Assessment of in vitro sperm characteristics after flow cytometric sorting of frozen-thawed bull spermatozoa

Looking back at five decades of embryo technology in practice

This paper covers developments from International Embryo Technology Society (IETS) meetings over the past 50years. The IETS was officially 'born' at a meeting in Denver, Colorado on Sunday 26 May 1974. There have been 51 IETS meetings (the first was in May 1974), and the first conference at which papers were presented was in 1975. The name of the IETS was changed from International Embryo Transfer Society to International Embryo Technology Society in 2016. The annual IETS conferences are held once a year in January. Embryo technology advanced from the laborious and slow techniques of surgical recovery and transfer of bovine embryos to non-surgical recovery and transfer in many species. Cryopreservation of embryos was initially a slow process that met with only moderate technical success before the development of technology that resulted in high embryo survival rates. The polymerase chain reaction was used to determine the sex of embryos following laminar flow cytology, which came into use to separate X- and Y-bearing spermatozoa. IVF has grown to become widely used, and several factors make it more useful than superovulation and embryo recovery/transfer. Perhaps the most important tool of all was the system that allowed the genetic 'mapping' of the genomic sequence of Bos taurus cattle. Charting key DNA differences by scientists, 'haplotypes' are now used routinely to identify animals with desirable traits of economic importance, discover new genetic disorders, and track carrier status of genotyped animals. Future technology is discussed.

Relationship between quality parameters and fertilizing ability of cryopreserved sexed bull sperm

The aim of the experiment was to assess the correlation between sperm quality parameters and field fertility after AI with sex-sorted (X-bearing) bull semen. A total of 32 ejaculates from 26 Holstein-Friesian bulls were analyzed to assess sperm motility parameters (CASA), viability (SYBR-14/PI), apoptotic-like changes (YO-PRO-1/PI), chromatin structure (SCSA), and ATP content. In order to determine sperm fertilizing ability, 816 heifers and 727 cows were inseminated. Ultrasound diagnosis of pregnancy was performed on day 35 after insemination. For each ejaculate, the percentage of pregnant females was calculated separately. The results revealed that the pregnancy rate ranged from 20.0 to 85.7% for heifers and from 7.7 to 66.7% for cows. On the basis of the pregnancy rate (PR) obtained, the ejaculates were divided into 3 groups: high PR (about 25% of ejaculates), medium PR (about 50% of ejaculates) and low PR (about 25% of ejaculates). Significant differences were detected for amplitude of lateral head displacement (ALH) and beat cross frequency (BCF) between high- and low-fertility ejaculates in heifers. Pearson’s correlation analysis showed a significant relationship between the BCF and the pregnancy rate for heifers (r = 0.53, P<0.01) and there was a trend towards significance for ALH (r = −0.37, P = 0.07). There was no relationship between the sperm quality parameters and pregnancy rate of cows. In conclusion, the present study identified markers of sexed bull sperm function that were related to the fertility of inseminated heifers.

Effects of single layer centrifugation (SLC) on bull spermatozoa prior to freezing on post-thaw semen characteristics

Single layer centrifugation (SLC) has been shown to select the most robust spermatozoa from the ejaculate in several species. Here the effects of SLC prior to freezing on various parameters of frozen-thawed bovine sperm quality are reported. Semen from 8 bulls was layered on top of a species-specific colloid, Bovicoll. After centrifugation for 20 min at 300 g, the resulting sperm pellet was resuspended in OPTIXcell^® (IMV Technologies, l'Aigle, France); the SLC-selected sperm samples and uncentrifuged controls were frozen. On thawing, all sperm samples were analysed for membrane integrity, production of reactive oxygen species, mitochondrial membrane potential (MMP) and chromatin integrity. The SLC-treated samples had a higher percentage of live, superoxide-positive spermatozoa than uncentrifuged samples (27.9 ± 5.1% versus 21.7 ± 6.7%; p = .03). They had a higher proportion of spermatozoa with high mitochondrial membrane potential than uncentrifuged samples (55.9 ± 8.2% versus 40.5 ± 15.1%; p = .03) and also a lower proportion of spermatozoa with low mitochondrial membrane potential than non-treated samples (42.0 ± 8.5% versus 55.9 ± 14.4%; p = .04). No significant effects of treatment were found for membrane integrity or chromatin integrity. The effect of bull was significant on the proportions of dead, superoxide-positive spermatozoa and live, hydrogen peroxide-negative spermatozoa, as well as on membrane integrity, but it was not significant for mitochondrial membrane potential or chromatin integrity. These results suggest that SLC selects the most metabolically active bull spermatozoa from the rest of the population in normal ejaculates; the pattern of reactive oxygen species production may be different in SLC-selected spermatozoa compared to unselected samples.

Pregnancy rate and birth rate of calves from a large-scale IVF program using reverse-sorted semen in Bos indicus, Bos indicus-taurus, and Bos taurus cattle

Obtaining sexed sperm from previously frozen doses (reverse-sorted semen [RSS]) provides an important advantage because of the possibility of using the semen of bulls with desired genetic attributes that have died or have become infertile but from whom frozen semen is available. We report the efficiency of RSS on the pregnancy rate and birth rate of calves in a large-scale program using ovum pick-up and in vitro embryo production (IVEP) from Bos indicus, Bos indicus-taurus, and Bos taurus cattle. From 645 ovum pick-up procedures (Holstein, Gir, and Nelore), 9438 viable oocytes were recovered. A dose of frozen semen (Holstein, Nelore, Brahman, Gir, and Braford) was thawed, and the sperm were sex-sorted and cooled for use in IVF. Additionally, IVF with sperm from three Holstein bulls with freeze-thawed, sex-sorted (RSS) or sex-sorted, freeze-thawed (control) was tested. A total of 2729 embryos were produced, exhibiting a mean blastocyst rate of 29%. Heifers and cows selected for adequate body condition, estrus, and health received 2404 embryos, and 60 days later, a 41% average pregnancy rate was observed. A total of 966 calves were born, and 910 were of a predetermined sex, with an average of 94% accuracy in determining the sex. Despite the lower blastocyst rate with freeze-thawed, sex-sorted semen compared with sex-sorted semen, (P < 0.05), the pregnancy rate (bull I, 45% vs. 40%; II, 35% vs. 50%; and III, 47% vs. 48% for RSS and control, respectively; P > 0.05) and sex-sorted efficiency (bull I, 93% vs. 98%; II, 96% vs. 94%; and III, 96% vs. 97% for RSS and control, respectively; P > 0.05) were similar for each of the three bulls regardless of the sperm type used in the IVF. The sexing of previously frozen semen, associated with IVEP, produces viable embryos with a pregnancy rate of up to 40%, and calves of the desired sex are born even if the paternal bull has acquired some infertility, died, or is located a long distance from the sexing laboratory. Furthermore, these data show the feasibility of the process even when used in a large-scale IVEP program.

Colloid centrifugation removes seminal plasma and cholesterol from boar spermatozoa

The objective of the present study was to investigate the effect of Single-Layer Centrifugation (SLC) on boar spermatozoa, namely the effect of removal of seminal plasma proteins and cholesterol from the surface of spermatozoa. The presence of porcine seminal plasma proteins I and II (PSP-I/PSP-II) before and after SLC was studied using immunofluorescence, whereas the removal of cholesterol was shown qualitatively by thin-layer chromatography (TLC). Finally, the integrity of the sperm plasma membrane was observed by electron microscopy. It was shown that the seminal plasma proteins PSP-I and -II were removed from spermatozoa during SLC but could be restored by adding seminal plasma to the SLC-selected sperm samples. Some cholesterol was also lost from the spermatozoa during SLC but the plasma membrane itself appeared to be morphologically intact. Further studies are underway to examine the relevance of these findings to boar sperm cryopreservation and sperm fertility.

Practical applications of sperm selection techniques as a tool for improving reproductive efficiency

Modern biotechnologies are used extensively in the animal breeding industry today. Therefore, it is essential that sperm handling procedures do not modulate the normal physiological mechanisms occurring in the female reproductive tract. In this paper, the different selection mechanisms occurring in vivo are described briefly, together with their relevance to artificial insemination, followed by a detailed description of the different selection processes used in reproductive biotechnologies. These selection methods included fractionated semen collection, cryopreservation, biomimetic sperm selection, selection based on hyaluronic acid binding, and last, but not least, sperm sex selection. Biomimetic sperm selection for AI or for cryopreservation could improve pregnancy rates and help to reverse the decline in fertility seen in several domestic species over the recent decades. Similarly, selection for hyaluronic acid binding sites may enable the most mature spermatozoa to be selected for IVF or ICSI.

Birth of offspring after artificial insemination of heifers with frozen-thawed, sex-sorted, re-frozen-thawed bull sperm

Two field trials were conducted to determine the fertilising capacity of (i) frozen-thawed, sex-sorted re-frozen-thawed (FSF) dairy bull sperm inseminated close to the time of ovulation, (ii) FSF sperm following large dose insemination, and frozen-thawed, sex-sorted (FS) sperm inseminated within 12h after sorting. In Trial 1, 24 heifers in synchronised oestrus were observed for standing heat over a 30-min period once every 3h. Upon observation of standing heat, the size of the pre-ovulatory follicle was tracked by ultrasound every 6h until ovulation was judged to be imminent. Heifers were inseminated with 4 x 10(6) X-bearing FSF or Control sperm within 6h of ovulation. Ovaries were scanned 6h after AI to ensure ovulation had occurred. All 24 heifers displayed standing oestrus and 20 of these subsequently ovulated. The mean length of standing oestrus was 16.8+/-0.4h and ovulation occurred 27.6+/-1.1h after the onset of standing heat from a pre-ovulatory follicle with a diameter of 16.1+/-0.3mm. All 12 heifers that received FSF sperm returned to oestrus<26d after AI. Of 8 heifers that received Control sperm, 6 (75%) were confirmed pregnant by ultrasound 7 wk after AI, confirming that the method of AI and herd fertility were sound. In Trial 2 the number of sperm inseminated and the effect of eliminating the post-sort cryopreservation step were investigated. Heifers (n=21) were synchronised for oestrus, and inseminated 24h after the onset of standing oestrus with 10 x 10(6) X-bearing FSF, 4 x 10(6) X-bearing FS, or 10 x 10(6) non-sorted frozen-thawed (Control) sperm. Heifers were observed for return to oestrus from 21d, and diagnosed for pregnancy 7 wk after AI. Of the 7 heifers that received FSF sperm, one was confirmed pregnant (14.3%) and delivered a female calf. Four heifers inseminated with control sperm became pregnant and calved, but no pregnancies were obtained using FS sperm. The birth of a calf following AI with FSF sperm demonstrates the potential of sorting from frozen-thawed semen, and with further work, may be a promising technique that will give producers access to sexed sperm from a greater range of bulls.

Retained Functional Integrity of Bull Spermatozoa after Double Freezing and Thawing Using PureSperm® Density Gradient Centrifugation

The main aim of this study was to compare the motility and functional integrity of bull spermatozoa after single and double freezing and thawing. The viability and morphological integrity of spermatozoa selected by PureSperm density gradient centrifugation after cryopreservation of bovine semen in two commercial extenders (Experiment 1) and the function of bull spermatozoa before and after a second freezing and thawing assisted by PureSperm selection (Experiment 2) were examined. On average, 35.8 +/- 12.1% of sperm loaded onto the PureSperm density gradient were recovered after centrifugation. In Experiment 1, post-thaw motility and acrosome integrity were higher for spermatozoa frozen in Tris-egg yolk extender than in AndroMed, whether the assessments were made immediately after thawing [80.4 +/- 12.7 vs 47.6 +/- 19.0% motile and 78.8 +/- 8.3 vs 50.1 +/- 19.5% normal apical ridge (NAR), p < 0.05] or after preparation on the gradient (83.3 +/- 8.6 vs 69.4 +/- 15.9% motile and 89.5 +/- 7.2 vs 69.1 +/- 11.4% NAR, p < 0.05). For semen frozen in Tris-egg yolk extender, selection on the PureSperm gradient did not influence total motility but significantly improved the proportion of acrosome-intact spermatozoa. After the gradient, both the total motility and percentage of normal acrosomes increased for spermatozoa frozen in AndroMed (Minitüb Tiefenbach, Germany). In Experiment 2, there was no difference in sperm motility after the first and second freeze-thawing (82.9 +/- 12.7 vs 68.8 +/- 18.7%). However, the proportion of acrosome-intact spermatozoa was significantly improved by selection through the PureSperm gradient, whether measured by phase contrast microscopy (78.9 +/- 9.7 vs 90.4 +/- 4.0% NAR, p < 0.05) or flow cytometry (53.4 +/- 11.7 vs 76.3 +/- 6.0% viable acrosome-intact spermatozoa, p < 0.001). The improvement in the percentage of spermatozoa with normal acrosomes was maintained after resuspension in the cooling extender and cooling to 4 degrees C (88.2 +/- 6.2) and after re-freezing and thawing (83.6 +/- 6.56% NAR). However, flow cytometric assessment of the sperm membranes revealed a decline in the percentage of viable spermatozoa with intact membranes after the second freezing and thawing compared with after gradient centrifugation (76.3 +/- 6.0% vs 46.6 +/- 6.6%, p < 0.001) to levels equivalent to those obtained after the first round of freeze-thawing (53.4 +/- 11.7% viable acrosome-intact spermatozoa). Sperm movement characteristics assessed by computer-assisted analysis were unaffected in the population selected on the PureSperm gradients but declined after cooling of the selected and extended spermatozoa to 4 degrees C. There was no further change in these kinematic measurements after the cooled spermatozoa had undergone the second round of freeze-thawing. These results demonstrate that bull semen can be frozen and thawed, followed by a second freeze-thawing cycle of a population of spermatozoa selected by PureSperm, with retained motility and functional integrity. This points to the possibility of using double frozen spermatozoa in bovine artificial insemination programmes and to the potential benefits of PureSperm density gradient centrifugation for the application of cryopreserved bull spermatozoa to other biotechnological procedures such as flow cytometric sex sorting followed by re-freezing and thawing.

Birth of offspring of pre-determined sex after artificial insemination of frozen-thawed, sex-sorted and re-frozen-thawed ram spermatozoa

The fertility of ram spermatozoa cryopreserved prior to, and following, sex-sorting by flow cytometry was assessed after insemination of mature Merino ewes at a synchronised estrus. Ewes were inseminated with spermatozoa from three rams, split into four treatment groups: 50 x 10(6) motile non-sorted, frozen-thawed (Control50), 15 x 10(6) motile non-sorted, frozen-thawed (Control15), 15 x 10(6) motile sex-sorted, frozen-thawed (SF15) or 15 x 10(6) motile frozen-thawed, sex-sorted, re-frozen-thawed (FSF15) ram spermatozoa. Separation of SF15 and FSF15 treatments into X- and Y-chromosome-bearing populations was achieved using a high-speed sperm sorter. The percentage of ewes lambing after insemination was similar for Control15 (36/74; 48.6%), SF15 (35/76; 46.1%) and FSF15 (26/72; 36.1%) groups (P>0.05). A higher percentage of ewes produced lambs in the Control50 (38/70; 54.3%) than the FSF15 group (P<0.05). Fifty-one of the 55 (92.7%) lambs derived from fresh, sex-sorted frozen-thawed spermatozoa were of the predicted sex, as were 41/43 (95.3%) lambs derived from frozen-thawed, sex-sorted, re-frozen-thawed spermatozoa. This study demonstrated for the first time in any species that frozen-thawed spermatozoa, after sex-sorting and a second cryopreservation step, are capable of producing offspring of the predicted sex following artificial insemination.

Flow cytometric sexing of mammalian sperm

This review reexamines parameters needed for optimization of flow cytometric sexing mammalian sperm and updates the current status of sperm sexing for various species where this technology is currently being applied. Differences in DNA content have provided both a method to differentiate between these sex-determining gametes and a method to sort them that can be used for predetermining sex in mammals. Although the DNA content of all cells for each mammalian species is highly conserved, slight but measurable DNA content differences of sperm occur within species even among cattle breeds due to different sizes of Y-chromosomes. Most mammals produce flattened, oval-headed sperm that can be oriented within a sorter using hydrodynamic forces. Multiplying the percentage the difference in DNA content of the X- or Y-chromosome bearing sperm times the area of the flat profile of the sperm head gives a simple sorting index that suggests that bull and boar sperm are well suited for separation in a flow sorter. Successful sperm sexing of various species must take into account the relative susceptibilities of gametes to the stresses that occur during sexing. Sorting conditions must be optimized for each species to achieve acceptable sperm sexing efficiency, usually at 90% accuracy. In the commercial application of sperm sexing to cattle, fertility of sex-sorted bull sperm at 2 x 10(6)/dose remains at 70-80% of unsexed sperm at normal doses of 10 to 20 x 10(6) sperm. DNA content measurements have been used to identify the sex-chromosome bearing sperm populations with good accuracy in semen from at least 23 mammalian species, and normal-appearing offspring have been produced from sexed sperm of at least seven species.

In vitro characteristics of fresh and frozen - thawed ram spermatozoa after sex sorting and re-freezing

The in vitro function of sex-sorted, frozen–thawed ram spermatozoa derived from fresh or frozen semen was investigated. Sorted, frozen–thawed spermatozoa had higher (P < 0.05) motility, viability, acrosome integrity and mitochondrial activity than non-sorted, frozen–thawed controls immediately following thawing and after incubation at 37°C for 3 and 6 h. Similarly, frozen–thawed, sorted, re-frozen–thawed spermatozoa outperformed (P < 0.05) non-sorted controls upon thawing (mitochondrial activity) and following a 3-h incubation (motility, viability/acrosome integrity and mitochondrial activity), but there were no differences after incubation for 6 h (P > 0.05). Velocity characteristics (computer assisted sperm assessment 0–6 h post-thaw) of sorted spermatozoa derived from either fresh or frozen semen remained inferior (P < 0.05) to non-sorted spermatozoa, as did their ability to penetrate artificial cervical mucus after thawing. Direct comparison of cryopreserved spermatozoa derived from either fresh or frozen semen revealed that frozen–thawed, sorted, re-frozen–thawed spermatozoa had comparable (P > 0.05) motility, viability/acrosome integrity, mitochondrial activity, average path velocity and oviducal binding capacity immediately post-thaw, but reduced (P < 0.05) quality after 3 and 6 h of incubation. These findings indicate that, under the tested in vitro conditions, sex-sorted spermatozoa derived from fresh semen are superior in some respects to those derived from frozen semen. Further, that the use of either technique, while reducing velocity characteristics and cervical mucus penetration, results in comparable, if not enhanced motility, membrane and mitochondrial function in the post-thaw population of spermatozoa when compared with non-sorted, frozen–thawed controls.

Integration of sperm sexing technology into the ART toolbox

Sex-sorting of mammalian spermatozoa has applications for genetic improvement of farm animals, in humans for the control of sex-linked disease, and in wildlife as a captive management strategy and for the re-population of endangered species. Considerable research has been undertaken worldwide on the Beltsville sperm sexing technology, the only effective method for pre-selection of sex of offspring. The combination of this method with assisted reproductive technologies has resulted in the birth of offspring in a wide range of animals, including cattle, the only livestock species in which sperm sexing is used commercially. Major improvements in the efficiency of sorting, in particular the development of high speed sorting (15 million X and Y spermatozoa per hour) have led to the production of offspring using conventional and low dose AI and the successful cryopreservation of sorted spermatozoa in cattle, sheep, horses and elk. A major limitation remains the short viable lifespan of sorted spermatozoa in the female genital tract, in most species necessitating sperm deposition deep in the uterus, and close to the expected time of ovulation, for acceptable fertility after in vivo insemination. Special deep uterine insemination technology has been employed to produce offspring in pigs and horses using low sperm doses. Considerable attention has been paid to reduction of the damage and capacitation-like changes to spermatozoa that result from flow cytometric sorting and from freezing and thawing. However, high-purity sorting of liquid-stored or frozen-thawed spermatozoa for immediate use, or re-cryopreservation for later use, does not reduce its fertilizing capacity in vitro, allowing its combination with in vitro fertilization or juvenile in vitro embryo transfer to produce blastocysts, and offspring in sheep and cattle after embryo transfer. Further research into sorting and preservation methods that incorporate strategies to prevent destabilization of sperm membranes may improve the fertilizing lifespan of flow cytometrically sorted spermatozoa. With continued improvement in sorting instrumentation and biological handling, sorting efficiency should reach a point where commercially acceptable pregnancy rates may be achieved in a number of species after conventional or deep uterine insemination.