Small human sperm vacuoles observed under high magnification are pocket-like nuclear concavities linked to chromatin condensation failure

Exploring the impact of environmental factors on male reproductive health through epigenetics

Male infertility has become an increasingly severe global health issue, with its incidence significantly rising over the past few decades. This paper delves into the crucial role of epigenetics in male reproductive health, focusing particularly on the effects of DNA methylation, histone modifications, chromatin remodeling and non-coding RNAs regulation on spermatogenesis. Exposure to various environmental factors can cause sperm DNA damage, leading to epigenetic abnormalities. Among these factors, we have discussed heavy metals (including Zinc, Cadmium, Arsenic, Copper), phthalates, electromagnetic radiation, and temperature in detail. Notably, aberrations in DNA methylation are closely associated with various symptoms of male infertility, and histone modifications and chromatin remodeling are essential for sperm maturation and function. By synthesizing existing literature and experimental data, this narrative review investigates how environmental factors influence male reproductive health through epigenetic mechanisms, thus providing new theoretical foundations and practical guidelines for the early diagnosis and treatment of male infertility.

First-line infertility treatment in normal or subnormal sperm: Interest of a simplified pre-IMSI test

BACKGROUND

In the field of male infertility, when sperm is normal/subnormal, a few "add-on" routine tests can complete the basic semen examination.

OBJECTIVES

The aim of this study was to develop and evaluate a faster, simplified motile sperm organelle morphology examination (MSOME) technique for selected infertile patients with apparently normal/subnormal sperm and, in their background: failure of two or three intrauterine insemination (IUI) cycles, repeatedly fragmented embryos, embryonic development to blastocyst-stage failures, repeated miscarriages, a long period of infertility or 2 or more IVF attempts without pregnancy. Our test results were correlated with IUI, conventional in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI) and intracytoplasmic morphologically selected sperm injection (IMSI) outcomes.

MATERIALS AND METHODS

We validated an adapted version of the MSOME analysis called the pre-IMSI test (PIT), based on vacuole evaluation alone. 248 infertile patients from our assisted reproductive technology (ART) Center were retrospectively selected and split into three PIT score subgroups (patients with ≤8% (score I), 9 to 15% (score II) and ≥16% normal spermatozoa (score III)) based on the correlation between PIT results and each ART technique outcome. The choice of one or another of these ART techniques had been made according to the usual clinico-biological criteria.

RESULTS

Clinical outcomes for each of the three PIT subgroups were compared individually for the different ART techniques. For ICSI, the effect of the PIT score subgroup was significant for clinical pregnancies (p = 0.0054) and presented a trend for live births (p = 0.0614). Miscarriage rates of IVF attempts were statistically different depending on the PIT score (p = 0.0348). Furthermore, the odds ratios of clinical pregnancy rates were significantly different according to PIT score subgroup when comparing ICSI vs. IMSI or IVF vs. ICSI attempts.

DISCUSSION

IMSI appears to be recommended when sperm belongs to PIT score I, ICSI when it belongs to PIT score II and IVF or IUI when sperm is of PIT score III quality in selected infertile couples. The lack of statistical power in these PIT subgroups means that we must remain cautious in interpreting results.

CONCLUSION

Our results support the interest of this simplified test for certain couples with normal/subnormal sperm to help choose the most efficient ART technique, even as first-line treatment.

Associations between Sperm Epigenetic Age and Semen Parameters: An Evaluation of Clinical and Non-Clinical Cohorts

The well-documented relationship between chronological age and the sperm methylome has allowed for the construction of epigenetic clocks that estimate the biological age of sperm based on DNA methylation, which we previously termed sperm epigenetic age (SEA). Our lab demonstrated that SEA is positively associated with the time taken to achieve pregnancy; however, its relationship with semen parameters is unknown. A total of 379 men from the Longitudinal Investigation of Fertility and Environment (LIFE) study, a non-clinical cohort, and 192 men seeking fertility treatment from the Sperm Environmental Epigenetics and Development Study (SEEDS) were included in the study. Semen analyses were conducted for both cohorts, and SEA was previously generated using a machine learning algorithm and DNA methylation array data. Association analyses were conducted via multivariable linear regression models adjusting for BMI and smoking status. We found that SEA was not associated with standard semen characteristics in SEEDS and LIFE cohorts. However, SEA was significantly associated with higher sperm head length and perimeter, the presence of pyriform and tapered sperm, and lower sperm elongation factor in the LIFE study (p < 0.05). Based on our results, SEA is mostly associated with defects in sperm head morphological factors that are less commonly evaluated during male infertility assessments. SEA shows promise to be an independent biomarker of sperm quality to assess male fecundity.

Human Sperm Head Vacuoles Are Related to Nuclear-Envelope Invaginations

Nuclear vacuoles are specific structures present on the head of the human sperm of fertile and non-fertile men. Human sperm head vacuoles have been previously studied using motile sperm organelle morphology examination (MSOME) and their origin related to abnormal morphology, abnormal chromatin condensation and DNA fragmentation. However, other studies argued that human sperm vacuoles are physiological structures and consequently, to date, the nature and origin of the nuclear vacuoles remains to be elucidated. Here, we aim to define the incidence, position, morphology and molecular content of the human sperm vacuoles using transmission electron microscopy (TEM) and immunocytochemistry techniques. The results showed that ~50% of the analyzed human sperm cells (n = 1908; 17 normozoospermic human donors) contained vacuoles mainly located (80%) in the tip head region. A significant positive correlation was found between the sperm vacuole and nucleus areas. Furthermore, it was confirmed that nuclear vacuoles were invaginations of the nuclear envelope from the perinuclear theca and containing cytoskeletal proteins and cytoplasmic enzyme, discarding a nuclear or acrosomal origin. According to our findings, these human sperm head vacuoles are cellular structures originating from nuclear invaginations and contain perinuclear theca (PT) components, allowing us to define a new term of 'nuclear invaginations' rather than 'nuclear vacuoles'.

Human Sperm Morphology as a Marker of Its Nuclear Quality and Epigenetic Pattern

Background: Human sperm chromatin condensation is a sum of epigenetic events that allows for the near-complete replacement of histones with protamines. Under high-magnification microscopy, nuclear vacuoles have been described as thumbprints with poor chromatin condensation. The objective of this study is to examine whether vacuolated spermatozoa carry specific epigenetic marks, which may influence embryo development. Methods: The presence and three-dimensional distribution of ten epigenetic marks (protamine-P2, histone-H3, H3K4me1/me2/me3, H3K9me1/me2/me3, H3K27me3, H4k20me2) were evaluated and compared in morphometrically normal spermatozoa according to the presence or absence of a large vacuole occupying more than 15% of the head surface (n = 4193). Results: Vacuolated spermatozoa were significantly more frequently labelled with H3 and H3K4me3 than normal spermatozoa (88.1% ± 2.7 and 78.5% ± 5.2 vs. 74.8% ± 4.8 and 49.1% ± 7.4, respectively; p = 0.009 and p < 0.001) and significantly less marked by P2 and H3K27me3 (50.2% ± 6.2 and 63.9% ± 6.3 vs. 82.1% ± 4.4 and 73.6% ± 5.1, respectively; p < 0.001 and p = 0.028). In three dimensions, vacuoles are nuclear concavities filled with DNA carrying the H3K4me3 marker. Conclusion: High-magnification microscopy is a simple tool to estimate in real time the sperm epigenetic profile. The selection of normal spermatozoa without vacuoles and the deselection of spermatozoa with vacuoles appear to be epigenetically favorable to embryo development and safe offspring.

Using Deep Learning Algorithm: The Study of Sperm Head Vacuoles and Its Correlation with Protamine mRNA Ratio

Objective

It is necessary to evaluate fertility effective agents to predict assisted reproduction outcomes. This study was designed to examine sperm vacuole characteristics, and its association with sperm chromatin status and protamine-1 (PRM1) to protamine-2 (PRM2) ratio, to predict assisted pregnancy outcomes.

Materials and Methods

In this experimental study, ninety eight semen samples from infertile men were classified based on Vanderzwalmen’s criteria as follows: grade I: no vacuoles; grade II: <2 small vacuoles; grade III: <1 large vacuole and grade IV: large vacuole with other abnormalities. The location, frequency and size of vacuoles were assessed using high magnification, a deep learning algorithm, and scanning electron microscopy (SEM). The chromatin integrity, condensation, viability and acrosome integrity, and protamination status were evaluated for vacuolated samples by toluidine blue (TB) staining, aniline blue, triple staining, and CMA3 staining, respectively. Also, Protamine-1 and protamine-2 genes expression was analysed by reverse transcription-quantitative polymerase chain reaction (PCR). The assisted reproduction outcomes were also followed for each cycle.

Results

The results show a significant correlation between the vacuole size (III and IV) and abnormal sperm chromatin condensation (P=0.03 and P=0.02, respectively), and also, protamine-deficient (P=0.04 and P=0.03, respectively). The percentage of reacting acrosomes was significantly higher in the grades III and IV spermatozoa in comparison with normal group. The vacuolated spermatozoa with grade IV showed a high protamine mRNA ratio (PRM-2 was underexpressed, P=0.01). In the IVF cycles, we observed a negative association between sperm head vacuole and fertilization rate (P=0.01). This negative association was also significantly observed in pregnancy and live birth rate in the groups with grade III and IV (P=0.04 and P=0.03, respectively).

Conclusion

The results of our study highlight the importance sperm parameters such as sperm head vacuole characteristics, particularly those parameters with the potency of reflecting protamine-deficiency and in vitro fertilization/ intracytoplasmic sperm injection (IVF/ICSI) outcomes predicting.

Molecular Profiling of Spermatozoa Reveals Correlations between Morphology and Gene Expression: A Novel Biomarker Panel for Male Infertility

Choosing spermatozoa with an optimum fertilizing potential is one of the major challenges in assisted reproductive technologies (ART). This selection is mainly based on semen parameters, but the addition of molecular approaches could allow a more functional evaluation. To this aim, we used sixteen fresh sperm samples from patients undergoing ART for male infertility and classified them in the high- and poor-quality groups, on the basis of their morphology at high magnification. Then, using a DNA sequencing method, we analyzed the spermatozoa methylome to identify genes that were differentially methylated. By Gene Ontology and protein-protein interaction network analyses, we defined candidate genes mainly implicated in cell motility, calcium reabsorption, and signaling pathways as well as transmembrane transport. RT-qPCR of high- and poor-quality sperm samples allowed showing that the expression of some genes, such as AURKA, HDAC4, CFAP46, SPATA18, CACNA1C, CACNA1H, CARHSP1, CCDC60, DNAH2, and CDC88B, have different expression levels according to sperm morphology. In conclusion, the present study shows a strong correlation between morphology and gene expression in the spermatozoa and provides a biomarker panel for sperm analysis during ART and a new tool to explore male infertility.

Effects of lead, cadmium, copper and zinc levels on the male reproductive function

This study aimed to investigate the effects of heavy metals on measures of male fertility. One hundred and two infertile men with occupational exposure and thirty fertile men were included in this study. Blood and urinary levels of lead, cadmium, zinc and copper were measured by atomic absorption spectrophotometry. Semen parameters and a motile sperm organelle morphology examination were also performed. Measures of hormonal levels, oxidation-reduction potential, DNA fragmentation index and chromatin condensation were assessed for all participants. Heavy metals levels, oxidative stress and DNA quality were significantly higher in the infertile group compared to controls. FSH and testosterone levels were lower in the infertile group. A urinary cadmium level was positively associated with abnormal sperm morphology (r = .225, p < .05). Normal morphology was inversely correlated with the duration of the exposure (r = -.227, p = .022). The blood lead level was positively related to the level of testosterone (r = .223, p = .031). Cadmium and lead blood levels were positively correlated with the level of chromatin decondensation (r = .528, p < .001; r = .280, p = .017). Our study showed that occupational exposure to heavy metals is very harmful to reproductive health. DNA quality and oxidative stress investigations must be recommended for reprotoxic exposed patients prior to in vitro fertilisation treatment.

Defect in the nuclear pore membrane glycoprotein 210-like gene is associated with extreme uncondensed sperm nuclear chromatin and male infertility: a case report

After the two meiotic divisions, haploid round spermatids undergo dramatic changes to become mature spermatozoa. One of the main transformations consists of compacting the cell nucleus to confer the sperm its remarkable hydrodynamic property and to protect its DNA from the oxidative stress it will encounter during its reproductive journey. Here, we studied an infertile subject with low sperm count, poor motility and highly abnormal spermatozoa with strikingly large heads due to highly uncondensed nuclear sperm DNA. Whole-exome sequencing was performed on the subject's DNA to identify the genetic defect responsible for this severe sperm anomaly. Bioinformatics analysis of exome sequence data uncovered a homozygous loss of function variant, ENST00000368559.7:c.718-1G>A, altering a consensus splice site expected to prevent the synthesis of the nucleoporin 210 like (NUP210L) protein. High-resolution mass spectrometry of sperm protein extracts did not reveal any NUP210L peptide sequence in the patient's sperm, contrary to what was observed in control donors, thus confirming the absence of NUP210L in the patient's sperm. Interestingly, homozygous Nup210l knock-out mice have been shown to be infertile due to a reduced sperm count, a high proportion of round-headed sperm, other head and flagella defects and a poor motility. NUP210L is almost exclusively expressed in the testis and sequence analogy suggests that it encodes a nuclear pore membrane glycoprotein. The protein might be crucial to regulate nuclear trafficking during and/or before spermiogenesis, its absence potentially impeding adequate nuclear compaction by preventing the entry of histone variants/transition proteins/protamines into the nucleus and/or by preventing the adequate replacement of core histones. This work describes a new gene necessary for male fertility, potentially improving the efficiency of the genetic diagnosis of male infertility. The function of NUP210L still remains to be resolved and its future investigation will help to understand the complex mechanisms necessary for sperm compaction.

Teratozoospermia with amorphous sperm head associate with abnormal chromatin condensation in a Chinese family

Male infertility affects approximately 7% of the male population. In about 40% of affected patients, the etiology remains unknown. Here, we report the cases of two infertile brothers who have a uniquely prevalent sperm phenotype with completely amorphous sperm heads. To investigate the mechanisms of familial teratozoospermia with amorphous sperm heads, chromatin condensation was assessed by aniline blue staining, western blot, sperm chromatin structure assay and atomic force microscopy in both the two brothers and 40 control fertile donors. Our results showed an abnormal condensation of chromatin with amorphous headed sperm. We suggest that abnormal chromatin condensation which was induced by disturbances in the process of histone-protamine replacement may be a possible cause of familial teratozoospermia with amorphous head, and the elasticity of sperm nuclei could be a new index to assess sperm quality. Additionally, for the first time, the current study provided a new biomechanics strategy for evaluating pathological sperm contributes to our understanding of teratozoospermia.Abbreviations: SCSA: sperm chromatin structure assay; AFM: atomic force microscopy; ICSI: intracytoplasmic sperm injection; HDS: high DNA stainability; DFI: DNA fragmentation index; PBS: phosphate-buffered saline; DTT: dithiothreitol; FITC: fluorescein isothiocyanate; DAPI: 4',6-diamidino-2-pheneylindole; SSC: standard saline citrate.

Impact of high magnification sperm selection on neonatal outcomes: a retrospective study

PURPOSE

The aim of this study was to compare the effect of the deselection of spermatozoa presenting vacuole-like structures using IMSI (intracytoplasmic morphologically selected sperm injection) with ICSI (intracytoplasmic sperm injection) by means of neonatal outcomes.

METHODS

In a retrospective two-center analysis, a total of 848 successful IMSI or ICSI cycles ending with a live birth, induced abortion, or intrauterine fetal death (IUFD) were included.

RESULTS

The IMSI and ICSI groups included 332 and 655 babies or fetuses, respectively. The parents were older in the IMSI group than in the ICSI group (mothers were 35.1 vs 32.9 years, and fathers were 39.1 vs 36.2 years). The multiple pregnancy rate was higher in the IMSI group. The mean pregnancy duration and mean birth weight were almost identical in both groups. There was no significant difference in major congenital malformations between the two groups. However, this rate was decreased in the IMSI group compared to that in the ICSI group (1.8 vs 3.2%), the difference being mainly found in singletons (1.4 vs 3.3%). Boys were more often affected than girls in both groups. The percentages of chromosomal abnormalities did not differ between the IMSI and ICSI groups (0.6 and 0.8%). The reported congenital malformations mainly affected the heart, urogenital, and musculoskeletal systems.

CONCLUSIONS

In the present study, the malformation rates observed in the IMSI and ICSI groups were not significantly different, even if slightly lower after IMSI. However, the observed difference followed the same trends observed in previous reports, indicating the possible impact of IMSI on decreasing congenital malformation occurrences. This highlights the necessity to prospectively evaluate the impact of IMSI on neonatal outcome after IVF treatment.

Mammalian sperm nuclear organization: resiliencies and vulnerabilities

Sperm cells are remarkably complex and highly specialized compared to somatic cells. Their function is to deliver to the oocyte the paternal genomic blueprint along with a pool of proteins and RNAs so a new generation can begin. Reproductive success, including optimal embryonic development and healthy offspring, greatly depends on the integrity of the sperm chromatin structure. It is now well documented that DNA damage in sperm is linked to reproductive failures both in natural and assisted conception (Assisted Reproductive Technologies [ART]). This manuscript reviews recent important findings concerning - the unusual organization of mammalian sperm chromatin and its impact on reproductive success when modified. This review is focused on sperm chromatin damage and their impact on embryonic development and transgenerational inheritance.

Three-dimensional sperm surface reconstruction: a novel approach to assessing sperm morphology

OBJECTIVE

To create a rapid, inexpensive, efficient, and reproducible real-time three-dimensional (3-D) analysis of viable spermatozoa. Previous studies have demonstrated that abnormal semen profiles are associated with a modest increase in the frequency of sperm chromosomal abnormalities, and that sperm with aberrations in the shape and contours of the head may be carriers of chromatinic defects. Although high-power magnification and enhanced video-generated magnification have been suggested, these techniques are inherently limited by the clarity of the image, the time required for the analysis, and the risk of variable head-positioning during imaging.

DESIGN

In vitro experiment.

SETTING

University-affiliated infertility research laboratory.

PATIENT(S)

Anonymous sperm donors.

INTERVENTION(S)

Individual motile sperm were identified, analyzed at ×600 magnification, and a 10-second digital video was obtained.

MAIN OUTCOME MEASURE(S)

Image-tracking software captured serial photographs of sperm from recorded videos. Images were automatically extracted from each video frame using enhanced correlation coefficient maximization; the general shape of the sperm was extracted via space-carving. The reconstructed image was rotated to permit viewing from any direction, and the final image was rendered through interpolation.

RESULT(S)

This technique yielded images that enable noninvasive, 3-D, real-time, in vitro assessment of sperm surface morphology.

CONCLUSION(S)

This proof-of-principle demonstrates that by keeping spermatozoa in a fluid environment, a 3-D sperm-surface reconstruction can be created. This technique can be automated, requires minimal computing power, and utilizes equipment already available in most embryology laboratories.

Seipin deficiency increases chromocenter fragmentation and disrupts acrosome formation leading to male infertility

The Berardinelli–Seip congenital lipodystrophy type 2 (Bscl2, seipin) gene is involved in adipogenesis. Bscl2^−/− males were infertile but had normal mating behavior. Both Bscl2^−/− cauda epididymis sperm count and sperm motility were ~20 × less than control. Bscl2^−/− seminiferous tubules had relatively normal presence of spermatogonia and spermatocytes but had reduced spermatids and sperm. Spatiotemporal expression analyses in Bscl2^+/+ testes demonstrated prominent Bscl2 transcriptional activity in spermatocytes with a plateau reached around postnatal day 28. Seipin protein localization was most abundant in postmeiotic spermatids, suggesting translational repression of Bscl2 mRNA in spermatocytes. In situ end-labeling plus detected increased spermatid apoptosis in Bscl2^−/− testis and annexin V detected increased percentage of positive Bscl2^−/− round spermatids compared with control. Immunofluorescence of marker proteins synaptonemal complex proteins 3 and 1 (SYCP3 and SYCP1), and H3K9me3 (histone H3 trimethylated at lysine 9) in germ cell spreads detected normal meiotic chromosome pairing and homologous chromosome synapsis in Bscl2^−/− spermatocytes, but significantly increased percentages of round spermatids with chromocenter fragmentation and late spermatids and sperm with chromatin vacuoles, indicating defective chromatin condensation in Bscl2^−/− spermatids. Bscl2^−/− late spermatids were disorganized within the seminiferous epithelium, despite normal appearance of Sertoli cells detected by vimentin immunofluorescence. Peanut agglutinin staining revealed various abnormalities of acrosomes in Bscl2^−/− late spermatids, including the absence, irregular-shaped, and fragmented acrosomes, indicating defective acrosome formation in Bscl2^−/− late spermatids, which may affect late spermatid orientation in the seminiferous epithelium. Mitotracker strongly stained the midpiece of control sperm but only very weakly labeled the midpiece of Bscl2^−/− sperm, indicating defective mitochondrial activity that most likely contributed to reduced Bscl2^−/− sperm motility. These data demonstrate novel roles of seipin in spermatid chromatin integrity, acrosome formation, and mitochondrial activity. Increased spermatid apoptosis, increased chromocenter fragmentation, defective chromatin condensation, abnormal acrosome formation, and defective mitochondrial activity contributed to decreased sperm production and defective sperm that resulted in Bscl2^−/− male infertility.

Investigation of the association between the outcomes of sperm chromatin condensation and decondensation tests, and assisted reproduction techniques

The main purpose of this prospective study is to examine possible influences of abnormalities of sperm nuclear condensation and chromatin decondensation with sodium dodecyl sulphate (SDS)-EDTA on outcomes of intrauterine insemination (IUI) or intracytoplasmic sperm injection (ICSI) cycles. Semen samples from 122 IUI and 236 ICSI cycles were evaluated. Before semen preparation for IUI or ICSI, basic semen analysis was performed and a small portion from each sample was spared for fixation. The condensation of sperm nuclear chromatin was evaluated with acidic aniline blue, followed by sperm chromatin decondensation by SDS-EDTA and evaluation under light microscope. Ongoing pregnancy rate was 24% and 26.2% in the IUI and ICSI groups respectively. The chromatin condensation rate was significantly higher in the ongoing pregnancy-positive group compared to the negative group, both in IUI (P = 0.042) and ICSI groups (P = 0.027), and it was positively correlated with ongoing pregnancy rate in both IUI and ICSI groups (P = 0.015, r = 0.214 and P = 0.014, r = 0.312 respectively). Chromatin decondensation rates were not significantly different in neither of the groups. These results indicate that IUI and ICSI outcome is influenced by the rate of spermatozoa with abnormal chromatin condensation. Sperm chromatin condensation with aniline blue is useful for selecting assisted reproduction techniques (ART) patients.

Clinical and structural features of sperm head vacuoles in men included in the in vitro fertilization programme

The human sperm head vacuoles and their role in male infertility are still poorly understood. The aim of this study was to identify the clinical and ultrastructural features of human sperm head vacuoles in men included in the in vitro fertilization programme: men with normal (normozoospermia) and impaired sperm morphology (teratozoospermia). The sperm samples were observed under 6000-time magnification using motile sperm organelle morphology examination (MSOME). The proportion of sperm with head vacuoles was evaluated and related to the outcome of in vitro fertilization. The sperm of men with impaired sperm morphology was characterized by a higher proportion of sperm head vacuoles. The sperm head vacuoles were related to impaired semen quality (sperm concentration, motility, and morphology) but were not influenced by male factors (semen volume, height, age, weight, or body mass index). Moreover, sperm head vacuoles were related to impaired fertilization rate merely after classical in vitro fertilization (IVF), while there was no relation to pregnancy. In a subgroup of men, the sperm was fixed and observed by transmission electron microscopy (TEM). The ultrastructural study revealed that sperm head vacuoles are large nuclear indentations of various sizes and positions, packed with membranous material organized in membrane whorls (MW).

High-magnification selection of spermatozoa prior to oocyte injection: confirmed and potential indications

Intracytoplasmic morphologically selected sperm injection (IMSI) involves the use of differential interference contrast microscopy at high magnification (at least ·6300) to improve the observation of live human spermatozoa (particularly by showing sperm head vacuoles that are not necessarily seen at lower magnifications) prior to intracytoplasmic sperm injection (ICSI) into the oocyte. However, a decade after IMSI’s introduction, the technique’s indications and ability to increase pregnancy and/or birth rates (relative to conventional ICSI) are subject to debate. In an attempt to clarify this debate, this work performed a systematic literature review according to the PRISMA guidelines. The PubMed database was searched from 2001 onwards with the terms ‘IMSI’, ‘MSOME’ and ‘high-magnification, sperm’. Out of 168 search results, 22 relevant studies reporting IMSI outcomes in terms of blastocyst, pregnancy, delivery and/or birth rates were selected and reviewed. The studies’ methodologies and results are described and discussed herein. In view of the scarcity of head-to-head IMSI versus ICSI studies, the only confirmed indication for IMSI is recurrent implantation failure following ICSI. All other potential indications of IMSI require further investigation.

The nature of human sperm head vacuoles: a systematic literature review

Motile sperm organelle morphology examination (MSOME) involves the use of differential interference contrast microscopy (also called Nomarski contrast) at high magnification (at least 6300x) to improve the observation of live human spermatozoa. In fact, this technique evidences sperm head vacuoles that are not necessarily seen at lower magnifications - particularly if the vacuoles are small (i.e. occupying <4% of the sperm head’s area). However, a decade after MSOME's introduction, it is still not clear whether sperm head vacuoles are nuclear, acrosomal and/or membrane-related in nature. In an attempt to clarify this debate, we performed a systematic literature review in accordance with the PRISMA guidelines. The PubMed database was searched from 2001 onwards with the terms "MSOME", “human sperm vacuoles”, "high-magnification, sperm”. Out of 180 search results, 21 relevant English-language publications on the nature of human sperm head vacuoles were finally selected and reviewed. Our review of the literature prompted us to conclude that sperm-head vacuoles are nuclear in nature and are related to chromatin condensation failure and (in some cases) sperm DNA damage.