Follicular fluid responds endothermically to aqueous dilution

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.

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 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.

Putative human sperm Interactome: a networks study

Background

For over sixty years, it has been known that mammalian spermatozoa immediately after ejaculation are virtually infertile. They became able to fertilize only after they reside for long time (hours to days) within female genital tract where they complete their functional maturation, the capacitation. This process is finely regulated by the interaction with the female environment and involves, in spermatozoa, a myriad of molecules as messengers and target of signals. Since, to date, a model able to represent the molecular interaction that characterize sperm physiology does not exist, we realized the Human Sperm Interactme Network3.0 (HSIN3.0) and its main component (HSNI3.0_MC), starting from the pathway active in male germ cells.

Results

HSIN3.0 and HSIN3.0_MC are scale free networks, adherent to the Barabasi-Albert model, and are characterised by an ultra-small world topology. We found that they are resistant to random attacks and that are designed to respond quickly and specifically to external inputs. In addition, it has been possible to identify the most connected nodes (the hubs) and the bottlenecks nodes. This result allowed us to explore the control mechanisms active in driving sperm biochemical machinery and to verify the different levels of controls: party vs. date hubs and hubs vs. bottlenecks, thanks the availability of data from KO mice. Finally, we found that several key nodes represent molecules specifically involved in function that are thought to be not present or not active in sperm cells, such as control of cell cycle, proteins synthesis, nuclear trafficking, and immune response, thus potentially open new perspectives on the study of sperm biology.

Conclusions

For the first time we present a network representing putative human sperm interactome. This result gives very intriguing biological information and could contribute to the knowledge of spermatozoa, either in physiological or pathological conditions.

Electronic supplementary material

The online version of this article (10.1186/s12918-018-0578-6) contains supplementary material, which is available to authorized users.

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.

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.

Presence and significance of temperature gradients among different ovarian tissues

After recalling male gonadal physiology in respect of tissue temperatures within the scrotal sac, and raising questions concerning abdominal testes, attention turned to mature Graafian follicles and ovarian stroma. Temperature gradients between such tissues were summarized for human, rabbit, pig, and cow, and generally fell in the range of 1.3-1.7 degrees C: follicles were always cooler than stroma. Measurements were made principally by means of a thermo-sensing camera at midventral laparotomy, but also using microelectrodes or thermistor probes sited in the follicular antrum of rabbits and pigs, respectively. When thermo-imaged under the fimbriated extremity of the Fallopian tube, mature pig follicles and stroma could still be distinguished. Such follicles cooled slightly more rapidly during the first 10 s of a 60-s recording interval, after which curves for the two tissues remained parallel. Arresting ovarian blood supply for 5 min had a negligible influence on the temperature differentials. Endoscopy in three models recorded mean differentials of 0.6 +/- 0.1 degrees C - 1.1 +/- 0.1 degrees C between follicles and stroma, but such follicles had not attained mature diameter. Temperature gradients were thought to be generated at least in part by endothermic reactions within mature follicles, reflecting hydration of large extracellular matrix molecules such as proteoglycans. A contribution to the cooling process from the products of leukocyte activity in the follicle wall and antrum could also be involved. Temperature gradients would be maintained locally by counter-current heat exchange mechanisms and, in this context, the microvasculature and lymphatic flow of individual follicles were found to be appropriate. Observations on the temperature of preovulatory follicles appear relevant to procedures of in vitro maturation and in vitro fertilization.

Sperm thermotaxis

Thermotaxis--movement directed by a temperature gradient--is a prevalent process, found from bacteria to human cells. In the case of mammalian sperm, thermotaxis appears to be an essential mechanism guiding spermatozoa, released from the cooler reservoir site, towards the warmer fertilization site. Only capacitated spermatozoa are thermotactically responsive. Thermotaxis appears to be a long-range guidance mechanism, additional to chemotaxis, which seems to be short-range and likely occurs at close proximity to the oocyte and within the cumulus mass. Both mechanisms probably have a similar function--to guide capacitated, ready-to-fertilize spermatozoa towards the oocyte. The temperature difference between the site of the sperm reservoir and the fertilization site is generated at ovulation by a temperature drop at the former. The molecular mechanism of sperm thermotaxis waits to be revealed.

Sperm guidance in mammals - an unpaved road to the egg

Contrary to the prevalent view, there seems to be no competition in the mammalian female genital tract among large numbers of sperm cells that are racing towards the egg. Instead, small numbers of the ejaculated sperm cells enter the Fallopian tube, and these few must be guided to make the remaining long, obstructed way to the egg. Here, we review the mechanisms by which mammalian sperm cells are guided to the egg.

Pre-ovulatory temperature gradients within mammalian ovaries: a review

The existence of a temperature gradient between the testis and deep body temperature has been accepted for many years. It is based on two simultaneous principles: cooling of the testis through the scrotal wall and transfer of heat between the testicular blood vessels. The ovary is positioned in the abdomen; a temperature difference parallel to the male system therefore seems less likely. However, the temperature of large follicles has been found to be 0.5 to 1.5 degrees C cooler than the ovarian stroma in rabbits, pigs and, probably, women. The temperature difference seems to be based on a heat-consuming process in the expanding follicullar fluid, and a local transfer of heat between intra-ovarian blood vessels. The reason for the temperature gradient is not yet known; one may speculate of a common reason for the cooling of the gamete in male and female.

Periovulatory increase in temperature difference within the rabbit oviduct

BACKGROUND

Earlier studies demonstrated a small temperature difference between the sperm storage and fertilization sites within the oviducts of rabbits and pigs. Our aim was to reveal the time dependence of this temperature difference relative to ovulation, and to determine how this difference is generated-by temperature elevation at one of these sites or by temperature decrease at the other site.

METHODS

The temperature at the sperm storage site (at the isthmus near the uterotubal junction) and at the fertilization site (the isthmic-ampullary junction) of rabbit oviducts were measured before, during, and after ovulation by two probes, connected to digital thermometers. Rectal temperature was constantly measured and served as a control for body temperature.

RESULTS

The temperature difference between the fertilization site and the storage site was 0.8+/-0.2 degrees C before ovulation. This difference increased at ovulation, reaching 1.6+/-0.1 degrees C after ovulation (P<0.03). This increased difference was mainly due to temperature decrease in the sperm storage site.

CONCLUSION

The temperature-difference increase within the rabbit oviduct is generated at ovulation by a reduced temperature at the sperm storage site. This temperature gradient may play a role in mammalian reproduction via sperm thermotaxis.

Counter-current transfer in reproductive biology

Heat and substances, including gases, steroids and peptide hormones, can pass from venous blood, interstitial fluid and lymph to the arterial blood; the process is called local counter-current transfer. It has been found in various reproductive organs in many animal species and in man: from the testis to the testis and epididymis; from the ovary to the ovary, tube and tubal corner of the uterus; from the tube and uterus to the ovary; from vagina to uterus; and even between brain blood vessels. Local transfer within the ovary has also been found. Local cooling that creates temperature gradients between organs or within an organ is one aspect of the transfer. Physiologically, the transfer also facilitates local feedback regulation of organ function in a process situated between general distribution of hormones through the systemic circulation and paracrine regulation. Counter-current transfer of drugs after local application opens up new possibilities for treatment.

Measurement of the speed of sound in follicular fluid

BACKGROUND

Measurement of ovarian follicles by ultrasound is common practice in fertility treatment. However, the effect of the speed of sound is not taken into account. We present results from a study aimed at measuring this.

METHODS

The speed of sound was measured in samples of follicular fluid aspirated from patients undergoing fertility treatment. The transmitted and received pulses from a single element ultrasound transducer were recorded using a digital oscilloscope for a pulse passed through a sample of the fluid. The distance over which the pulse travelled was known from calibration with pure water. Variation with temperature was investigated in the range 25-45 degrees C. Dependence on ultrasound frequency, patient and time from aspiration were also investigated.

RESULTS

The speed of sound in follicular fluid was found to be 1550+/-3 m/s at 37.3 degrees C using 5.0 MHz ultrasound. The speed varied from 1528+/-3 m/s at 24.8 degrees C to 1561+/-3 m/s at 44.8 degrees C. Variation with patient, time and frequency were not detected.

CONCLUSION

The speed of sound in follicular fluid at body temperature is 1550 m/s. This small difference from the speed assumed by the ultrasound machine corresponds to the systematic bias in volume measurement evident in previously published results.

Vital aspects of Fallopian tube physiology in pigs

This essay reviews four topical aspects of Fallopian tube physiology that bear on either successful fertilization or early development of the zygote. An initial focus is on glycoprotein secretions of the duct that accumulate as a viscous mucus in the caudal isthmus. Because this is the site of the pre-ovulatory sperm reservoir, an involvement of the secretions is considered in: preventing uterine and ampullary tubal fluids from entering the functional sperm reservoir; removing residual male secretions from the sperm surface; deflecting spermatozoa towards endosalpingeal organelles and reducing flagellar beat before ovulation. The subtle prompting of flagellar movement with impending ovulation is examined in terms of potential reactivation mechanisms, with overall control attributed to increasing secretion of progesterone. The site of full capacitation and the acrosome reaction in a fertilizing spermatozoon is then debated, with strong arguments pointing to completion of these processes in the specific fluids at the ampullary-isthmic junction. Finally, the synthetic activity of cumulus cells released at ovulation as a paracrine tissue in the Fallopian tube is highlighted with reference to steroid hormones, peptides and cytokines. Not only does the suspension of granulosa-derived cells influence the process of fertilization, but also it may amplify oocyte or embryonic signals to the endosalpinx and ipsilateral ovary.