Ultrastructure of gametes after intracytoplasmic sperm injection

Microinjection induces changes in the transcriptome of bovine oocytes

Gene knockdown techniques are widely used to examine the function of specific genes or proteins. While a variety of techniques are available, a technique commonly used on mammalian oocytes is mRNA knockdown by microinjection of small interfering RNA (siRNA), with non-specific siRNA injection used as a technical control. Here, we investigate whether and how the microinjection procedure itself affects the transcriptome of bovine oocytes. Injection of non-specific siRNA resulted in differential expression of 119 transcripts, of which 76 were down-regulated. Gene ontology analysis revealed that the differentially regulated genes were enriched in the biological processes of ATP synthesis, molecular transport and regulation of protein polyubiquitination. This study establishes a background effect of the microinjection procedure that should be borne in mind by those using microinjection to manipulate gene expression in oocytes.

Preventing polyspermy in mammalian eggs-Contributions of the membrane block and other mechanisms

The egg's blocks to polyspermy (fertilization of an egg by more than one sperm) were originally identified in marine and aquatic species with external fertilization, but polyspermy matters in mammalian reproduction too. Embryonic triploidy is a noteworthy event associated with pregnancy complications and loss. Polyspermy is a major cause of triploidy with up to 80% of triploid conceptuses being the result of dispermic fertilization. The mammalian female reproductive tract regulates the number of sperm that reach the site of fertilization, but mammals also utilize egg-based blocks to polyspermy. The egg-based blocks occur on the mammalian egg coat (the zona pellucida) and the egg plasma membrane, with apparent variation between different mammalian species regarding the extent to which one or both are used. The zona pellucida block to polyspermy has some similarities to the slow block in water-dwelling species, but the mammalian membrane block to polyspermy differs substantially from the fast electrical block that has been characterized in marine and aquatic species. This review discusses what is known about the incidence of polyspermy in mammals and about the mammalian membrane block to polyspermy, as well as notes some lesser-characterized potential mechanisms contributing to polyspermy prevention in mammals.

Risk of chromosomal aberration in spermatozoa during intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) has become critical for the treatment of severe male infertility. The principal feature of ICSI is the direct injection of spermatozoon into an oocyte, which facilitates the production of fertilized embryos regardless of semen characteristics, such as sperm concentration and motility. However, the chromosomal integrity of ICSI zygotes is degraded compared to that of zygotes obtained via in vitro fertilization. This chromosomal damage may occur due to the injection of non-capacitated, acrosome-intact spermatozoa, which never enter the oocytes under natural fertilization. Furthermore, it is possible that the in vitro incubation and pre-treatment of spermatozoa during ICSI results in DNA damage. Chromosomal aberrations in embryos induce early pregnancy losses. However, these issues may be overcome by embryo production using gametes with guaranteed chromosomal integrity. Because conventional chromosome analysis requires fixing cells to obtain the chromosome spreads, embryos cannot be produced using the nucleus that has been analyzed. On the other hand, genome cloning using androgenic or gynogenic embryos provides an additional nucleus for chromosome analysis following embryo production. Thus, this review aims to highlight the hazardous nature of chromosomal aberrations in sperm during ICSI and to introduce a method for the prezygotic examination for chromosomal aberrations.

Lower blastocyst quality after conventional vs. Piezo ICSI in the horse reflects delayed sperm component remodeling and oocyte activation

PURPOSE

The aim of this study was to evaluate the differential effects of conventional and Piezo-driven ICSI on blastocyst development, and on sperm component remodeling and oocyte activation, in an equine model.

METHODS

In vitro-matured equine oocytes underwent conventional (Conv) or Piezo ICSI, the latter utilizing fluorocarbon ballast. Blastocyst development was compared between treatments to validate the model. Then, oocytes were fixed at 0, 6, or 18 h after injection, and stained for the sperm tail, acrosome, oocyte cortical granules, and chromatin. These parameters were compared between injection techniques and between sham-injected and sperm-injected oocytes among time periods.

RESULTS

Blastocyst rates were 39 and 40%. The nucleus number was lower, and the nuclear fragmentation rate was higher, in blastocysts produced by Conv. Cortical granule loss started at 0H after both sperm and sham injection. The acrosome was present at 0H in both ICSI treatments, and persisted to 18H in significantly more Conv than Piezo oocytes (72 vs. 21%). Sperm head area was unchanged at 6H in Conv but significantly increased at this time in Piezo; correspondingly, at 6H significantly more Conv than Piezo oocytes remained at MII (80 vs. 9.5%). Sham injection did not induce significant meiotic resumption.

CONCLUSIONS

These data show that Piezo ICSI is associated with more rapid sperm component remodeling and oocyte meiotic resumption after sperm injection than is conventional ICSI, and with higher embryo quality at the blastocyst stage. This suggests that there is value in exploring the Piezo technique, utilized with a non-toxic fluorocarbon ballast, for use in clinical human ICSI.

Combination of density gradient centrifugation and swim-up methods effectively decreases morphologically abnormal sperms

Density gradient centrifugation (DGC) and swim-up techniques have been reported for semen preparation in assisted reproductive techniques in humans. We investigated whether semen preparation using a combination of DGC and swim-up techniques could effectively decrease morphologically abnormal human sperms at the ultrastructural level. Semen samples were obtained from 16 infertile males and fractionated by swim-up following DGC. Ultrastructural abnormalities of sperms obtained from original semen, lower layer of swim-up following DGC, and upper layer of swim-up following DGC were analyzed by transmission electron microscopy. The correlation among ultrastructural head abnormality in sperms from the upper layer of swim-up, fertilization in in vitro fertilization, and pregnancy after embryo transfer was also investigated. Furthermore, sperms with DNA fragmentation in the samples processed via a combination of DGC and swim-up was assessed in a sperm chromatin structure assay. Ultrastructural abnormalities in sperm heads and tails in the upper layer after swim-up following DGC was the lowest among the three groups. Sperms with nuclear vacuoles were the most difficult to eliminate using a combination of DGC and swim-up in all types of head abnormalities. A negative correlation was confirmed between the fertilization rates of intracytoplasmic sperm injection and head abnormality of sperms obtained from the upper layer of the swim-up following DGC. Sperms with DNA fragmentation were effectively decreased using the combination of two techniques. In conclusion, the combination of DGC and swim-up effectively decreased the number of sperms with ultrastructural abnormalities both in the head and in the tail. However, sperms with ultrastructural abnormalities that cannot be completely decreased using a combination of DGC and swim-up may impair fertilization in some cases of intracytoplasmic sperm injection.

Ultrastructure of human gametes, fertilization and embryos in assisted reproduction: a personal survey

This extensively illustrated review will cover the progression of recent research on the ultrastructure of human gametes, fertilization and embryos performed in collaboration with colleagues in In vitro fertilization (IVF) centers over the past three decades, in Australia, Singapore, India, England, Sri Lanka, Spain and Italy. It will also include some aspects of gametogenesis and embryogenesis, particularly in relation to the centrosome that activates embryonic development, and is inherited from the father at fertilization. Assessment of both normal and abnormal gametes and embryos and some clinical aspects of assisted reproduction will be discussed. Full reference will also be made to the contribution of other groups to the ultrastructure of reproduction, particularly in humans.

ICSI: Hype or hazard?

Ever since its introduction in clinical practice more than 10 years ago, intracytoplasmic sperm injection (ICSI) has been the subject of ongoing debate regarding its indications and safety. ICSI is hyped because of its potential to give couples with severe male factor infertility a chance to conceive, and because of its apparently low fertilization failure rate compared with 'classic' in vitro fertilization (IVF). Concerns about ICSI are related to technical, biological and genetic hazards. ICSI has been branded 'the ultimate rape of the oocyte', as the oocyte membrane is mechanically pierced, appearing to bypass all biological and genetic selection. ICSI has been linked in a number of reports to an increased incidence of chromosomal anomalies, congenital abnormalities and perinatal hazards in offspring conceived with this technique. The etiology of the increased risk of chromosomal anomalies in ICSI offspring, especially sex-chromosome anomalies, is thought to be partly multifactorial, partly andrological, related to paternal karyotypic abnormalities and/or abnormal sperm. The majority of studies on ICSI and IVF offspring have, setting aside inconsistencies in methodology and classification, not shown significant differences between the two techniques in terms of congenital abnormalities, however, compared to naturally conceived offspring there does show an increased risk. This risk is attributed mainly to parental factors such as maternal age, poor sperm quality and infertility as an independent risk factor. Perinatal hazards may include low birth weight and perinatal mortality. Behavioural and psychological development is carefully monitored in ICSI and IVF children with no significant differences to the development of naturally conceived children, but many factors are involved including demographics. Follow-up studies are essential to the technique of ICSI. It is our duty to inform patients of the concerns and benefits to this treatment, based on the latest data available.

Application study of intracytoplasmic sperm injection for golden hamster and cattle production

This paper describes several technical improvements and our results in hamster intracytoplasmic sperm injection (ICSI), hamster round spermatid injection (ROSI) and bovine ICSI. The hamster is the mammalian species in which ICSI was first tried to produce fertilized oocytes. However, until recently, no live offspring following ICSI have ever been obtained. We reported the birth of live offspring following hamster ICSI. Improved points to success were 1) performing hamster ICSI in a dark room with a small incandescent lamp and manipulating both oocytes and fertilized eggs under microscope with a red light source and 2) injecting sperm heads without acrosomes. Under controlled illumination, the majority of the oocytes injected with acrosomeless sperm heads were fertilized normally, cleaved, and developed into morulae. Nine live offspring (19%) were born by transfer of hamster ICSI-derived embryos. Furthermore, we reported the birth of live offspring following hamster ROSI. About 70% of oocytes injected with round spermatids broken before injection were fertilized normally and about half of them developed to morulae and blastocysts. Three (5%) live young were born by transfer of hamster ROSI-derived embryos. On the other hand, in cattle, the main improvements were 1) injection of spermatozoa immobilized by scoring their tail just before injection into oocytes, and 2) additional ethanol activation 4 h after ICSI. About 70% of oocytes injected were activated 4 h after ICSI, and about 30% of them developed to blastocysts. Twenty-four live calves (39%) were born by non-surgical transfer of ICSI-derived embryos. Those results shows that, at present, live offspring are able to be obtained following hamster ICSI, ROSI and bovine ICSI, but further improvement is required due to higher production efficiency of offspring.

Intracytoplasmic injection of spermatozoa and spermatogenic cells: its biology and applications in humans and animals

Intracytoplasmic sperm injection (ICSI) has become the method of choice to overcome male infertility when all other forms of assisted fertilization have failed. Animals in which ICSI has produced normal offspring include many species. Success rate with normal spermatozoa is well above 50% in the mouse but ICSI success rates in other animals have been low, ranging from 0.3 to 16.5%. Mouse ICSI revealed that spermatozoa that cannot participate in normal fertilization can produce normal offspring by ICSI, provided their nuclei are genomically intact. Human ICSI using infertile spermatozoa has been highly successful perhaps because of the intrinsic instability of human sperm plasma membrane. The health of children born after ICSI and other assisted fertilization techniques is of major concern. Careful analyses suggest that higher incidences of congenital malformations and/or low birth weights after assisted fertilization are largely attributable to parental genetic background and increased incidence of multiple births, rather than to the techniques of assisted fertilization. Since the physiological and nutritional environments of developing embryos may cause persisting alteration in DNA methylation, extreme caution must be exercised in handling gametes and embryos in vitro. In the mouse, round spermatid injection (ROSI) has been routinely successful but its use in humans is controversial. Whether human ROSI and assisted fertilization involving younger spermatogenic cells are medically safe must be the subject of further investigations.

Centrosomal function assessment in human sperm using heterologous ICSI with rabbit eggs: A new male factor infertility assay

Sperm centrosomal function was assessed by immunocytochemical analysis after the injection of human sperm into mature rabbit eggs. Three hours after intracytoplasmic sperm injection (ICSI), an astral microtubule array from the base of the human sperm was observed in the rabbit eggs. This sperm aster expanded in the egg cytoplasm, concomitant with pronuclear formation, and a dense microtubule array was organized at the time of pronuclear centration. Using fertile donor sperm, the sperm aster formation rate at 3 hr after ICSI was 35.0 +/- 1.5%. Using sperm from infertile patients, the average aster formation rate was lower (25.4 +/- 14.8%, P<0.05). Among infertile cases, there was no correlation between sperm aster formation rates and conventional parameters of semen analysis. However, the sperm aster formation rate correlated with the embryonic cleavage rate following human in vitro fertilization (IVF). These data suggest that this assay reflects sperm function during embryonic development after sperm entry and that reproductive success during the first cell cycle requires a functional sperm centrosome. Furthermore, sperm centrosomal function cannot be predicted from conventional parameters of semen analysis. We propose that insufficient centrosomal function could be the cause of certain cases of idiopathic infertility. These assays may lead to the discovery of new types of infertility, which have previously been treated as "unexplained infertility," and may also lead to the treatment of infertility incurable even by ICSI. Consequently, an accurate and relevant assay to help assure couples of the success of fertilization is warranted, perhaps prior to ICSI therapy.

Interactions of sperm perinuclear theca with the oocyte: implications for oocyte activation, anti-polyspermy defense, and assisted reproduction

Perinuclear theca (PT) is the cytoskeletal coat of mammalian sperm nucleus that is removed from the sperm head at fertilization. PT harbors the sperm borne, oocyte-activating factor (SOAF), a yet-to-be-characterized substance responsible for triggering the signaling cascade of oocyte activation, thought to be dependent on intra-oocyte calcium release. The present article reviews the current knowledge on the biogenesis and molecular composition of sperm PT. Possible functions of sperm PT during natural and assisted fertilization, and in the initiation of embryonic development are discussed. Furthermore, evidence is provided that SOAF is transferred from the sperm PT to oocyte cytoplasm through the internalization and rapid solubilization of the post-acrosomal PT. It is shown that during natural fertilization the sperm PT dissolves in the oocyte cytoplasm concomitantly with sperm nuclear decondensation and the initiation of pronuclear development. SOAF activity is preserved in the differentially extracted sperm heads only if the integrity of PT is maintained. After intracytoplasmic sperm injection (ICSI), activation occurs only in those oocytes in which the injected spermatozoon displays complete or partial dissolution of PT. In the latter case, the residual PT of the sub-acrosomal and/or post-acrosomal sperm region may persist on the apical surface of the sperm nucleus/male pronucleus and may cause a delay or arrest of zygotic development. We propose that the sperm PT harbors SOAF in the post-acrosomal sheath, as this is the first part of the sperm cytosol to enter the oocyte cytoplasm and its disassembly appears sufficient to initiate the early events of oocyte activation. Dissolution of the sub-acrosomal part of the PT, on the other hand, appears necessary to insure complete DNA decondensation in the internalized sperm nucleus and initiate DNA synthesis of both pronuclei. The release of the SOAF from the sperm head into oocyte cytoplasm at fertilization ultimately leads to the activation of oocyte mechanism including the completion of the meiotic cell cycle, pronuclear development and anti-polyspermy defense.

Intracytoplasmic sperm injection: stiletto conception or a stab in the dark

To describe the importance of molecular and cellular analyses in intracytoplasmic sperm injection (ICSI) the authors review the literature on biological challenges in ICSI and associated techniques. Several matters can be proposed in molecular and cellular challenges in ICSI for safety and efficacy: (1) a reliable and convenient animal model for understanding the molecular and cellular basis of human ICSI must be established, and molecular and cellular analysis of the first cell cycle of human fertilization should be better understood; (2) a proper assay for human sperm function that contributes to the indication for ICSI should be developed; and (3) de novo and transmitted genetic security in ICSI should be examined.

Impact of intracytoplasmic sperm injection on the activation and fertilization process of oocytes

Intracytoplasmic sperm injection (ICSI) is used worldwide to treat preferentially severe cases of male factor infertility. In this review, data regarding the processes of oocyte activation and fertilization in non-assisted conception, conventional IVF and ICSI are discussed. The second messenger calcium shows a typical pattern after ICSI, which is different from that after subzonal insemination (SUZI), which is closer to the conditions of normal fertilization. The onset of calcium spikes is delayed. Sometimes a monophasic calcium pattern, in animals typical for parthenogenetic activation, is observed despite normal subsequent oocyte activation. Furthermore, the frequency of spikes is higher after SUZI, and only one phase instead of two is observed after ICSI for the second onset of calcium release. These alterations may be explained by the differences in oocyte activation after ICSI, since no oolemma-sperm contact is present. Sperm decondensation also follows another pattern after ICSI: as long as residuals of the acrosome are present on the sperm head, no sperm decondensation takes place at that site. Therefore, decondensation is delayed and pronucleus formation, especially that of the male pronucleus, takes longer after ICSI as compared with conventional IVF. Since studies have shown that gonosomes are located preferentially in the apical part of the sperm nucleus, this was proposed to be an explanation for a higher incidence of gonosomal aberrations in offspring after ICSI. However, other explanations, taking clinical data like the background risk of the parents into account, can also be offered for this phenomenon. These alternative theories are more likely to be associated with a slight instead of a frank increase in gonosomal aberrations. The inheritance of paternal mitochondrial DNA seems not to be a problem after ICSI, as shown by different studies. Mitochondrial DNA can be demonstrated in embryos after conventional IVF as after ICSI up to the blastocyst stage but not in children born after ICSI. Finally, lesion of the meiotic spindle by the ICSI procedure seems not to be a problem when data from different studies are taken into account. As assumed also at the beginning of the ICSI era, the meiotic spindle is almost always located in an area of < 90 degrees deviation from the polar body axis. Therefore, intrusion of the microinjection needle at the 90 degrees position might not endanger the spindle apparatus. To conclude, several studies using different approaches might show differences in the oocyte activation pattern, the 'choreography of fertilization' and pronucleus formation after ICSI. However, this different pattern does not necessarily mean that ICSI per se is a problem for embryonal development. The different pattern can be explained by the fact that ICSI uses another means of oocyte entry than the normal fertilization process. The clinical data of a high fertilization, cleavage and implantation rate, and especially the data from newborn babies, show that ICSI is a reliable procedure.

Timing and ultrastructure of events following intracytoplasmic sperm injection in a marsupial, the tammar wallaby (Macropus eugenii)

The aim of the present study was to determine the timing of oocyte activation, sperm decondensation and pronucleus formation after intracytoplasmic sperm injection (ICSI) in the tammar wallaby and to determine the fate of sperm structures at an ultrastructural level. Metaphase II-stage tammar wallaby oocytes were injected with spermatozoa and cultured for 1 (n = 15), 2 (n = 24), 4 (n = 30), 6 (n = 14), 8 (n = 32), 10 (n = 25), 12 (n = 29) or 19 h (n = 12). Oocytes were assessed using light, fluorescence and electron microscopy. The timing of oocyte activation and sperm decondensation after ICSI in the tammar wallaby is relatively similar to that of some eutherian species. Resumption of meiosis II was observed from 1 h and the first female pronucleus was seen 6 h after ICSI. Most oocytes (88%) possessed a female pronucleus by 10 h. Intact acrosomes persisted with intact sperm heads up to 2 h after ICSI. At 10 h, 80% of oocytes possessed a male pronucleus. The sperm tail had undergone considerable degeneration by 10 h after ICSI, including breakdown of the fibrous sheath dense fibres. The identification of sperm tail and midpiece remnants adjacent to pronuclei confirms that the events observed in wallaby oocytes after ICSI are not due to parthenogenetic activation.

Intracytoplasmic sperm injection in assisted reproductive technology: an evaluation

Since the first reports of successful pregnancies in humans after treatment with intracytoplasmic sperm injection (ICSI), intensive investigations have focused on several important aspects of this form of assisted reproductive technology. In addition to the technical development of ICSI and increasing understanding of the biochemical and biophysical processes involved during fertilization after injection of an immobilized sperm, studies have aimed to define the indications for patients for a first-line ICSI treatment. One of the major concerns is of course the safety of the technique in terms of the health and reproductive life of the babies born after ICSI. The rhesus monkey is an excellent model to investigate all aspects of this micromanipulation technique. This article provides an evaluation of ICSI.

Should ICSI be the treatment of choice for all cases of in-vitro conception? Considerations of fertilization and embryo development, cost effectiveness and safety

There is now considerable discussion whether intracytoplasmic sperm injection (ICSI) should be used in all cases of IVF. A critical and balanced view of the current literature is presented. The difficult question is how to identify men with apparently normal semen who are likely to fail to achieve a pregnancy using IVF. In conclusion, from both the safety and scientific viewpoint, ICSI should only be used in cases where success at IVF is regarded as unlikely.

Oocyte activation: lessons from human infertility

During fertilization, the spermatozoon penetrates through the cumulus cells and the zona pellucida that surrounds the oocyte, before it binds and fuses with the oocyte plasma membrane to induce activation. In vitro fertilization (IVF) studies performed in non-human mammals have contributed extensive knowledge regarding the mechanisms by which the spermatozoon activates the meiotic-arrested oocyte to resume meiosis, cleave and develop into an embryo. Although IVF has been used extensively for treating subfertile couples, not all of them were able to benefit from this procedure. In intracytoplasmic sperm injection (ICSI), one viable spermatozoon only is sufficient for successful fertilization of a single oocyte. Moreover, the injected fertilizing spermatozoon bypasses several physiological barriers, compared with IVF, which together could explain the high success rate for this procedure. ICSI has also allowed the identification of sperm components that are required for successful fertilization.

ICSI choreography: fate of sperm structures after monospermic rhesus ICSI and first cell cycle implications

We have dissected the initial stages of fertilization by intracytoplasmic sperm injection of single spermatozoa into prime oocytes from fertile rhesus monkeys (Macaca mulatta). DNA decondensation was delayed at the apical portion of the sperm head. It is possible that this asynchronous male DNA decondensation could be related to the persistence of the sperm acrosome and perinuclear theca after injection. However, incomplete male pronuclear formation did not prevent sperm aster formation, microtubule nucleation and pronuclear apposition. In contrast, DNA synthesis was delayed in both pronuclei until the sperm chromatin fully decondensed, indicating that male pronuclear formation constitutes an important checkpoint during the first embryonic cell cycle.

Biological aspects of testicular sperm extraction

Since the first reports of successful pregnancies after treatment with intra cytoplasmic sperm injection (ICSI) in humans, intensive investigations focused on the use of testicular spermatozoa and immature sperm cells in the treatment of azoopsermic patients. Several studies explore the technical development of the preparation, isolation and cryo storage of testicular germ cells. Other studies focus on ICSI itself and try to identify the biochemical and biophysical processes involved in fertilisation after injection of a testicular sperm cell into a human oocyte. Indications for azoospermic patients to whom this first line treatment can be offered are becoming more defined. But one of the major concerns is of course the safety of the technique, especially, for the health and reproductive life of the babies born after application of ICSI with testicular germ cells. An evaluation of ICSI with testicular germ cells is presented in this manuscript.

Atypical decondensation of the sperm nucleus, delayed replication of the male genome, and sex chromosome positioning following intracytoplasmic human sperm injection (ICSI) into golden hamster eggs: does ICSI itself introduce chromosomal anomalies?

OBJECTIVE

To examine nuclear decondensation, positioning of sex chromosomes, and the S-phase in human sperm nuclei following intracytoplasmic sperm injection (ICSI) into hamster eggs.

DESIGN

Prospective analysis of hamster eggs and human sperm following ICSI.

SETTING

Division of Reproductive Sciences, Oregon Health Sciences University and Oregon Regional Primate Research Center.

PATIENT(S)

Fertile donor sperm from a commercial source.

INTERVENTION(S)

Human sperm were examined by immunofluorescence stain, bromodioxyuridine (BrdU) uptake assay and fluorescence in situ hybridization following ICSI into hamster eggs.

MAIN OUTCOME MEASURE(S)

Immunostaining and fluorescence in situ hybridization.

RESULT(S)

Decondensation of human sperm nuclei occurred initially in the basal region, and perinuclear theca of sperm persisted around the condensed apical region. In some sperm nuclei, following ICSI the sex chromosomes were in the apical region, remaining condensed for longer than in the basal region. S-phase entry of human sperm nuclei following ICSI was delayed compared to the zona-free hamster egg penetration assay.

CONCLUSION(S)

These results force questions about the mechanism of male pronuclear formation after ICSI and suggest new strategies for understanding the basis of chromosomal anomalies leading to birth defects as well as continuing improvements in the safety and efficacy of infertility therapies.

SNAREs in mammalian sperm: possible implications for fertilization

Soluble N-ethylmalameide-sensitive factor attachment protein receptor (SNARE) proteins are present in mammalian sperm and could be involved in critical membrane fusion events during fertilization, namely the acrosome reaction. Vesicle-associated membrane protein/synaptobrevin, a SNARE on the membrane of a vesicular carrier, and syntaxin 1, a SNARE on the target membrane, as well as the calcium sensor synaptotagmin I, are present in the acrosome of mammalian sperm (human, rhesus monkey, bull, hamster, mouse). Sperm SNAREs are sloughed off during the acrosome reaction, paralleling the release of sperm membrane vesicles and acrosomal contents, and SNARE antibodies inhibit both the acrosome reaction and fertilization, without inhibiting sperm-egg binding. In addition, sperm SNAREs may be responsible, together with other sperm components, for the asynchronous male DNA decondensation that occurs following intracytoplasmic sperm injection, an assisted reproduction technique that bypasses normal sperm-egg surface interactions. The results suggest the participation of sperm SNAREs during membrane fusion events at fertilization in mammals.

Paternal contributions to the mammalian zygote: fertilization after sperm-egg fusion

Mammalian fertilization has traditionally been regarded as a simple blending of two gametes, during which the haploid genome of the fertilizing spermatozoon constitutes the primary paternal contribution to the resulting embryo. In contrast to this view, new research provides evidence of important cytoplasmic contributions made by the fertilizing spermatozoon to the zygotic makeup, to the organization of preimplantation development, and even reproductive success of new forms of assisted fertilization. The central role of the sperm-contributed centriole in the reconstitution of zygotic centrosome has been established in most mammalian species and is put in contrast with strictly maternal centrosomal inheritance in rodents. The complementary reduction or multiplication of sperm and oocyte organelles during gametogenesis, exemplified by the differences in the biogenesis of centrosome in sperm and oocytes, represents an intriguing mechanism for avoiding their redundancy during early embryogenesis. New studies on perinuclear theca of sperm revealed its importance for both spermatogenesis and fertilization. Remodeling of the sperm chromatin into a male pronucleus is guided by oocyte-produced, reducing peptide glutathione and a number of molecules required for the reconstitution of the functional nuclear envelope and nuclear skeleton. Although some of the sperm structures are transformed into zygotic components, the elimination of others is vital to early stages of embryonic development. Sperm mitochondria, carrying potentially harmful paternal mtDNA, appear to be eliminated by a ubiquitin-dependent mechanism. Other accessory structures of the sperm axoneme, including fibrous sheath, microtubule doublets, outer dense fibers, and the striated columns of connecting piece, are discarded in an orderly fashion. The new methods of assisted fertilization, represented by intracytoplasmic sperm injection and round spermatid injection, bypass multiple steps of natural fertilization by introducing an intact spermatozoon or spermatogenic cell into oocyte cytoplasm. Consequently, the carryover of sperm accessory structures that would normally be eliminated before or during the entry of sperm into oocyte cytoplasm persist therein and may interfere with early embryonic development, thus decreasing the success rate of assisted fertilization and possibly causing severe embryonic anomalies. Similarly, foreign organelles, proteins, messenger RNAs, and mitochondrial DNAs, which may have a profound impact on the embryonic development, are propagated by the nuclear transfer of embryonic blastomeres and somatic cell nuclei. This aspect of assisted fertilization is yet to be explored by a focused effort.

Light and electron microscopic analysis of human testicular spermatozoa and spermatids from frozen and thawed testicular biopsies

The morphological changes caused by freezing and thawing human testicular spermatozoa have been assessed here. Retrieval of testicular biopsies was carried out on six patients with obstructive azoospermia preparatory to intracytoplasmic sperm injection (ICSI). Light microscope analysis was carried out on testicular cells and ultrastructural analysis was carried out on spermatozoa and different spermatid stages before and after the freezing procedure. Upon examination under light microscopy, all germ cells presented increased vacuolization in their cytoplasm and shrinkage or swelling of the nuclei and cytoplasmic membranes. These altered structures were accentuated in the spermatocyte I cell which often presented disrupted membranes. The ultrastructural findings under transmission electron microscopy demonstrated that after freezing and thawing the major types of cryoinjury were the swelling and rupture of inner and outer acrosomal and plasma membranes. The acrosome material often appeared as dispersed material or as condensed spots or was even lost. Such damage was observed mainly at the spermatozoa and late spermatid stages. We conclude that the freezing and thawing of testicular biopsies causes similar morphological damage to testicular spermatozoa and frozen-thawed ejaculated spermatozoa. It is still unclear whether these changes in testicular spermatozoa after freezing and thawing may compromise its use in the ICSI procedure.