Hoechst 33342 is the fluorophore used routinely to measure DNA in X- and Y-chromosome-bearing mammalian sperm so they can be separated by flow sorting. A difference of <3% in DNA mass can be detected. This synthetic dye consists of two adjacent benzimidazole rings with N-methyl-piperazine and phenolic groups at the ends. The molecule permeates the cell membrane of living cells and binds selectively to A-T base pairs exposed in the minor-groove of double stranded DNA. Capability to distinguish and separate X- and Y-chromosome-bearing sperm has led to artificial insemination of somewhere around a million female mammals. Offspring with obvious abnormalities are no more frequent than after insemination of unsorted sperm into cows, horses, humans, pigs, sheep, rabbits, dolphins and other mammals. There is no apparent genotoxic effect from exposure of sperm to Hoechst 33342, although information on cellular toxicity or development of embryos resulting from Hoechst 33342-stained sperm is less reassuring. Little is known about the fate of sperm-delivered Hoechst dye in the female reproductive tract or on progeny of resultant offspring.

Although the basic principles controlling the sex of mammalian offspring have been known for a relatively long time, recent application of certain modern cellular methodologies has led to development of a flow cytometric system capable of differentiating and separating living X- and Y-chromosome-bearing sperm in amounts suitable for AI and therefore, commercialization of this sexing technology. After a very long history of unsuccessful attempts to differentiate between mammalian sperm that produce males from those that produce females, a breakthrough came in 1981 when it was demonstrated that precise DNA content could be measured. Although these initial measurements of DNA content killed the sperm in the process, they led to the ultimate development of a sperm sorting system that was capable, not only of differentiating between live X- and Y-sperm, but of sorting them into relatively pure X- and Y-sperm populations without obvious cellular damage. Initial efforts to predetermine the sex of mammalian offspring in 1989 required surgical insemination, but later enhancements provided sex-sorted sperm in quantities suitable for use with IVF. Subsequent advances in flow sorting provided minimal numbers of sperm sufficient for use in AI. It was not until the flow cytometric sorting system was improved greatly and successful cryopreservation of sex-sorted bull sperm was developed that efficacious approaches to commercialization of sexed semen could be implemented worldwide in cattle. A number of companies now offer sex-sorted bovine sperm. Innovative approaches by a diverse group of scientists along with advances in computer science, biophysics, cell biology, instrumentation, and applied reproductive physiology provided the basis for commercializing sexed semen in cattle.

A major obstacle in successful cooled semen storage is the reactive oxygen species generated in the media by the sperm cells. Recently, many reports document the antioxidant, anti-carcinogenic and anti-mutagenic properties of flavonoids when used in vitro or in vivo. The objective of this study was to determine whether supplementing diluents with the flavonoids, silibinin or catechin, would aid in maintaining the viability of caprine sperm during cooling and storage at 5°C. Semen samples were collected from seven post-pubertal bucks using electro-ejaculation and aliquots of sperm (40×106cells, replicated 3 times), were diluted in 9% non-fat dried skim milk extender supplemented with 0 (control), 25, 50, 75, or 100μM of either silibinin or catechin. Diluted samples were cooled and maintained at 5°C prior to microscopic analysis (250×) using a Makler counting chamber. A minimum of 200 sperm cells per sample were counted to determine the percentage of motile sperm following 24, 48, 72 and 96h of incubation. An ANOVA was performed on arcsine-transformed data to determine differences in sperm motility between control and treatment levels for each flavonoid. No differences in motility were detected for the silibinin-treated samples at any of the concentrations across the four time periods. Significant differences (P<0.05) in motility were detected between the 0 (control, 34%) and the 25, 50, 75 and 100μM concentrations of catechin (57, 53, 55 and 64% motile cells, respectively) at 96h. These results demonstrate that the flavonoid catechin, but not silibinin, aids in maintaining the motility of cooled goat sperm in a dose dependent manner.

The fluorophore 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl carbocyanine iodide (JC-1) localizes to the mitochondria and is affected by membrane potential, fluorescing bright orange when the membrane potential is high and green when mitochondrial membrane potential is low. The present study used flow cytometric analysis of JC-1 staining patterns of large numbers of spermatozoa to detect chemical-induced alterations of sperm mitochondrial membrane potential. Cauda epididymal rat spermatozoa were incubated with pentachlorophenol (PCP; 0.1 microM or 1.0 microM), a known uncoupler of mitochondrial oxidative phosphorylation. Microscopic evaluation showed that the midpiece (mitochondrial location) of live, highly motile spermatozoa stained bright orange, while the midpiece of live, non-motile spermatozoa stained green. The midpiece of slightly or non-progressively motile spermatozoa stained a faint orange-green. The percentage of spermatozoa stained bright orange and the total percentage of spermatozoa stained orange (bright orange+faint orange) in the control samples of spermatozoa were significantly higher (P<0.001) than in the 0.1 microM and 1.0 microM PCP treated samples. These data indicate that sperm mitochondrial membrane potential is highly sensitive to the uncoupling effects of PCP and that JC-1 staining and flow cytometric analysis may be a sensitive assay to detect the effect of toxicants on rat sperm mitochondrial function.

Development of flow cytometry for sorting mammalian sperm according to their sex chromosomes began in the late 1970s and early 1980s. This technology, which has recently been commercialized for bovine sperm, is based on the differences in DNA content between X- and Y-chromosome-bearing sperm. Under ideal conditions, 5000 live bovine sperm of each sex can be sorted per second at 90% accuracy. Pregnancy rates of 50% have been achieved routinely in well-managed heifers with sex-sorted, cryopreserved bovine sperm compared to 60–80% with unsexed control sperm. About 90% of offspring have been of the selected sex. Sorting sperm according to sex chromosome content is similarly successful in many other mammals including exotic species, but sorting efficiencies are somewhat less for sperm from some species. Key words: Mammals, sex chromosomes, flow cytometer, cell sorter, DNA content, X and Y sperm, Hoechst 33342

Thousands of offspring have now been produced via artificial insemination with spermatozoa sexed by flow cytometry and cell sorting. We are unaware of any other practical approach to sexing spermatozoa that maintains fertility. Accuracy of sexing usually is 85-95% in most species, but somewhat lower with human spermatozoa. Spermatozoa are sexed in series, one at a time, at routine rates of about 3000 live spermatozoa of each sex per second for most species, and nearly twice that rate under optimal conditions for some species. Owing to various constraints and statistical considerations, there appears to be an upper theoretical limit to sexing spermatozoa of about 10,000 live spermatozoa of each sex per second with current methodology. About a quarter of the spermatozoa processed are sexed; the rest are discarded in the process or lost due to logistical constraints. Spermatozoa undergo some damage during sorting, although much less in terms of viability than with routine cryopreservation; fertility is lower with sexed than control spermatozoa. Offspring from sexed spermatozoa appear to have no more abnormalities than do controls, and both groups grow and thrive similarly. Despite high costs and complex procedures, sexing spermatozoa, usually followed by cryopreservation, is being used commercially for cattle and horse production in several countries, and is used to produce girls to avoid X-chromosome-linked genetic diseases.

Fluorescent assessment of cellular integrity and mitochondrial function by flow cytometry can provide a rapid and precise means of determining the functional status of large numbers of spermatozoa. In the present study, rat sperm viability was assessed with SYBR-14 and PI and sperm mitochondria were differentially labeled with JC-1. Sperm samples of variable viability were prepared using varying proportions of fresh and frozen spermatozoa. SYBR-14 stained sperm correlated well with expected sperm viability (r = 0.98). Motile sperm stained with JC-1 appeared orange in the midpiece indicating a high mitochondrial membrane potential whereas immotile sperm with a low membrane potential stained green. The percentage of spermatozoa staining orange was highly correlated (r = 0.99) with expected sperm viability. Flow cytometry using specific fluorescent probes is a useful technique for detecting changes in rat sperm plasma membrane integrity and mitochondrial function in large numbers of spermatozoa.

Dilution of semen to low cell numbers/dose can result in a bull-dependent reduction in the post-thaw viability of cryopreserved bovine spermatozoa. It is possible that essential seminal plasma components are lacking at the greater dilution rates, thereby contributing to the deleterious effects of semen dilution. Ejaculates of 6 Holstein bulls were diluted to 120 x 10(6) sperm/mL in an egg yolk citrate extender (EYC). Split samples were further diluted to 80, 40, 20 and 4 x 10(6) sperm/mL in EYC extender with (+SP) and without (-SP) the addition of frozen/thawed seminal plasma previously obtained from a vasectomized bull. Serial dilutions for the +SP treatments were calculated and performed such that each dilution contained a volume of seminal plasma equal to the original 120 x 10(6) sperm/mL dilution. Samples were then loaded into 0.5-mL French straws yielding final sperm concentrations of 30, 20, 10, 5 and 1 x 10(6)/dose. Straws from each dilution were analyzed using 2 stain combinations: the sperm viability stain, SYBR-14 and propidium iodide (PI); or the mitochondrial-specific, membrane potential-dependent stain JC-1 along with PI. Split-plot analysis of variance indicated that within bulls, there were greater proportions of viable spermatozoa in aliquots containing added seminal plasma than in aliquots without added seminal plasma (P < 0.05). Contrast analyses showed that sperm viability significantly decreased as sperm concentration decreased in the -SP samples. Although the dilution effect was also observed in the +SP samples, the magnitude of the effect was less than in the -SP samples. At most sperm concentrations, the proportions of spermatozoa that stained with JC-1 were correlated (r > 0.84; P < 0.05) with the percentages of SYBR- 14 stained spermatozoa. Furthermore, the proportions of JC-1-stained spermatozoa were greater in the +SP aliquots than in the -SP samples at a concentration of 10 x 10(6) sperm/0.5 mL. These results suggest that the addition of seminal plasma can be beneficial to sperm viability when semen is diluted to low cell numbers/dose.

The fluorescent carbocyanine dye, JC-l, labels mitochondria with high membrane potential orange and mitochondria with low membrane potential green. Evaluation of mitochondrial membrane potential with JC-1 has been used in a variety of cell types, including bull spermatozoa; however, JC-1 staining has not yet been reported for equine spermatozoa. The aim of this study was to apply JC-l staining and assessment by flow cytometry or a fluorescence microplate reader for evaluation of mitochondrial function of equine spermatozoa. Six ejaculates from four stallions were collected and centrifuged through a Percoll gradient (PERC). Spermatozoa were resuspended to 25 x 10(6) cells/mi, samples were split, and one sample was repeatedly flash frozen (FF) in LN2 and thawed. The following gradients of PERC:FF were prepared: 100:0 (100), 75:25(75), 50:50 (50), 25:75 (25) and 0:100(0). Samples were stained with 2.0 mu M JC-1 and assessed for staining by flow cytometry and by a fluorescence microplate reader. A total of 10,000 gated events was analyzed per sample with flow cytometry. The mean percentage of cells staining orange for the 100, 75, 50, 25 and 0 treatment was 92.5, 72.8, 53.4, 27.3 and 7.3, respectively. The expected percentage of spermatozoa forming JC-1 aggregates was correlated with the actual percentage of orange labeled sperm cells determined by flow cytometry (r(2) = 0.98). Conversely, JC-l monomer formation was negatively correlated with expected mitochondrial membrane potential (r(2) = -0.98).

The mitochondrial probe 5,5′,6,6′‐tetrachloro‐1,1′,3,3′‐tetraethylbenzimidazolyl‐carbocyanine iodide (JC‐1) not only identifies mitochondria exhibiting low membrane potentials by the emission of green fluorescence (range, 510–520 nm) but also differentiates these from mitochondria exhibiting relatively high membrane potentials. This discrimination occurs because JC‐1 forms aggregates at high membrane potentials. These J‐aggregates emit a bright red‐orange fluorescence at 590 nm. In this study, JC‐1 was combined with the classical dead cell stain, propidium iodide (PI), to identify a spectrum of functional sperm along with degenerate sperm. Flow cytometric analysis of bull sperm showed that the aggregate:monomer ratio differed among bulls before cryopreservation (P < 0.001) but not afterwards (P > 0.05). The effects of stain equilibration time, sperm concentration, and live:dead ratios were examined. The addition of SYBR‐14 to the JC‐1 and PI combination enhanced the distinction between the red PI‐stained and red‐orange JC‐1–stained populations. This discrimination between J‐aggregates and the PI‐stained sperm was affected by sperm concentration. These studies show that JC‐1 can be useful in monitoring mitochondrial function in bovine sperm. Mol. Reprod. Dev. 53:222–229, 1999. © 1999 Wiley‐Liss, Inc.

ContentsEight bull ejaculates were split to evaluate the effects of glycerol on sperm organelle function. Although glycerol protects sperm membranes during cryopreservation, preliminary data suggested that glycerol was detrimental to sperm organelles to varying degrees. To assess the compartmental effects, three organelle‐specific fluorophores were used to analyze with (G+) and without glycerol (G–) in spermatozoa stored for 24 h at 5°C in an egg‐yolk‐based extender. The mitochondrial probe, 5,5′,6,6′‐tetrachloro‐1,1′,3,3′‐tetraethyl benzimidazolylcarbocyanine iodide (JC‐1) was used to examine the level of mitochondrial metabolic function by it’s discrimination between high (J‐aggregate staining, red‐orange) and low membrane potentials (JC‐1 monomeric staining, green); while acosomal reacted spermatozoa were identified using fluorescein‐labelled lectin from arachis hypogaea, PNA‐FITC. The proportions of living and dead spermatozoa were determined by staining with the combination of SYBR‐14 and propidium iodide (PI). Split‐plot analysis of variance revealed that within bulls, glycerol altered the proportions of sperm staining with each organelle‐specific fluorophore to varying degrees. The proportion of spermatozoa labelled with SYBR‐14, indicating intact plasmalemmae, were not affected by the addition of glycerol (p = 0.11). Although the total proportion of JC‐1‐labeled spermatozoa were similar in G– and G+ samples (p = 0.90), the presence of glycerol decreased the proportion of spermatozoa that exhibited J‐aggregate staining (p < 0.01) while producing an increase in monomeric staining (p = 0.02). The proportions of acrosome‐reacted spermatozoa, however, were greater in the G– samples than in the G+ samples as indicated by PNA‐FITC (p = 0.03). These findings suggest that mitochondria, acrosomes and the plasmalemmae of unfrozen spermatozoa vary in their response to the addition of the cryoprotectant glycerol.

Associations of abnormal spermatozoa with bull fertility have yielded varying results. Manual methods of analysis are subjective and highly variable within and between technicians, which may account for these differences. Computer-aided sperm head morphometry appears to be a precise method of assessing sperm head dimensions; however, the effects of replication and technician on sperm head morphometry have not been assessed. The objective of this study was to determine the inter- and intra-analysis and technician variation associated with computer-aided bull sperm head morphometry analysis. Semen from 10 bulls was diluted to 200 x 10(6) sperm/mL, and slide smears were prepared and stained using haematoxylin and rose bengal. Each of two technicians analysed 250 images from each slide, 3 times, using computer-aided sperm head morphometry analysis. The morphometric dimensions of area, perimeter, length, width and width/length for individual sperm heads of each analysis were assessed by GLM-ANOVA for effects of bulls, replications and technicians. The coefficient of variation was recorded for each analysis and across replications. The mean coefficients of variation within and between analyses were compared between technicians by GLM-ANOVA. No differences (p > 0.1) between technicians were found between or among bulls for area (29.63 vs. 29.26 micron 2), perimeter (23.73 vs. 23.86 microns), length (8.73 vs. 8.71 microns), width (4.47 vs. 4.46 microns), or width/length (0.51 vs. 0.51). No differences (p > 0.1) between replicates for sperm head dimension were detected within or among bulls for either technician. No intra- or inter-analysis differences (p > 0.1) between technicians on CVs were observed. The mean intra-analysis CVs for all bulls for both technicians were area = 6.9%, perimeter = 4.9%, length = 4.5%, width = 5.6% and width/length = 6.5%. The mean interanalysis CVs for both technicians were area = 3.0%, perimeter = 2.4%, length = 2.0%, width = 2.0%, and width/length = 1.7%. The results indicate that ASMA is a repeatable and objective method of assessing bull sperm head morphometry within and between technicians. No differences between replications were detected, and hence replicate analyses are not necessary to acquire accurate morphometric data.

Flow cytometry was used to compare the functional status of fluorescently stained sperm organelles from 12 Holstein bulls after storage for 24 h at 5 degrees C and after cryopreservation. The organelle-specific stains, SYBR-14 and LysoTracker Green DND-26, identified spermatozoa with intact plasmalemma and those with intact acrosomes, respectively. The mitochondria-specific stain, 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl-carbocyan ine iodide (JC-1), identified two populations of spermatozoa. One population stained red-orange because the JC-1 accumulated in the mitochondria as aggregates (characteristic of cells exhibiting a high membrane potential); a second population stained green because of JC-1 monomers within the mitochondria (characteristic of cells exhibiting a lower membrane potential). Analysis of variance revealed that within bulls, the properties of sperm viability, intact acrosomes, and mitochondrial status differed in spermatozoa stored for 24 h (p < 0.001) but not in cryopreserved spermatozoa (p > 0.11). Linear regression analyses resulted in significant models in which the proportions of stained spermatozoa stored for 24 h were indicative of those proportions observed in the cryopreserved fractions. These findings suggest that the plasmalemma, the acrosome, and the mitochondria of unfrozen spermatozoa varied as to their functional status. The cryopreservation process, however, resulted in a more uniform status of sperm organelles.

Artificial insemination using cryopreserved semen is a common management tool of the contemporary livestock producer. However, cryopreservation is detrimental to sperm function and fertility, killing some 50% of the spermatozoa during the process. Prediction of cryopreservation damage from prefreeze samples remains elusive. Computer-automated sperm head morphometry was used in this study to determine the effects of cryopreservation on bovine sperm head morphometry. Semen was collected from 18 bulls and was divided. One portion was extended to 200 x 10(6) sperm/ml and a microscope slide was prepared, while the remaining portion was cryopreserved in a Triscitrate-yolk extender. After thawing, the cryopreserved samples were prepared on microscope slides. All slides were air dried and were stained with hematoxylin and rose bengal. The morphometric dimensions for length, width, width/length, area, and perimeter for a minimum of 200 sperm heads were analyzed from each slide by computer-aided sperm head morphometry analysis, and the mean measurements were recorded. Bull sperm heads were significantly (P < 0.01) smaller in cryopreserved spermatozoa than in the companion extended samples for length (8.56+/-0.07 vs. 8.63+/-0.08 microm), width (4.39+/-0.05 vs. 4.48+/-0.05 microm), area (28.42+/-0.07 vs. 29.14+/-0.08 microm), and perimeter (23.33+/-0.21 vs. 23.70+/-0.23 microm) for all bulls. Width/length was also different (0.513 vs. 0.519). In addition, differences (P < 0.05) were found within 14 of 18 bulls for at least four of the morphometric parameters. The percent change in measures after cryopreservation were correlated (P < or = 0.05) to the variability of the extended sample. Variations in sperm head measurements were lower (P < or = 0.05) in extended samples of the four bulls in which no changes occurred than in extended samples of the remaining 14 bulls. These data suggest that the variability in sperm head measurements of individual bulls, or ejaculates, may be an indicator of sperm cryosurvivability.

The combination of specific fluorometric staining and flow cytometry provides a rapid and precise means of assessing the functional status of cells. We sought to utilize this approach to quantify two important seminal characteristics, acrosomal integrity and sperm viability, and to compare these with classical microscopic measurements of acrosomal integrity and sperm motility. Samples of thawed, cryopreserved sperm packaged in 0.5-ml French straws were obtained from 12 Holstein bulls. Classical acrosomal assessments and sperm motility estimates were made using differential interference contrast microscopy. Fluorescent acrosomal probes included LysoTracker Green DND-26 (LYSO-G), fluorescein-labeled peanut agglutinin, the biotinylated isocoumarin serine protease inhibitor Bi-Aca-Aca-OMe-IC that was secondarily labeled with fluorescein-avidin, and rabbit antibodies to bovine acrosin that were secondarily labeled with fluoresceinated anti-rabbit immunoglobulin. The fluorescent probes for sperm viability were a combination of SYBR-14 and propidium iodide (PI) or of SYTO-17 and PI. Significant differences were found among methods and among bulls, but not among straws (n = 3). All four fluorescent measures of acrosomal integrity showed highly significant correlations with both classical measurements. These data indicated that the quality of cryopreserved bovine sperm samples could be readily quantified using a variety of organelle-specific fluorescent staining techniques.