Micromanipulation as a Clinical Tool ppt

14 Micromanipulation as a Clinical Tool Jacques Cohen Tyho-Galileo Research Laboratories and Reprogenetics, West Orange, New Jersey, U.S.A. INTRODUCTION Micromanipulation involves a well-integrated set of technologies in assisted reproductive technology (ART). Its applications are diagnostic as well as therapeutic, and it is practiced in mature gametes and all stages of preimplantation embryos. It is used in biopsy for preimplantation genetic diagnosis (PGD), intra-cytoplasmic sperm injection (ICSI), assisted hatching, and in other more controversial areas such as egg freezing via zygote reconstitution, cryopreservation of isolated testicular spermatozoa, and cytoplasmi c or mitochondrial transfer for reversal of cytoplasmic and potentially nuclear incompetence (1–6). In this context, it becomes increasingly difficult to dis- cuss micromanipulation as a separate subject. In fact, the need for a separate assessment appears almost artificial. When different fields merge in science, exciting developments can be expected. This has occurred numerous times in ART, first in the integration of biochemistry and reproductive endocrinology, and later when cryobiol- ogy and applied genetics emerged as tools to improve efficiency and safety. No reproductive specialist could have predicted that the field of experimental micromanipulation would ha ve such an enormous impact on assisted reproduction less than two decades after the first relatively simple but elegant applications appeared (1–3) . Since then, hundreds of thousands of 283 babies have been born worldwide from micromanipulative methods aimed at alleviating male infertility and enhancing implantation and the exclusion of chromosomal and single gene disorders. Some laboratories now have three or more complete stations for micromanipulation. Rather than having some embryologists sub-specialize in the area of micromanipulation, the practice in some laboratories, most embryologists now aim to become pro- ficient in one or more micromanipulation techniques. The main emphasis here will not be already integrated procedures such as ICSI and in part embryo and polar body biopsy, which are covered in depth by other chapters of this book, but on other innovative techniques. The different procedures and concepts will be discussed in two sections; the first will focus on gamete micromanipulation and the second will deal with the manipulation of embryos. It is also important to note that the topics discussed in this chapter are often considered controversial or unpro- ven and are seen by some as hazardous to clinical care and even the human species at large. No review would be complete if it does not refer to the alternative opinion. A recent 2004 opinion by Cummins gives a very different perspective of some of the technologies described below (7). GAMETE MICROMANIPULATION Cryopreservation of Spermatozoa Under Zonae Men who are azoospermic can be successfully treated through surgical iso- lation of spermatozoa from their testicles or reproductive tract (8–10). Repeated surgical procedures, however, may not only be costly but invasive, especially in the case of testicular sperm extraction (11). Repetition can, in some cases, be avoided by normal cryopreservation of spermatozoa, but only when sufficient numbers of functional cells are isolated (12,13) Freezing methods are now available that can freeze and recover very few or even single spermatozoa, and avoid the need for repeated surgical sperm extraction (4,14,15). Single sperm can be frozen by insertion of spermatozoa into animal or human evacuated zonae pellucidae or through variations of this method such as freezing in cryoloops. Recovery rates of spermatozoa frozen and thawed in evacuated human or an imal zonae are high, with motility recovery rates in excess of 75% (4,15). In standard freezing protocols, centrifugation is essential. But in this protocol, washing can be accomplished by individually pipetting and removing the cryoprotectant. In addition to reducing the need for repeated sperm retrievals and perhaps donor sperm, this approach has the advantage of avoiding the uncertain outcome of surgical extraction by freezing spermatozoa and retrieving eggs at diff erent times (4,16). The time-consuming search for spermatozoa can be conducted independent of an egg retrieval. 284 Cohen Details of Procedure and Choice of Technical Details Micromanipulation can be performed using polyvinyl pyrrolidone (PVP) as a tool to slow the insertion of spermatozoa into the zonae and to withdraw spermatozoa from the zonae after thawing. Two different solutions of PVP are recommended: (i) an 8 to 10% solution for sperm capture and insertion into empty zonae (this is produced by a number of manufacturers with vary- ing results) and (ii) a 10 to 12% solution for sperm recovery from the thawed zonae. The ICSI procedures using thaw ed spermatozoa should be performed at 37  C, but all other micromanipulations can be performed at room temperature in order to reduce sperm velocity and perhaps prolong survival. The microtools needed for zona opening, avoidance of zona collapse, cell extraction, and ICSI have been described (3,4). The pilot experiments were conducted using spermatozoa from surgical retrievals and spermatozoa not cryopreserved from men with normal semen analysis to test the fertilizing ability of donated research oocytes by ICSI. Human-evacuated zonae can be obtained from multiple sources: immature eggs, unfertilized ICSI eggs, and abnormal embryos that were not exposed to sperm suspensions. It was shown that two small incis ions in the zonae improved extraction of the egg ooplasm and insertion of the sperm cells into the evacuated zonae by preventing the collapse of the zona during suction and excessive inflation. With gained experience, a single-hole technique may be effective as well. At first, holes were made chemically in some pre-fertilization zonae by releasing acidified Tyrode’s solution from a 10 mm open microneedle. However, progressively motile spermatozoa escaped, resulting in poor recovery rates. This can be avoided by cutting a hole in the zona mechanically using partial zona dissection with a spear-shaped closed microneedle. Alternatively, one can use a laser-mediated opening of the zona pellucida, but the efficiency of this needs to be shown in clinical trials (17). Cytoplasm can be extracted using a larger micropipette, connected in turn to a suction device. The zona is positioned so that one of the two incisions is at the 3 o’clock position. The beveled microtool is inserted through the aperture using the sharp edge on the lower end. The tool is moved through the oolemma, and the cytoplasm is fully aspirated until the zona is empty. The pipette is occasionally emptied outside the zonae as more medium is sucked up to remove any sticky cytoplasm from the pipette tip. Mouse and hamster zonae as well as human eggs and embryos can also be prepared in the same fashion. Spermatozoa can be released into the 10% PVP solution prior to insertion into empty zonae. They are individually taken from small 2- to 5-mL droplets of sperm suspension using an ICSI microtool. Spermatozoa can be injected into the empty zona while motile or can be immobilized before Micromanipulation as a Clinical Tool 285 injection and freezing (17). In the latter case, the spermatozoa remain viable at a high frequency, but fertilization and pregnancy have not yet been demonstrated. Considering the absence of clinical data, motile spermatozoa are recommended for this purpose. The use of human zonae, while appropriate for creating strict xeno-free conditions, is problematic as spermatozoa attach to the inside of the zona pellucida and may immobilize before freez - ing. Recently, we developed and tested a combinatorially derived ligand pieczenik peptide sequence 2 from an artificial target that specifically binds to human zonae pellucidae (18). When injected inside evacuated human zonae, this ligand efficiently prevented sperm attachment by possibly inter- fering with sperm/zona pellucida (ZP3) interaction. Whether this ligand could be used for the purpose of freezing spermatozoa in evacuated zonae and maintaining high motility requires further evaluation. Injected zonae can be frozen in a simple 8% glycerol solution using a phosphate-buffered solution supplemented with 3% human serum albumin. Alternatively, one can use buffer containing TES (N-tris[hydroxymethy]- methyl-2-aminoethanesufonic acid) and Tris (Tris[hydroxymethyl]amino- methane) yolk buffer, but the recovery rate is not superior compared to earlier studies (1 6). The zonae can be frozen s ingly in sta ndard plastic straws between two small air bubbles to indicate their position. One end of the straw is closed using sealant polyvinyl alcohol powder, whereas the other end is heat sealed. The freezing procedure is based on a simple standard semen cryopreservation protocol (4 ). Thawed straws can be inserted into a medium droplet as the cryopreservation medium containing the zona slowly releases. The zona is washed to remove cryoprotectant and moved into an ICSI dish containing droplets of N-2-Hydroxyethylpiperazine-N 0 -2-Ethanesulfonic acid (HEPES) buffered medium and a central droplet of 10 to 12% PVP in supplemented intra- cellular solution. Some sperm cells may show considerable motility within the zona. The zona is positioned using the holding pipette and a PVP-filled ICSI microtool so one incision and a sperm cell line up. This allows for the penetration of the ICSI needle and aspiration of the sperm cell (mechanical recovery method). Minimum suction is used for this process. These cells can be aspirated by positioning the needle at the contralateral side and applying suction when the sperm cell passes the needle aperture. All spermatozoa are removed and released gently in the PVP solution. Any still motile can then be immobilized for ICSI. The mechanical sperm recovery method is preferred, and other methods involving zona digestion have proved less successful. Recovered sperm are washed and immediately injected into eggs; thaws should be planned accordingly. Zonae are rarely lost with this method as they are still heavy enough to drop to the bottom of a dish when the straw’s contents are released. The number of sperm lost through holes after thawing ranges from 2 to 30% and is depen- dent on the technique (4,15,16). Spermatozoa are not lost before freezing 286 Cohen through the narrow mechanical PZD incisions, but some may be lost when using acidified Tyrode’s solution or laser opening. Loss after thawing through narrow incisions may also occur because of inadvertent excess suction applied through the holding pipette. This can be avoided by visualizing both holes prior to micromanipulation and sperm recovery. The rate of sperm loss through the incisions diminishes markedly with increased operator experience. Motility Recovery and General Efficiency Motility recovery rates are defined as the percentage of motile cells seen, and vary between 73 and 100%. Rates of recovery over 80% are found in experiments involving rodent zonae and human zonae that are embryonic in origin, or spermatozoa that are immobilized prior to freezing. Some cells that are motile without marked velocity become progressively more motile after aspiration and exposure to medium. Some spermatozoa inserted into human zonae may become caught up in cytoplasmic remnants or crevices inside the glycoprotein matrix, but this occurs less frequently when animal zonae are used. Aggregation of spermatozoa also appears to inhibit individ- ual motility, but is avoided when three or less spermatozoa are inserted per zona. The use of only one to three sperm cells per zona pellucida appears optimal, yet insertion of up to 15 spermatozoa has been reported (16). General Considerations of Single Sperm Freezing Although single sperm freezing is performed with spermatozoa from men with extreme oligospermia, average motility recovery rates are considerably higher than those generally reported for moderately abnormal semen and are comparable to results of donor semen or moderately oligospermic patients treated with a combination of cryoseeds and dithiothreitol (DTT) (19–21). The use of the zona pellucida or perhaps cryoloops as a vehicle avoids the known loss in motility associated with post-thaw dilution and sperm washing seen in frozen donor semen. It is possible that the presence of multiple sperm in a small volume exerts an internally deleterious effect during freezing, thawing, and centrifugation, which may be avoided by freezing sperm singly or in small groups. This method makes it feasible to perform surgical extractions independently from the time and place of egg retrieval. Another distinct advantage is that animal zonae, such as those from mice and hamsters, can be used for storage. This application has been rejected by some because of the desire to culture under xeno-free conditions. Although it is immunologically unlikely that there will be any cross-species hazards or reactive compounds, it is possible that compounds may adhere to the sperm cell and become incorporated in the oocyte. Whether these have any metabolic effects on the embryo has yet to be clarified. Nevertheless, the absence of reverse transcriptase appears evident, thereby rigorously reducing the possibility of any process resembling transgenesis. Micromanipulation as a Clinical Tool 287 The use of combinatorially derived ligands as described above for inhi- biting sperm–egg binding in the homologous application may be preferred, but should be tested in pre-clinical models first. Clinical Application The experimental procedure was first tested in an azoospermic patient who required testicular biopsy and had scant motile cells (22). Extensive searching for sperm yielded enough spermatozoa for ICSI. Other motile spermatozoa (n ¼ 23) were isolated and trapped into two evacuated zonae derived from the patient’s immature eggs. The patient did not become pregnant and the couple returned for another ICSI attempt, but this time without sperm retrieval by testicular biopsy. One zona with 12 trapped spermatozoa was thawed first, washed, and prepared for sperm extraction. All but one sperm could be recovered. Ten were motile, and five were injected into the patient’s mature eggs four hours after egg retrieval. The patient delivered healthy twins. A total of 104 procedures have been performed, and 52 of them returned for transfer. Of these, 49 had transfers and 26 became clinically pregnant. The implantation rate was nearly 25%. There were three miscar- riages and two biochemical pregnancies. Although a small data set, the work is encouraging but awaits confirmation by other teams. Ooplasmic Transplantation Transplantation of ooplasm for clinical application involves a set of experimental techniques designed to address oocyte-specific deficits in a defined and limited group of infertile patients who previously failed ART (5,23). The first clinical application was based on oocyte donation, although the use of homologous ooplasm or mitochondria from the patient’s cumulus cells has also been suggested (24–26). The latter proposal was first made by Tzeng’s team at Taipei Medical University in Taiwan and this has led to the birth of more than 20 babies. There are a number of ways to trans- plant ooplasm, but the one initially used was a minor modification of the standard clinical ICSI technique. A small portion of ooplasm was trans- ferred from a donor to the patient’s oocytes. Other groups have reported on the experimental application of modified versions of ooplasmic transfer, including the use of cryopreserved donor oocytes, polyspermic zygotes, and cumulus cell mitochondrial suspensions as the source of the donor infusion (24,27,28). In the work performed from 1997 to 2001 at Saint Barnabas Medical Center’s Institute for Reproductive Medicine and Science, two instances of abnormal karyotypes were reported following the application of ooplasmic transfer (both 45, XO). One resulted in an early spontaneous miscarriage and the second in an elective reduction at 15 weeks following an abnormal ultrasonography (29). A single case of pervasive developmental disorder (a spectrum of autism-related diagnoses which has an incidence of one in 250 children) was also diagnosed at 18 months in a ooplasmic 288 Cohen transfer infant, a boy from a mixed sex twin. Other centers applying ooplasmic transplantation have not reported any potential side effects. This may be because there were no side effects or the studies were incomplete. Due to the very small sample size, any direct connection between the abnormali- ties and the technique itself has yet to be established (30). In 2001, we voluntarily agreed to cease clinical application of ooplasmic transfer pending the application and review of an investigative new drug (IND) protocol with the U.S. Food and Drug administration. The pre-IND process was concluded in 2003, but we did not pursue the application further for non-clinical reasons. The proce dure was not prohibited as Schultz and Williams (31) and others have suggested. It is currently unknown whether other groups have applied for a permit. The clinical application an d sci ence o f o oplasmic transplantation hav e been extensively criticized by ethicists and scientists (7,31–33). Some careless articles of the lay pre ss, fueled by some scientists, have likened th e technique to gene tr a nsfer and described a detrimental artificial s cenario likely to change the genetics of mankind. Several recent publications have presented viewpoints based on a poor understanding of t he issu es. T he deb ate sh ould b e bas ed o n a factual understanding of the procedure and the clinical and scientific realities involved. The technique’s underlying experimental nature must be stressed and the many scientific and clinical ‘‘unkno wns’’ that surround it must be presented. Ooplasmic Transfer Procedures Preliminary clinical investigations applying the technique to a defined group of patients with ‘‘normal’’ ovarian reserve who exhibit consistent developmental problems and implantation failure have been published (29,34). The final 27 couples treated had exhibited a record of failure in over 95 prior assisted reproduction treatment cycles. Unlike what some crit ics have suggested, this is an unavoidable confounding factor in attempting to design and conduct true controlled trials (33,35). In this case, the ‘‘control’’ treatment is already consistently known to result in failure and the cause is not unknown since embryo morphology was clearly marginal in all these cases. The 43% pregnancy rate and the delivery of 17 babies following ooplasmic transfer was charact erized solely on this prior failure criteria. The issue of controlled trials is obviously complex, and to suggest that this is a simple deficit that we have failed to address is incorrect. There is a considerable ethical issue in forcing patients to engage in treatment modalities that offer them no hope of success for the sake of these trials. In a subsequent study, patients with diminished ovarian reserve were treated unsuccessfully with cytoplasmic transfer (36). It is known that in these patients most eggs are aneuploid, a condition that is irreversible at the MII stage (37). Ooplasmic transfer was conceived as a simple but critical extension of the standard egg donation protocol that is a clinical option available for patients. It theoretically provides for a beneficial donor egg while Micromanipulation as a Clinical Tool 289 maintaining the patient’s genetic contribution. The technique is suitable for eggs ‘‘having a normal nuclear genome, but ooplasm that is abnormal or defi- cient due to maternally mediated factors’’ (38). Recently, several authors have incorrectly represented our publications as suggesting that ooplasmic transfer is based on a correction of deficits in mitochondrial function or ATP content (32,33). One publication goes so far as to raise this purported ATP deficit as a ‘‘straw man’’ argument and, in rejecting it, raises the question that ooplasmic transfer might not be ‘‘biologically plausible.’’ We have clearly stated in our publications that there are a multitude of causative factors potentially underlying ooplasmic-related developmental deficits. Energy metabolism is certainly one of these factors. However, the entire concept of ooplasmic transfer is to infusethepotentiallycompromised patientoocytewithawholesourceofhealthy donor ooplasm. We have never suggested that a manipulation of ooplasmic ATP or any specific factor underlies any positive effect of ooplasmic transfer. Others have suggested that mitochondria transfer from isolated cumulus cells would be a safer alternative, but we reject the direct comparison based on the arguments outlined here (24,25). Mitochondria transfer has been considered sub-optimal compared to ooplasmic transfer in one artificial mouse model (39). It is likely that a subset of infertile patients exhibit reproductive dysfunction derived from oocyte-related deficits and that replacing compro- mised oocytes with donor substitutes is not only ‘‘biologically plausible’’ but also a technique compatible with early development and pregnancy (40). Mitochondrial Issues An area of controversy concerns the transfer and persistence of donor mitochondria following ooplasmic transfer. The transfer of heterologous mitochondria was not detectable in the first ooplasmic transfer cases, and therefore initial reports and discussions reflected this (38). The oosplasmic transfer protocol has included an analysis of mitochondrial DNA and, in subsequent treatment cycles, donor mitochondrial DNA was identified in pre- and postnatal samples derived from ooplasmic transfer offspring. To date, donor mitochondrial DNA has been positively identified in three of the 13 tested ooplasmic transfer babies (34). This suggests a heteroplasmic condition with two populations of mitochondria (donor and recipient) present. However, current scientific evidence does not support the concept that ooplasmic transfer-related heteroplasmy constitutes a potentially deleterious condition. The only heteroplasmy that has been observed in a small subset of ooplasmic transfer patients is in the form of benign polymorphisms in the non-coding hyper-variable region of the otherwise highly conserved mitochondrial genome (34). This form of heteroplasmy is now known to be a common phenomenon in the n ormal human population a nd may not have an associ- ation with mitochondrial disease or dysfunction (41). Indeed, this form of benign heteroplasmy may have little significance. Many mammalian species are routinely heteroplasmic in the replication control region, and this 290 Cohen heteroplasmy may be conserved and transmitted from generation to generation (42,43). This benign form of heteroplasmy is different from that associated with deleteriouscodingregionmutationsrelatedto‘‘agingandininheritedmitochon- drial disease’’ as incorrectly suggested in one critical opinion (33). Selection of young and healthy oocyte donors providing gametes for ooplasmic transfer is based on the same criteria as standard oocyte donation. Ooplasmic transfer patients have no unique risk of the transmission of such rare deleterious mitochondrial DNA mutations as whole oocyte donation is the only other clinical option available for them. Suggestions that ooplasmic transfer donors should be uniquely screened for mitochondrial mutations are more logically directed at oocyte donation where 100% of the mitochondrial genome is derived from the donor (32). No incidence of mitochondrial disease transmission has been reported over 10 of 1000 of oocyte donation cycles, although it is expected that 1 of 8000 children will develop mitochondrial disease de novo. A substantial body of animal research has been concerned with the manipulative creation of heteroplasmy in mice and large animals [reviewed by Malter and Cohen (29)]. Although the opinion piece by St John (32) men- tions this research, it fails to point out the underlying fact that much of this research is based on the efficient generation of hundreds of healthy heteroplasmic animals that have been produced through cytoplasmic transfer and maintained over 15 generations with no obvious developmental or physiological problems. From a genetic standpoint, many of these experiments are also based on a much more drastic heteroplasmic scenario as the mixed mitochondrial populations are essentially derived from two ancestrally different species. While one should not place tremendous confidence in modeling a complex phenomenon through research animals, in this case such research has demonstrated that a much more extreme heteroplasmic condition than would be possible through clinical ooplasmic transfer is compatible with normal mammalian development. Other basic research suggests considerable flexibility in nuclear/mitochondrial interaction, particularly among primates. In the type of cellular hybrid experiments also discussed by St John (32), chimp and gorilla mitochondria could readily replace human mitochondria (a cross-genus ‘‘mismatch’’), and create fully functional cells with human nuclear genomes and non-human primate mitochondria exhibit- ing unremarkable mitochondrial protein synthesis and function (44). Although the specific nature of the heteroplasmy observed in a small fraction of ooplasmic transfer offspring is not fully understood, an honest review of current research in this area does not suggest a potential for negative developmental or physiological outcomes. This obviously does not mean that future patients should not be informed of the uncertainties that remain. Epigenetic Aspects and Animal Models Ooplasmic transfer, by design, generates a recipient oocyte that contains donor-derived components such as proteins, messenger RNAs, mitochondria, Micromanipulation as a Clinical Tool 291 and other cytoplasmic constituents. In theory, this infusion of healthy donor components can have a positive effect on important ooplasmic functions during early development. However, a recent opinion piece by Hawes et al. suggests a potential for adverse developmental outcomes resulting from the simple creation of a mixed ooplasmic state (35). This argument is based on abnormal developmental syndromes that have been identified in unique inbred mouse strains [reviewed by Malter and Cohen (29)]. These syndromes result from genetic incompatibilities between inbred strains apparently manifested via unique epigenetic events in the cytoplasm of their oocytes and early embryos. Studies with these inbred strains have been critical to understanding the early cytoplasmic genome modification events that create the mature functional embryonic genome (29). The epigenetic processing that occurs during this period is necessary for proper development. This supports the concept that positive developmental effects could be obtained by a moderate infusion of healthy ooplasm, although other outcomes, including negative effects, are plausible as well. The scenarios and experimental manipulations involved with the manifestation of these aberrant developmental outcomes are also essentially compatible with normal development in the mouse (and other species) outside of these experiments. In our own research, similar manipul ation to the early cytoplasm of F1 hybrid mouse embryos resulted in a significant improvement in certain developmental parameters compared to non-manipulated controls [Levron et al. (6)]. It has been suggested that similar alleleic combinations for these unique incompatible murine gene products could be present in the human, yet there is no proof of this. The aberrant developmental outcomes observed in the inbred mouse strains are unique epigenetic anomalies with clear genetic causes (45,46). These specific deleterious anomalies are unknown outside of the unique inbred combinations used and any other animal model system. Inbred mice are genetically anomalous strains considered to be homozygous at all loci. They manifest a great variety of adverse developmental and physiological conditions including reduced fertility (29). Even between related inbred strains, drastic differences in morphology, physiology, and behavior make drawing cross-strain conclusions questionable. Such strains do not constitute valid models for complex processes (particularly those related to fertility and development) in other mammalian species. Furthermore, many differences between the developmental processes in humans and other mammalian species have been well characterized, including a recent finding which demon- strates clear differences in the methylation-based imprinting system (47). In fact, the authors of this study go so far as to suggest that concerns about the aberrant epigenetic-processing (and resulting developmental effects) observed after in vitro manipulation in other mammalian species and model systems may simply not apply to the human. 292 Cohen [...]... applications is that only the efficacy has been questioned and safety has been evaluated in terms of pregnancy rates The results reported appear promising, apparently demonstrating simple, repeatable, and appropriate zona ablation with no obvious detrimental effects, at least none that are reported (72,73) In some studies, implantation and clinical pregnancy rate may have been Micromanipulation as a. .. needle as soon a break-through of the zona occurs Assisted Hatching with Laser The non-contact infrared laser has emerged as the methodology perhaps best suited to mammalian zona-cutting applications (70) Several commercial systems are now available with Food and Drug Administration (FDA) permits using IR diode lasers, and these have been put to use in human clinical embryology procedures such as assisted... is assumed that assisted Micromanipulation as a Clinical Tool 295 hatching is not widely practiced based on the fact that there are only about 250 scientific papers regarding this topic in the literature, in sharp contrast with, for instance, the ICSI literature Whereas there is a broad consensus about efficacy regarding the latter, this is clearly not the case with assisted hatching A large proportion... done gently and patiently Continuous changes of focusing adjustment on target material and adjacent blastomeres are necessary because fragments are not all in the same plane as the pipettetip and the artificial opening The zona should be turned after removal of some fragments, as a different angle increases the likelihood that more fragments can be removed Fragments in between blastomeres and those opposite... Micromanipulation as a Clinical Tool 301 increased following laser-mediated assisted hatching and a relatively large group of healthy babies have now been born The positive results and efficacy reported for laser-based zona cutting have been intriguing; however, basic safety studies have been rare and those that exist (71) are rarely documented by researchers investigating clinical applications Time will... extraction In this respect and using our model, implantation rates could drop as low as 14% (again presuming that the expected rate was 20%) Interestingly, in two recently published clinical trials by the Brussels team, implantation rates in the PGD for aneuploidy and control groups were similar, indicating that the genetic diagnosis Micromanipulation as a Clinical Tool 305 had made up for the initial... concern among lawmakers and ethicists alike, especially as it involves the tools enabling scientists to perform nuclear transplantation Laws were passed in haste in several countries banning cloning after Dolly’s birth, but a few more contemplative lawmakers pointed at the potential benefits that cloning technology could foster and assumed that further evaluation was warranted Yet to date, many countries... produced for assisted hatching Similarly, no monozygotic twins have been found after transferring blastocysts from which the zona pellucida was removed (Coughlin Wagner and Maravilla, Highland Park, Chicago, personal communication) Though our practice indicates that assisted hatching improves implantation, controversy continues to impede its widespread and dependable application with as many studies... of a hole that is nearly rectangular The inside layer of the zona pellucida is frequently more resistant to reduced pH than the outer layers Care should be taken, therefore, to assure that zona breakthrough occurs over a sufficiently wide area of at least 20 mm, and not at a single small point As soon as the zona is breached, the flow through the assisted hatching needle must be immediately reversed All... extremists Micromanipulation as a Clinical Tool 307 ACKNOWLEDGMENTS ´ I am grateful to Santiago Munne, John Garrisi, Mina Alikani, Tim Schimmel, Steen Willadsen, and Henry Malter for countless contributions I am thankful for editorial comments by Rashmi Dalai REFERENCES 1 Handyside AH, Kontogianni EH, Hardy K, Winston RML Pregnancies from biopsied human preimplantation embryos sex by Y-specific DNA amplification . than 20 babies. There are a number of ways to trans- plant ooplasm, but the one initially used was a minor modification of the standard clinical ICSI technique. A small portion of ooplasm was. the hatching needle as soon a break-through of the zona occurs. Assisted Hatching with Laser The non-contact infrared laser has emerged as the methodology perhaps best suited to mammalian zona-cutting. In some studies, implantation and clinical pregnancy rate may have been 300 Cohen increased following laser-mediated assisted hatching and a relatively large group of healthy babies have now been born.

Định dạng
Số trang	30
Dung lượng	222,93 KB