temperatures (30-33 °C), while the migration persistence length stayed constant throughout the temperature range. Coupled with the increased speed observed at high temperatures, this suggests that neutrophils are primed to migrate more effectively at the elevated temperatures characteristic of inflammation. Temperature gradients exist on both cell and tissue scales. Taking this into consideration, we also investigated the ability of differentiated HL-60 cells to sense and react to the presence of temperature gradients, a process known as thermotaxis. Using a two-dimensional temperature gradient chamber with a range of 27-43 °C, we observed a migration bias parallel to the gradient, resulting in both positive and negative thermotaxis. To better mimic the extracellular matrix (ECM) environment in vivo, a three-dimensional collagen temperature gradient chamber was constructed, allowing observation of biased neutrophil-like differentiated HL-60 migration toward the heat source.

Cervix-rectum temperature differential at the time of insemination is correlated with the potential for pregnancy in dairy cows

This study sought to establish whether temperature gradients between the cervix, vagina, and rectum at and 7 days post-artificial insemination (AI) were associated with the incidence of pregnancy in lactating dairy cows (Experiment I; n = 90 ovulating cows) and to evaluate temperature gradient dynamics from the time of insemination to 7 days post-AI under heat stress conditions (Experiment II; n = 16 ovulating and 4 non-ovulating cows). In Experiment I, 39 cows (43.3%) became pregnant. The odds ratio for pregnancy was 2.5 for each one-tenth of a degree drop in cervical temperature with reference to the control rectal temperature at the time of AI (P = 0.01), whereas the same decrease in the cervix–rectum temperature differential 7 days post-AI resulted in an odds ratio of 0.44 (P = 0.02). In Experiment II, 5 of the ovulating cows (31.3%) became pregnant. The mean values of the vagina–rectum, vagina–cervix, and cervix–rectum temperature differentials at AI (day 0), 8 h, 24 h, and 7 days post-AI changed significantly from day 0 to day 7 (within-subject effect; P < 0.02) in ovulating cows but not in non-ovulating cows. Temperature differentials on days 0 and 7 were similar between ovulating cows and cows of Experiment I. Overall, our findings support the notion that a temperature differential between the caudal cervical canal and rectum at AI may be an indicator of the likelihood of pregnancy. Possible prospects of confirming estrus at the herd-level are also suggested.

Thermal Mechanisms Preventing or Favoring Multiple Ovulations in Dairy Cattle

Simple Summary

While cows are usually monovular, the incidence of dizygous twin births has recently increased considerably alongside increasing milk production. Genetic progress and improvements in nutrition and management practices have led to a continuous increase in milk yield and thus also to multiple ovulations and twin pregnancies. Twin pregnancies are undesirable as they seriously compromise the welfare of the cow and herd economy. A better understanding of the processes associated with multiple ovulations should help to reduce rates of twinning. During the stages of the sexual cycle, temperature gradients are established within the ovary and throughout the genital tract. Pre-ovulatory local cooling of the reproductive system favors male and female gamete maturation and subsequent fertilization. In fact, thermal mechanisms may prevent or favor multiple ovulations and thus twinning. The purpose of this review was to update this topic.

Abstract

While cows are predominantly monovular, over the past 30 years the incidence of multiple ovulations and thus twinning has increased considerably alongside milk production. Multiple pregnancies are not desirable as they negatively affect the health of cows and the herd economy. Although causal mechanisms associated with multiple ovulations have been extensively revised, the process of multiple ovulations is not well understood. Recent studies on the thermal biology of the reproductive system have shown how thermal mechanisms may prevent or favor multiple ovulations. This review focuses on this relationship between thermal dynamics and multiple pregnancies. Cooling of the pre-ovulatory follicle is able to regulate ovulation. In effect, pre-ovulatory local cooling of the female reproductive system favors male and female gamete maturation and promotes fertilization. Thermal stress is proposed here as a model of stress. Periods of high ambient temperature affect the processes of pre-ovulatory follicular cooling and multiple ovulations. While the ratio between unilateral and bilateral multiple pregnancies is normally close to one, under heat stress conditions, this ratio may be 1.4 favoring unilateral multiple pregnancies. A ratio approaching unity is here proposed as an indicator of cow wellbeing.

Temperature gradients in the mammalian ovary and genital tract: A clinical perspective

Temperature within mammalian reproductive tissues is noted to be a key component of fertility, and significant gradients in temperature can be demonstrated deep within the abdomen shortly before ovulation. Indeed, in the absence of such gradients in the ovary and genital tract, the processes of ovulation and fertilisation are severely compromised. This review aims to assess literature produced during the last five decades regarding temperature gradients in the mammalian ovary and genital tract. A large body of observations derived from rabbits, women, pigs and cattle is summarised in tabular form, ovarian follicular values being as much as 2.5 °C or more cooler than neighbouring ovarian tissues or deep rectal temperature. We highlight recent works demonstrating a positive correlation between pre-ovulatory follicular cooling and pregnancy. Understanding the significance of follicular cooling should help us (a) explain why so many potential pregnancies fail in vivo as in vitro and (b) inspire ways for improving the processes of fertilisation and establishment of a full-term pregnancy. Based on our findings in domestic animals, and most recently in cows whose Graafian follicles are comparable in size and timing of response to the LH peak with human follicles, we wish to encourage IVF and fertility preservation clinics to take advantage of this work. By so doing, the incidence of full-term pregnancies in women should be improved.

Evolutionary sequences in mammalian reproductive biology

Broad biological aspects and accepted evolutionary sequences may offer useful guidance towards a comprehensive explanation of the function of mammalian ovaries and oviducts and their vital contribution to the events of fertilization. Cooling of the preovulatory follicles before ovulation may well have its roots in the primitive stages of external fertilization in which aggregates of oocytes are shed into freshwater.

Local cooling of the ovary and its implications for heat stress effects on reproduction

The effects of season on the fertility of the dairy cow added to the metabolic stress of milk production are well known. We here present lactating dairy cows as a comparative model of this problem. This review examines the results of recent studies that have highlighted heat stress (HS) effects on pre-ovulatory follicles. From these studies, we draw information regarding the mechanisms giving rise to temperature gradients across reproductive tissues. Our review is completed by a discussion of approaches designed to reduce the negative effects of HS based on cooling strategies implemented before ovulation at or around estrus.

Pre-ovulatory follicular cooling correlates positively with the potential for pregnancy in dairy cows: Implications for human IVF

INTRODUCTION

In rabbits, pigs, cows and humans, pre-ovulatory Graafian follicles may be more than 1.0 °C cooler than ovarian stroma and both these ovarian compartments are cooler than deep rectal temperatures. This study examines the effect of follicular cooling on the incidence of pregnancy in dairy cows.

MATERIAL AND METHODS

Follicular measurements were compiled for cows with one ovulatory follicle (monovular) and cows with one ovulatory follicle per ovary (bi-ovular) and their corresponding uterine horn contents. The study sample consisted of 80 pre-ovulatory follicles in which antral temperatures were measured using a fine thermistor probe.

RESULTS

Mean (± S.D.) follicular fluid temperature of the ovulating follicles was 1.12 ± 0.86 °C significantly cooler (P < 0.0001) than rectal temperatures. No significant differences in temperatures were found for non-ovulating follicles. In follicles undergoing cooling (n = 58), a one-tenth of a degree drop in temperature with reference to control rectal temperature gave rise to a 3.6-fold increase (odds ratio) in the pregnancy rate (P = 0.003). The follicle-rectum temperature differential giving rise to pregnancy (n = 18; 1.51 ± 1.15 °C) was significantly greater (P = 0.004) than the differential recorded in cooling follicles at that did not result in a subsequent pregnancy (n = 40; 0.83 ± 0.57 °C).

CONCLUSION

Follicular cooling is needed to trigger ovulation and correlates positively with the potential for pregnancy in cows. This finding has interesting implications for human reproductive medicine.

The effect of different temperature conditions on human embryosin vitro: two sibling studies

RESEARCH QUESTION

What temperature is optimal for human embryo development up to day 5 or 6?

DESIGN

Two prospective sibling oocyte studies on culture temperature were conducted in a university-based tertiary referral centre. Eligibility critera for both studies: Study 1: 50 cycles between August and October 2015, with culture at a stable temperature (37.0°C ± 0.3°C) or culture using a circadian temperature rhythm (CTR) (1 am to 6 am: 36.6°C, gradual increase to 37.5°C; 11 am to 9 pm: 37.5°C; gradual decrease to 36.6°C); study 2: 99 cycles between April and November 2016, with stable culture at 36.6°C or 37.1°C.

PRIMARY OUTCOME MEASURES

fertilization and embryo development (top and good quality) up to day 5 or 6, and utilization rate (number of embryos transferred and cryopreserved per zygote). Secondary outcome measure: clinical pregnancy (number of pregnancies with at least one gestational sac).

RESULTS

An incubator with CTR was used for culture. An effect was found on embryo development (utilization rate: 42.1% versus 32.6%; P = 0.014), but not on clinical pregnancy rate (60.0% versus 45.5%; P = 0.670). Stable culture at 36.6°C or 37.1°C did not affect embryo development (utilization rate: 40.0% versus 40.4%; P = 0.905); clinical pregnancy rate was improved by culture at 37.1°C (46.4% versus 74.2%; P = 0.036).

CONCLUSION

Culture in an incubator with CTR does not improve fertilization rate or embryo quality. Embryo culture at 36.6°C or 37.1°C showed similar embryo development.

Pre-ovulatory follicular temperature in bi-ovular cows

In a previous study on monovular cows, follicles revealed a mean antral (follicular fluid) temperature 1.54°C cooler than rectal temperatures in ovulating cows, whereas no such temperature differences were detected in non-ovulating cows. The present study adds to our previous work, this time considering 24 bi-ovular cows (one follicle per ovary). In order to increase the number of pre-ovulatory follicles failing to ovulate, this study was performed under heat-stress conditions. Follicular temperatures of the ovulating follicles (n = 31) were 0.93°C significantly cooler (P < 0.0001) than rectal temperatures, whereas no significant differences in temperature were found in non-ovulating follicles (n = 17). Eight cows became pregnant. The results of the present study indicate that, similar to those in monovular cows, pre-ovulatory follicles in bi-ovular cows were cooler than deep rectal temperatures and those temperature gradients were not found in follicles showing ovulation failure.

In vivo oxygen, temperature and pH dynamics in the female reproductive tract and their importance in human conception: a systematic review

BACKGROUND

Despite advances in ART, implantation and pregnancy rates per embryo transfer still remain low. IVF laboratories strive to ensure that the process of handling gametes in vitro closely mimics the in vivo environment. However, there remains a lack of knowledge regarding the in vivo regulation and dynamic variation in biophysical parameters such as oxygen concentration, pH and temperature within the reproductive tract.

OBJECTIVE AND RATIONALE

To undertake a systematic review of the current understanding of the physico-chemical parameters of oxygen tension (pO2), pH and temperature within the female reproductive tract, and their potential implications in clinical and pathological processes related to fertility and those pertaining to limited reproductive capacity.

SEARCH METHODS

A comprehensive literature search was performed using electronic databases including Medline, Embase, Cochrane Library and Pubmed to identify original and review articles addressing the biophysical parameters (pO2, pH and temperature) in the female reproductive tract of any species. The search included all studies published between 1946 and November 2015. Search terms included 'oxygen', 'pH', 'hydrogen ion concentration', 'acid base' and others terms. We also used special features and truncations to identify synonyms and broaden the search. Studies were excluded if they only assessed embryo culture conditions, fetal acid-base status, oxidative stress, outcomes of pregnancy and measurements of these parameters in non-reproductive organs.

OUTCOMES

Our search generated 18 685 records and 60 articles were included. pO2 within the female reproductive tract shows cyclical variation and minute-to-minute oscillations, which may be influenced by uterine contractility, hormones, the autonomic system, cardiac pulsatility, and myometrial and smooth muscle integrity. Fine balanced control of pO2 and avoidance of overwhelming oxidative stress is crucial for embryogenesis and implantation. The pH in the female reproductive tract is graduated, with lowest pH in the vagina (~pH 4.42) increasing toward the Fallopian tubes (FTs) (~pH 7.94), reflecting variation in the site-specific microbiome and acid-base buffering at the tissue/cellular level. The temperature variation in humans is cyclical by day and month. In humans, it is biphasic, increasing in the luteal phase; with the caudal region of the oviduct 1-2 degrees cooler than the cranial portion. Temperature variation is influenced by hormones, density of pelvic/uterine vascular beds and effectiveness of heat exchange locally, crucial for sperm motility and embryo development. We have identified significant deficiencies and inconsistencies in the methods used to assess these biophysical factors within the reproductive tract. We have suggested that the technological solutions including the development of methods and models for real time, in vivo recordings of biophysical parameters.

WIDER IMPLICATIONS

The notion of 'back to nature' in assisted conception suggested 20 years ago has yet to be translated into clinical practice. While the findings from this systematic review do not provide evidence to change current in vitro protocols, it highlights our current inability to assess the in vivo reproductive tract environment in real time. Data made available through future development of sensing technology in utero may help to provide new insights into how best to optimize the in vitro embryo environment and allow for more precise and personalized fertility treatment.

Whither human IVF? Fertilisable oocytes selected on the basis of follicular temperature

Bearing in mind specific parallels between cow and human ovarian physiology, as noted in the manuscript, we have measured whether the temperature in a pre-ovulatory follicle is cooler than that in adjacent tissues. Using a novel approach not requiring anaesthetics or surgical procedures, we found that follicular fluid bathing cow oocytes shortly before ovulation is cooler than the neighbouring uterine surface and cooler than deep rectal temperature (the reference body temperature in cattle). By contrast, Graafian follicles of comparable size and ultrasonic image that do not subsequently ovulate do not have a reduced antral temperature. Human pre-ovulatory follicles have previously been reported to be cooler than other ovarian tissues, so the divergence between ovulatory and non-ovulatory follicle temperature suggests a valuable addition to selection procedures currently used in human in vitro fertilisation (IVF) clinics. In future, oocytes to be subjected to IVF might best be those taken from cooler follicles. Follicular antral temperature could become a more sensitive indicator of oocyte potential that a purely morphological assessment.

Temperature gradients in vivo influence maturing male and female gametes in mammals: evidence from the cow

Since 1980 several reports have indicated that temperatures vary between preovulatory follicles and other ovarian tissues in rabbit, cow, pig and human. However, these observations did not achieve prominence; they were regarded as artefacts due to the use of anaesthetics and open surgery (laparotomy). Recently, without resorting to anaesthesia or surgery, direct measurements of temperature in preovulatory follicles have been performed in the cow by means of a thermistor probe introduced into the antrum under ultrasonic guidance. Such follicles revealed a mean antral (follicular fluid) temperature 0.74°C and 1.54°C cooler than uterine surface and rectal temperatures respectively in ovulating cows, whereas no such temperature differences were detected in non-ovulating cows. Cows are predominantly monovular and preovulatory follicles attain a diameter of 15–22 mm or more. These features and the timescale of response to the preovulatory gonadotrophin surge make them a valuable model for the human preovulatory follicle. Temperature gradients are interpreted primarily in a context of final maturation of gametes immediately before the onset of fertilisation. Preovulatory follicular temperature in women could be assessed by a comparable approach and might become a valuable selection guide for oocyte viability.

The effect of scrotal reconstruction with skin flaps and skin grafts on testicular function

Due to its unique composition, the reconstruction of scrotal skin defects is a major clinical challenge. This study was designed to evaluate the effects of scrotal reconstruction, using skin grafts and skin flaps, on spermatogenesis. In Group 1, the rats did not undergo surgery and were used as controls. In Group 2, after removal of all of the scrotal skin to expose the testicles, the defect was repaired using a skin flap from the right groin region. In Group 3, the reconstruction was achieved using skin grafts. All the rats were killed at 2 months postoperatively and evaluated. The mean wet weights of the testicles in the control group were significantly higher compared with that of the graft group. The mean height of the germinal epithelium was significantly greater in the control and flap groups compared with that of the graft group. The Johnsen score for spermatogenesis in the control group was higher than that in the graft group. The use of flaps resulted in testicular function that was comparable to that of the control group, whereas the use of grafts resulted in diminished testicular function. Therefore, we suggest that flaps may be the first choice for scrotal reconstruction.

When is a cow in estrus? Clinical and practical aspects

Good detection of estrus is critically important in dairy husbandry. Incorrect detection of estrus is related to loss of profit due to extended calving intervals, milk loss, veterinary costs, etc. Detection of estrus remains a major problem despites enormous progress in the knowledge of reproductive physiology of the cow and in development of estrus detection aids. To achieve good estrus detection, many factors have to be taken into account. On one hand a cow has to express estrus and on the other hand the farmer has to detect it. Combined action of several hormones causes physiological changes that lead to ovulation and an environment in the uterus that allows sperm to fertilize the egg. Besides these internal actions, a number of external changes can be observed. When using visual observations, time of the day and time spend on observation have a great impact on detection rates. Many devices are available to aid in estrus detection, such as pedometers, mount devices, temperature, and hormone measurements. Expression of estrus can be influenced by many factors. Heritability, number of days postpartum, lactation number, milk production, and health are known to influence estrus expression. Environmental factors like nutrition, season, housing, herd size, etc. also play a role in estrus expression. To evaluate estrus detection, record keeping is very important; a number of formulas can be used to assess detection efficiency. Besides the farmer, the veterinarian and inseminator can play an important role in estrus confirmation and good insemination strategy. In the end, the time of ovulation and the age of the egg at sperm penetration is critical for conception. Therefore, emphasis in research needs to be on the timing of insemination relative to ovulation, and thus on the detection of ovulation.

Does heat stress provoke the loss of a continuous layer of cortical granules beneath the plasma membrane during oocyte maturation?

The objective of the present study was to evaluate the influence of heat stress on bovine oocyte maturation. Both nuclear stage and distribution of cortical granules (CG) were simultaneously evaluated in each oocyte. Oocyte overmaturation under standard conditions of culture was also evaluated. For this purpose, logistic regression procedures were used to evaluate possible effects of factors such as heat stress, overmaturation, replicate, CG distribution and metaphase II (MII) morphology on oocyte maturation. Based on the odds ratio, oocytes on heat stressed (HSO) and overmaturated (OMO) oocyte group were, respectively, 14.5 and 5.4 times more likely to show anomalous MII morphology than those matured under control conditions (CO). The likelihood for an oocyte of showing the CG distribution pattern IV (aging oocyte) was 6.3 and 9.3 times higher for HSO and OMO groups, respectively, than for the CO group. The risk of undergoing anomalous oocyte maturation, considering both nuclear stage and distribution of CG was 17.1 and 18 times greater in oocytes cultured in HSO and OMO groups, respectively, than those in the CO group. In conclusion, heat stress proved to be valuable in aging oocytes. Heat stress advanced age for nuclear and cytoplasmic processes in a similar form to that of oocyte overmaturation.

Embryo culture: can we perform better than nature?

Culture of preimplantation-stage embryos has always been a key element of laboratory embryology and has contributed substantially to the success of many assisted reproduction procedures. During the past decade, its importance has increased as extended in-vitro embryo culture and single blastocyst transfer have become indispensable parts of the approach to decreasing the chance of multiple pregnancy while preserving the overall efficiency of the treatment. However, in spite of the scientific and commercial challenge stimulating research worldwide to optimize embryo culture conditions, a consensus is missing even in the basic principles, including composition and exchange of media, the required physical and biological environment and even the temperature of incubation. This review attempts to summarize the controversies, demonstrate the fragility of some widely accepted dogmas and generate an open-minded debate towards rapid and efficient optimization. New approaches expanding the traditional frames of mammalian embryo culture are also discussed. Although some researchers suppose that the efficiency of the presently applied in-vitro culture systems have already approached the biological limits, authors are confident that substantial improvement may be achieved that may expand considerably the possibilities of future assisted reproduction in humans.

Metabolism of the viable mammalian embryo: quietness revisited

This review examines the 'Quiet Embryo Hypothesis' which proposes that viable preimplantation embryos operate at metabolite or nutrient turnover rates distributed within lower ranges than those of their less viable counterparts. The 'quieter' metabolism consistent with this hypothesis is considered in terms of (i) 'functional' quietness; the contrasting levels of intrinsic metabolic activity in different cell types as a consequence of their specialized functions, (ii) inter-individual embryo/cell differences in metabolism and (iii) loss of quietness in response to environmental stress. Data are reviewed which indicate that gametes and early embryos function in vivo at a lower temperature than core body temperature, which could encourage the expression of a quiet metabolism. We call for research to determine the optimum temperature for mammalian gamete/embryo culture. The review concludes by examining the key role of reactive oxygen species, which can induce molecular damage, trigger a cellular stress response and lead to a loss of quietness.

Physiological temperature variants and culture media modify meiotic progression and developmental potential of pig oocytes in vitro

Ovarian follicles in vivo are cooler than surrounding abdominal and ovarian tissues. This study investigated whether typical follicular temperatures influence the maturation and developmental potential of pig oocytes in vitro. Oocytes were synchronised at the germinal vesicle (GV) stage and incubated at 39, 37 or 35.5 degrees C. When compared with 39 degrees C, which is often used for in vitro studies, lower temperatures delayed spontaneous progression to the metaphase I and II (MI and MII) stages of meiosis. The MII was delayed by about 12 h per degrees C. All oocytes had normal morphology. Oocytes reaching GV breakdown (GVBD) at 39 degrees C were subsequently unaffected by cooling, demonstrating thermal sensitivity during the pre-GVBD stage only. Simultaneous assay of maturation-controlling kinases (maturation promoting factor (MPF) and MAPK) showed that cooling delayed kinase activation, provided it was applied prior to GVBD. Activity profiles remained coupled to the stage of meiosis. Neither enzyme was directly thermally sensitive over this temperature range. Following in vitro fertilisation, fewer blastocysts developed from embryos derived from 35.5 or 37 degrees C oocytes as compared with those from 39 degrees C oocytes. Manipulation of fertilisation timings to allow for delayed maturation showed that over-maturing or aging at lower temperatures compromises subsequent embryo development, despite normal nuclear maturation; the GV stage was again the thermally sensitive period. Cleavage rates were improved by the culture of oocytes with follicle-stimulating hormone (FSH) at 37 but not at 35.5 degrees C. Inclusion of 20% follicular fluid in the oocyte medium restored the blastocyst rate to that seen at higher temperatures. Thus, FSH and follicular fluid may allow oocytes to achieve normal developmental potential at in vivo temperatures.

Mathematical modelling of oxygen transport-limited follicle growth

Mathematical modelling was used to investigate oxygen transport in the developing ovarian follicle. In contrast to previous findings, the results show that oxygen can reach the oocyte in large preantral follicles. This is largely due to the inclusion of fluid voidage in the model and improved estimates of oxygen diffusion coefficients through the granulosa. The results also demonstrate that preantral follicles will eventually reach a size beyond which further growth will result in the follicle becoming increasingly anoxic. The predicted size range at which this occurs is consistent with the size range at which antrum formation is observed in many mammals. This suggests that the antrum formation stage of follicular growth may be pivotal to the further development and ultimate fate of the follicle, and that antrum formation itself may represent a mechanism by which the follicle can overcome oxygen limitations. This was supported through extension of the model to the antral follicle, which showed that antrum formation can provide a way in which the follicle can continue to grow and yet avoid becoming hypoxic. The results of the model were consistent with observed follicle development.