Changes in human sperm chromatin stability during preparation for in-vitro fertilization

This study was designed to define the effects of sperm preparation on sperm chromatin stability in relation to in-vitro fertilization (IVF). Semen samples used for IVF-embryo transfer (ET) in the treatment of infertility due to tubal factors were studied. Cases with semen variables below reference limits in previous samples were excluded. Sperm were prepared by a swim-up technique employing either of two different tissue culture media, Ham's F-10 or Earle's balanced salt solution. Sperm chromatin stability was tested by exposure both to sodium dodecyl sulphate (SDS) only and SDS together with a zinc-chelating agent, disodium ethylene diamine tetraacetate (SDS-EDTA). Sperm head swell scores were defined under different experimental conditions and the relationship to sperm motility, morphology, fertilization rate and pregnancy occurrence was tested. No differences were seen between the chromatin stability of sperm from the original sample and that after swim-up preparation, neither immediately after completion of the swim-up procedure, nor at the time of insemination of ova. With time, the chromatin became more stable, which occurred to a similar extent both in the original sample and in swim-up preparations using Ham's F-10. Otherwise, sperm chromatin stability was unaffected by either of the two media used for swim-up. At higher incubation temperatures, decondensation in SDS was enhanced. Altogether, no correlation was found between sperm chromatin stability or enhancement of decondensation by temperature and the success of IVF treatment expressed in fertilization rates or pregnancies. The results are reassuring in that only small changes in sperm chromatin stability occurred during the preparation for IVF. As long as semen of presumably good quality is used, these changes in chromatin stability do not seem to be of clinical importance.     

Multivariate discriminate analysis of the relationship between the hypo-osmotic swelling test and the in-vitro fertilizing capacity of human sperm

Multivariate discriminant analysis was used to evaluate the usefulness of routine semen parameters and the hypo-osmotic swelling test (HOST) as predictors of the in-vitro fertilizing capacity of human sperm as assessed by the zona-free hamster egg penetration assay (HEPA). Eighty-eight semen samples from untreated patients attending an infertility clinic were analysed. Semen samples were classified into the following three groups before statistical analysis: group 1--positive sperm penetration (greater than or equal to 10%, n = 39); group 2--borderline penetration rates for HEPA (greater than 0% but less than 10%, n = 39) and group 3--negative sperm penetration (0%, n = 10). The percentage of sperm with normal morphology and sperm count were found to be significant in discriminating between semen samples exhibiting different in-vitro fertilizing capacity. These two discriminating variables in combination gave an overall correct classification rate of 45.5%. The multivariate discriminant analysis was also performed after excluding the data of group 2 semen samples (n = 39), which exhibited borderline sperm penetration rates. As a result, three discriminating variables including semen volume, sperm count and the percentage of sperm with normal morphology were selected. These three variables in combination could accurately predict whether a semen sample would exhibit positive sperm penetration (group 1) or negative sperm penetration (group 3) with an overall accuracy of 75.5%. The percentage of swollen sperm after hypo-osmotic treatment was not related to the HEPA result, as determined by linear correlation and multiple regression analyses, and did not give additional information about the in-vitro fertilizing capacity of sperm as evaluated by multivariate discriminant analysis.     

An evaluation of the hypo-osmotic sperm swelling test as a predictor of fertilizing capacity in vitro

The ability of sperm to swell in hypo-osmotic conditions was examined in 211 semen samples from the partners of patients about to undergo oocyte retrieval for in-vitro fertilization (IVF). The test was performed using aliquots of semen, the remainder of which was then prepared for IVF. No significant difference was found, in either the percentage of swollen sperm or the type of swelling response, between samples that achieved fertilization in vitro and those that did not, or between any of the diagnostic categories of infertility (tubal damage, unexplained infertility, oligospermia). In samples which achieved fertilization in vitro there were correlations between sperm swelling and sperm motility (r = -0.51) and abnormal morphology (r = 0.33), but no such correlations were demonstrated in samples that failed to achieve fertilization. Moreover, there was no significant difference between the percentage of swollen sperm in semen (mean motility 64%), in samples immediately after preparation for IVF (mean motility 96%) or in capacitated sperm 24 h after preparation (mean motility 91%). These results demonstrate that the hypo-osmotic sperm swelling test does not assist in the prediction of the fertilizing capacity of human sperm in vitro.     

Clinical correlates of the biological variation of sperm DNA fragmentation in infertile men attending an andrology outpatient clinic

Determination of sperm DNA fragmentation, as assessed by the sperm chromatin structure assay (SCSA), has become an important tool for the evaluation of semen quality. The aim of the present study was to describe the biological variation of sperm DNA fragmentation in men attending an andrology clinic and to identify clinical correlates of the biological variation of sperm DNA fragmentation. For this study, two consecutive semen samples from 100 patients attending our andrology outpatient clinic were subjected to semen analysis, performed in parallel according to WHO guidelines and by SCSA. A good agreement between pairs of samples was found for SCSA-derived variables, as indicated by a significantly lower median coefficient of variation (CV) of the DNA Fragmentation Index (DFI) and the high DNA stainability (HDS) compared with WHO semen parameters. In half of the men attending our andrology clinic, however, the individual biological variation of DFI and HDS, expressed as CV of two samples, exceeded 10%. Dysregulation of spermatogenesis, as seen as testicular insufficiency or varicocele, was not associated with increased variability of DFI or HDS. A backward multiple linear regression analysis, however, indicated that the biological variation of DFI may be more profound in men with characteristics of normal spermatogenesis. In conclusion, we confirm previous reports that sperm DNA fragmentation has a lower biological variability than classical semen parameters. We hypothesize that the sperm chromatin structure may be more influenced in patients with normal spermatogenesis, whereas in men with disturbed spermatogenesis, the chromatin structure may be already so impaired that the effect of unidentified factors leading to variability of sperm DNA fragmentation in time may not be as profound.     

Sperm chromatin stability and zinc binding properties in semen from men in barren unions

Sperm chromatin stability and zinc binding properties were studied in semen samples from 115 men living in barren unions. Of these men, 26% had a high proportion of swelling sperm, i.e. less than 80% sperm with stable chromatin after exposure to the detergent sodium dodecyl sulphate. From 2-67% of seminal zinc was bound to high molecular weight ligands of vesicular origin (HMW). This shows that, among infertile men, liquefied seminal plasma has huge variations in zinc chelating properties. The relationship between prostatic palpatory status, the proportion of abnormal sperm, the percentage zinc bound to HMW (HMW-Zn), the time between ejaculation and analysis and chromatin stability were studied. Samples with low chromatin stability were found more frequently in men with low HMW-Zn levels in semen. The proportion of stable sperm decreased in samples with prolonged exposure to seminal plasma. Neither the proportion of stable sperm heads nor the percentage zinc bound to HMW could be used to predict the future chances of the infertile men fathering children when studied 15-180 min after ejaculation. To differentiate between initial zinc-dependent stability and superstability developed in seminal plasma, other more sensitive methods must be developed.     

Sperm chromatin structure assay （SCSA）： a tool in diagnosis and treatment of infertility

Diagnosis of male infertility has mainly been based on the World Health Organization (WHO) manual-based semen parameter's concentration, motility and morphology. It has, however, become apparent that none of these parameters are reliable markers for evaluation of the fertility potential of a couple. A search for better markers has led to an increased focus on sperm chromatin integrity testing in fertility work-up and assisted reproductive techniques. During the last couple of decades, numerous sperm DNA integrity tests have been developed. These are claimed to be characterized by a lower intraindividual variation, less intralaboratory and interlaboratory variation and thus less subjective than the conventional sperm analysis. However, not all the sperm chromatin integrity tests have yet been shown to be of clinical value. So far, the test that has been found to have the most stable clinical threshold values in relation to fertility is the sperm chromatin structure assay (SCSA), a flow cytometric test that measures the susceptibility of sperm DNA to acid-induced DNA denaturation in situ. Sperm DNA fragmentation as measured by SCSA has shown to be an independent predictor of successful pregnancy in first pregnancy planners as well as in couples undergoing intrauterine insemination, and can be used as a tool in investigation, counseling and treatment of involuntary childlessness. More conflicting data exist regarding the role of sperm DNA fragmentation in relation to fertilization, pre-embryo development and pregnancy outcome in in vitro fertilization and intracytoplasmic sperm injection (ICSI).     

Nuclear chromatin decondensation of spermatozoa in vitro: a method for evaluating the fertilizing ability of ovine semen

Spermatozoa obtained from testes, epididimydes and complete ejaculates of healthy rams during the breeding and non-breeding seasons were induced to show nuclear chromatin decondensation by controlled exposure to dithiotreitol (DTT) and sodium dodecyl sulphate (SDS) in vitro. A gradual resistance to decondensation was shown by sperm during epididymal transit, confirming a progressive increase in the prevalence of chromatinic disulphide bonds during sperm maturation in this species. A high % of stable (non-decondensed) sperm nuclei after treatment (79%) was found in semen from rams with normal fertility (64% non-return rate at first oestrus). Opposite changes were found in the semen from rams having low fertility rates (37%), as these showed only 31% of stable sperm nuclei. There were no differences in the spermiograms of these two groups. When semen from the same rams was tested during the non-breeding season, a similar relationship was found, although in both groups there was a higher % of sperm with stable nuclei than during the breeding season. The possible role of seminal plasma and of some of its constituents (e.g., zinc) on the decondensation of ram sperm nuclear chromatin was also studied. The presence of seminal plasma and the addition of zinc largely or completely inhibited the decondensation of ram sperm nuclear chromatin whilst the reverse situation was seen following the addition of chelating agents (e.g. EDTA) to the semen samples. The present results suggest that the induction of sperm nuclear decondensation by exposure to DTT and SDS under controlled conditions may provide a simple but reliable method for predicting in vitro the fertilizing ability of a ram semen sample.     

Evaluation of ICSI-selected epididymal sperm samples of obstructive azoospermic males by the CKIA system

The objective of this study was to evaluate nuclear normality in intracytoplasmic sperm injection (ICSI)-selected epididymal sperm from obstructive azoospermic (OA) patients. We evaluated whether the selection criteria used in routine ICSI (morphology and motility at a magnification of 400x) is adequate for selecting "normal" sperm from epididymal samples. Surgically retrieved spermatozoa from the caput epididymis of 15 OA patients and ejaculated sperm samples from 9 normospermic donors were evaluated with a DNA-specific stain (Feulgen) and in combination with the computerized karyometric image analysis (CKIA) system. Original (unselected) samples and ICSI-selected sperm were compared in donor and patient samples. In the original fraction, a larger variation in almost all measured parameters was found in epididymal sperm than in ejaculated sperm. After sperm selection, the morphometry was comparable between epididymal and ejaculated sperm. However, for those parameters related to the DNA stainability and chromatin texture (nuclear condensation), significant differences between patients and donors were observed. This result suggests that the size and form of the sperm do not necessarily hold similar internal structures. Thus, the frequency of "normal" sperm significantly increased after ICSI selection, but the improvement was more marked in donor than in OA patients' samples. In conclusion, at least a twofold increase in the number of normal spermatozoa was achieved after ICSI selection. The heterogeneity in the stainability and chromatin condensation of epididymal samples from OA patients indicates that some of the selected spermatozoa have a hypocondensed or hypercondensed chromatin. Even in the best of donor cases, no more than 55% of the selected sperm scored normal with CKIA, indicating that the present routine ICSI selection criteria are not sufficient for selecting normal condensed nuclei.     

Evaluation of gram stain as an alternative in the assessment of human spermatozoa quality

INTRODUCTION: During spermiogenesis, protaminosis and sperm chromatin condensation are important prerequisites for the preservation of DNA integrity in spermatozoa. The aim of this study is to assess Gram stain as an alternative technique for the evaluation of human sperm chromatin condensation status. Patients and METHODS: Aniline blue and Gram staining were applied to semen samples from 34 donors in order to determine the relationship between sperm chromatin condensation and infertility. In addition, the possible correlation between morphology and vitality (eosin-Y staining) of spermatozoa compared with their nuclear status (aniline blue and Gram staining) was studied. RESULTS: Chromatin condensation and sperm vitality were significantly higher in fertile men compared to the subfertile. A significant correlation was found between chromatin condensation and (a) sperm vitality (p < 0.01), and (b) nuclear protein status (p < 0.01). CONCLUSIONS: Gram staining may be used as a routine method in assisted reproduction laboratories and could assist in the evaluation of sperm quality as well as in the selection of the appropriate fertilization technique.     

Association of human sperm nuclear decondensation and in vitro penetration ability

Sperm nuclear decondensation is an integral step in fertilization which leads to the formation of the male pronucleus. The association between the in vitro spontaneous nuclear decondensation of human sperm and its fertilizing ability was studied in infertile male patients. The ability of sperm to fertilize an egg using the discontinuous two-layer Percoll method was significantly correlated to the percentage of decondensed swollen head (r = 0.43; P less than 0.005). The fertilizing ability of sperm processed with Test-Yolk buffer was correlated with the percentage of sperm at the fully decondensed stalk stage (r = 0.51; P less than 0.05). There were insignificant correlations for the whole-wash centrifugation, cryopreserved-thawed and swim-up methods. Samples of sperm that were positive (greater than 0% fertilization) in the sperm penetration assay had a higher percentage of decondensed sperm heads (66.7% vs. 20.6%) after Percoll wash or whole-wash centrifugation (60.5% vs. 44.3%) treatments compared with samples with no fertilization. Treatments that included Test-Yolk resulted in high percentages of decondensed swollen heads. The results suggest a positive association between sperm nuclear decondensation and the fertilizing ability of sperm, and affirm the importance of nuclear decondensation to the study of fertilization events.     

Fluorescence energy transfer shows that various physical and chemical treatments of human sperm induce unpacking of chromatin

Summary. Fluorescence resonance energy transfer (FRET) was used to study the changes which human sperm chromatin went through after various physical and chemical treatments. This technique showed a dilatation of the spatial relationship among chromatin linear arrays, with UV and DNAse among the treatments that gave rise to the highest increase. FRET image analysis showed that the chromatin linear arrays after treatment reach a spatial disarrangement similar to that brought about by sperm decondensation. Comparison of these results with the ability of human treated sperm to form pronuclei after microinjection into hamster eggs, suggests that the highly condensed spatial organization of sperm chromatin arrays may not be a necessary prerequisite for pronucleus formation.     

Better sperm selection for intracytoplasmic sperm injection with the side migration technique

The side migration technique (SMT) is a recent method for preparing very poor-quality semen samples to be used in intracytoplasmic sperm injection (ICSI). In most centres, the washing swim-up and Percoll gradient columns techniques have been routinely used. The present study is aimed at comparing the quality of oligozoospermic semen samples selected after these three methods. All three methods were found to select better percentage motility, normal morphology, viability, functional integrity of plasma membrane and nuclear chromatin integrity compared with the original semen samples. Among the three methods, however, SMT yielded better sperm quality, including morphology, viability, membrane integrity and nuclear chromatin integrity. The results of this study and our experience have confirmed that SMT is an effective and physiological method to prepare sperm for ICSI.     

Effect of freeze–thawing procedure on chromatin stability, morphological alteration and membrane integrity of human spermatozoa in fertile and subfertile men

Cryopreservation is known to impair sperm motility and decrease the fertilization rate by detrimental effects on acrosomal structure and acrosin activity. However, the consequences of cryopreservation on the integrity of the sperm nucleus, chromatin stability and centrosome are less clear. The present study was designed to determine the effect of the freeze-thawing procedure on chromatin condensation (aniline blue staining) and the morphology (strict criteria) and membrane integrity of human spermatozoa. The structural and functional characteristics of the sperm plasma membrane were measured by the eosin-test and hypo-osmotic swelling test which were done separately. Sperm cryopreservation was performed on semen samples from two groups of men classified as fertile (n = 20) and subfertile (n = 72), based on their reproductive history and semen analysis according to WHO guidelines. The mean percentage of condensed chromatin, morphologically normal spermatozoa and membrane integrity in all semen samples investigated (n = 92) decreased significantly (p = 0.0001) after freeze-thawing, in comparison to the value observed prior to freezing. By comparing the semen samples between fertile and subfertile patients, significantly (p = 0.0009) greater damage was demonstrated in the subfertile than in the fertile group. Furthermore, no significant difference was observed between the two groups with regard to the morphological alteration and structural as well as functional damage of the sperm membrane. In conclusion, the freeze-thawing procedure significantly affects chromatin structure and sperm morphology, especially in the head and the tail regions, and this may explain the lower fertilization rate and IVF/ICSI outcome when frozen-thawed spermatozoa are used. In addition, this study demonstrates that chromatin condensation is a sensitive parameter for the evaluation of cryodamage of semen samples from fertile and subfertile patients, though subfertile patients with very poor semen characteristics have yet to be studied. It is therefore recommended that chromatin condensation be used as an additional parameter for the assessment of sperm quality after freeze-thawing.     

Predictive value of sperm chromatin condensation (aniline blue staining) in the assessment of male fertility

A case control study was carried out to determine the value of sperm chromatin condensation in the assessment of male fertility. A total of 165 semen samples from 90 patients (cases) and 75 healthy donors (control) were examined for chromatin condensation (aniline blue staining), as well as conventional sperm parameters, notably sperm morphology, sperm count, and progressive motility. Whereas only 55 +/- 12.0% of the samples from the infertile patients were unstained by aniline blue (chromatin condensed), 78 +/- 19.0% of the samples in the control group did not take up the stain (chromatin condensed). A significant difference (p < .001) was observed between the two groups. Similarly, the difference between the mean percentage of morphologically normal spermatozoa for the infertile patients (12.1 +/- 1.2%) and the control (23.9 +/- 1.9%) was very significant (p < .001). In addition, only 50 out of the 90 patients (55.5%) had a normal sperm count, whereas all the 75 (100%) were normal in the control group. By comparing between the two groups a significant difference (p < .001) was also observed. Furthermore, a significant difference (p < .001) was also found between the cases and the control with regard to the percentage of spermatozoa illustrating linear progressive motility (40 +/- 9.7% vs. 70 +/- 12.3%). However, no correlation was found between sperm chromatin condensation and morphology, count, and motility. This study suggests that chromatin condensation constitutes a valuable parameter in the assessment of male fertility, completely independent of conventional sperm parameters. Consequently, the inclusion of chromatin condensation to routine laboratory investigations of semen prior to assisted reproduction is strongly recommended.     

Computerized semen analysis (CASA): effect of semen concentration and chamber depth on measurements

The purpose of this study was to examine the influence of sample concentration and chamber depth on the performance of a real-time analysis computer-assisted semen analysis system, the Hobson Sperm Tracker. Fresh semen samples were provided by patients or donors who attended the author's clinic. The samples were used to estimate total concentration, percentage motility, and sperm kinematics. A considerable variation was found in total concentration and motility recordings between manual and computerized analysis, which was more profound in high-density samples (>80 x 10(6) sperm/mL). The sperm motion parameters were significantly different between low- and high-density samples. This difference could not have been due to sample variation since it was also observed after 1:10 dilution of dense samples. The results indicate that a real-time analysis system can be used clinically for semen analysis over a wide range of sperm concentration. However, high sperm concentration can distort sperm count, motility, and sperm kinematics recordings.     

Effects of Chlamydia trachomatis infection on sperm chromatin condensation and DNA integrity

The present study was performed to investigate the relation of Chlamydia trachomatis infection to sperm chromatin/DNA integrity in a population of infertile men (male partner of infertile couples) from Iran. Blood, semen and first‐void urine samples were obtained from 250 infertile men. Data were analysed with regard to the results of (i) serological analysis for specific antibodies to C. trachomatis in serum; (ii) the presence of C. trachomatis and DNA in first‐void urine; and (iii) in the semen sample of the male partner, in addition to sperm analysis, four different tests (aniline blue, chromomycin A3, acridine orange and TUNEL) were used to detect sperm chromatin and DNA abnormalities. The main conclusions of the results were: (i) no evidence of C. trachomatis infection in semen samples was found; (ii) sperm DNA fragmentation and chromatin studies were not correlated with C. trachomatis diagnosis; (iii) the percentage of DNA fragmentation is positively correlated with the percentage of immotile sperm but negatively with semen volume, normal morphology; and (iv) in sperm chromatin evaluations, only the percentage of chromatin protamination was related to male age.     

Sperm Viability: A Comparison of Analytical Methods

Two methods of measuring human sperm viability, the stain exclusion assay and the hypoosmotic swelling (HOS) test, were evaluated. Human sperm were pretreated with 2.0% glutaraldehyde or 0.1% Triton X-100 and compared to untreated controls. Approximately one half of the sperm were found to be viable in the control samples and nearly all sperm were non-viable in the Triton X-100 treated samples by both the stain exclusion and HOS assays. After glutaraldehyde pretreatment, presumably inactivating the spermatozoa, the HOS test revealed that most sperm were not viable, while the stain exclusion test found no difference between glutaraldehyde pretreated sperm and control sperm. Investigations with scanning and transmission electron microscopy demonstrated that the HOS test caused the membrane of the sperm tail to swell and the tail fibers to coil several times within the swollen membrane. It is concluded that the stain exclusion assay merely measures the structural integrity of the sperm membrane, whereas the HOS test also provides an indication of the physiological integrity of the sperm membrane.     

Chromatin decondensation of human sperm in vitro and its relation to fertilization rate after ICSI

The inability of sperm chromatin to decondense has been implicated in the failure of fertilization, This study was undertaken to identify the relationship between sperm chromatin decondensation in vitro after incubation with follicular fluid at various points in time and fertilization or pregnancy rates after intracytoplasmic sperm injection. Moreover, an attempt was made to determine whether this test could be used as a predictive test for the outcome of ICSI. Thirty-two infertile couples undergoing ICSI therapy were included in this prospective study. One milliter of semen from each sample was mixed with 1 mL of follicular fluid obtained from ICSI patients at the time of oocyte retrieval and incubated for 24 h. Many smears were made directly after semen liquefaction at the following time intervals: 30, 60, and 120 min and 24 h. Chromatin decondensation was evaluated with acridine orange staining. The mean percentage of uncondensed chromatin of spermatozoa in the native semen samples was 25 +/- 18.3%, which increased within 24 h to 91 +/- 9.5%. On the other hand, the fertilization and ongoing pregnancy rates were 64 +/- 21.7% and 20%, respectively. However, no correlations were found between chromatin decondensation at various point of time (30, 60, and 120 min and 24 h) and fertilization rate. No correlation was shown between the chromatin decondensation and sperm counts in the ejaculate. morphology, or the percentage of condensed chromatin. In light of this study, chromatin decondensation in vitro cannot be recommended for predicting the fertilization potential of spermatozoa and pregnancy rates in the ICSI program. Further research is necessary, especially in cases where ICSI is being considered as a therapeutic option.     

The relationship between sperm morphology and chromatin integrity in the koala (Phascolarctos cinereus) as assessed by the Sperm Chromatin Dispersion test (SCDt)

Koala (Phascolarctos cinereus) sperm nuclei show a tendency to swell after cryopreservation, but it is uncertain whether this phenomenon is associated with DNA fragmentation. In this study, we validated a modified version of the sperm chromatin dispersion test (SCDt) for use with koala spermatozoa, which is the first use of the test for a marsupial. Cryopreserved spermatozoa (multiple straws) from a single koala were used to explore the relationship between sperm morphology, viability, chromatin dispersion, and DNA fragmentation. A SCDt prototype kit (Sperm Halomax) was specifically developed for koala spermatozoa with the use of a lysing solution that did not contain dithiothreitol. DNA fragmentation of lysed and nonlysed spermatozoa was examined in microgel slides and validated by means of in situ nick translation (ISNT). The SCDt was then applied to the analysis of extended and frozen-thawed semen samples of 3 different koalas. Spermatozoa were classified into 3 distinct koala sperm morphotypes (KSMs) after the SCDt: 1) KSM-1, rod-shaped cells with no halo of DNA; 2) KSM-2, rounded nuclei with various degrees of halo formation about a dense chromatin core; and 3) KSM-3, rod-shaped or rounded nuclei consisting of an inner chromatin core but with large dispersed halos of stellar chromatin. Although ISNT after the SCDt did not label KSM-1, both KSM-2 and KSM-3 stained positively for DNA fragmentation. ISNT was not able to differentiate between KSM-2 and KSM-3. Although application of the SCDt to the spermatozoa of another 3 koalas showed no difference in the percentage of the 3 sperm morphotypes found between extended and frozen-thawed semen, thawed spermatozoa incubated at 35 degrees C for 2 hours showed an increase in the incidence of KSM-3 and a corresponding decrease in KSM-2. We propose that KSM-1 and KSM-2 represent nuclei that show either no, or only limited, sperm DNA fragmentation, respectively. It is likely that the halos formed around KSM-2 are from DNA that is damaged as part of the normal processing of the spermatozoa and is a consequence of the lack of cysteine residues and associated stabilizing disulfide bonds in marsupial sperm DNA. "True" sperm DNA damage is most likely associated with KSM-3, which shows a massive dispersion of chromatin similar to that described in other species. A model of koala sperm chromatin structure is proposed to explain the behavior of the sperm nuclei after the SCDt. Further studies are required to determine whether DNA damage found in KSM-2 is indicative of single-stranded DNA breakage associated with an inherent lack of cysteine residues in marsupial sperm chromatin. Conversely, it will also be important to establish whether KSM-3 is caused by an increased incidence of double-stranded DNA breakage and whether this abnormality is correlated with impaired fertility as it is in other species.