Gamete and embryo selection: the Holy Grail of embryology

embryo selection

Published 19 November 2018

A wide ranging and very well attended Campus meeting on the assessment of gametes and embryos for IVF took place in November. The course covered the morphological and functional assessment of embryos, PGT‐A and mitochondrial DNA analysis, embryo culture, and emerging approaches to gamete and embryo selection.

The bottom line of this well attended Campus meeting staged by the SIG Embryology in November was that there's little that embryology can do to improve the quality of the gametes and embryos available for IVF. The many methods of assessment, therefore, reviewed in detail at this meeting, have proved their worth in selection; the better that selection, the more likely a healthy pregnancy (and a shorter time to pregnancy too). Hence the 'Holy Grail' of the IVF lab: the identification of the embryo with the greatest potential for successful implantation.

Not that implantation per se is any longer the ultimate aim of IVF. Denny Sakkas, Scientific Director of Boston IVF, proposed that the definition of 'success' in IVF is not just a healthy baby, nor even a healthy adult, but a healthy 'future generation'. 'Selecting the best embryo will be more than just picking a viable embryo,' he warned. And of course it's the viability of that selected embryo which makes single embryo transfer more and more likely, and the avoidance of IVF risks in multiple pregnancy.

Of the many technologies reviewed at this meeting only time-lapse imaging held out any promise (beyond the exclusion and 'deselection' of embryos) of improving IVF outcome (or even embryo quality) through its consistent culture conditions: a small incubation chamber, regulated temperature and minimal disturbance, especially with single‐step culture media. So, asked Thomas Freour from the University Hospital of Nantes, can time-lapse imaging improve embryo culture conditions ('Yes'), improve embryo quality assessment ('Yes'), improve clinical outcome ('Probably helps') and improve time to pregnancy ('Probably yes', though adding that 'no existing studies unequivocally and definitely prove that time-lapse improves clinical outcomes'). However, Freour persuasively reported that a 'growing body of evidence' shows that time-lapse does provide optimal culture conditions and thereby may help the selection of those embryos with high implantation potential (the Holy Grail!). Time-lapse, he added, also allows the exclusion of embryos with extremely poor implantation chances.

Several studies, described by Cristina Albanese from the SISMER Clinic in Bologna, also suggest (though not demonstrate) the potential of comprehensive aneuploidy screening technologies to improve implantation rates. She cited a recent review of SART PGT-A data showing that implantation rates 'hover around 50%' irrespective of maternal age, but without PGT decrease after the age of 35. There was a suggestion from this SART review that PGT-A might have the ability to reduce the effect of age in IVF. However, as has been the case for the past 20 years, unscrambling these many results to a consistent conclusion has proved highly controversial, not least because of mosaicism in the embryos biopsied and the possibility of self-correction as the embryo progresses towards the blastocyst.

Mosaicism was one 'headache' of PGT identified by Barcelona embryologist, and former ESHRE Chairman, Anna Veiga. There seems some consensus in the view that PGT-A will minimise miscarriage rate and possibly reduce time to pregnancy (though not increase live birth rate). Postponing biopsy from the cleavage to the blastocyst stage (ie, from one blastomere to several trophectoderm cells) has been shown in several trials (including one from Veiga's own group) to improve efficiency considerably, with fewer biopsies necessary (because of extended culture) and a higher rate of euploid embryos. The much cited study by Scott et al of 2013 showed sustained implantation rates of just 30% following day 3 biopsy (versus 50% in unbiopsied controls), but a 50% rate of sustained implantation for biopsied (and control) blastocysts. However, the presence of mosaicism not only raises the possibility of distorted results, but the possibility too that viable embryos are discarded. With evidence from several groups now suggesting that some moderately mosaic embryos have the ability to form viable pregnancies, recent policy statements have not opposed the transfer of mosaic blastocysts, while giving priority to those deemed euploid.

Polar body analysis would remove the consideration of mosaicism, as explained by Karen Sermon in her review of the recently completed ESTEEM trial - but again these results emphatically showed no benefit in delivery rates, though a welcome reduction in miscarriage rates.

And 'non-invasive' methods of selection would remove the need for biopsy. Denny Sakkas reviewed 'omic' tests of embryo metabolism and, citing the work of Gardner et al of 2012, proposed that an assessment of nutrient consumption (glucose) could predict viability and even live birth. Other studies with non-invasive lifetime imaging microscopy had also indicated embryo potential just hours after fertilisation. Similarly, novel proteomic tests have been shown to identify biomarkers expressed by the proteome of euploid (and not aneuploid) blastocysts.

Sakkas also considered the efficacy of emerging non-invasive tests of embryo ploidy, first by analysing cell-free DNA in used culture medium. Both he and his fellow speaker Elpida Fragouli described the technique as 'promising', but with still many questions to be answered. And, as Fragouli warned, even the transfer of a euploid embryo does not guarantee implantation. This was evident in her own work on measuring mitochondrial DNA in which blastocysts deemed to be euploid but with a mitochondrial load over a threshold of around 30% all failed to implant in studies from her group. It was also noteworthy that just a few weeks before this Campus meeting a report in Fertilty & Sterility from two centres in Rome concluded on the basis of a prospective blind comparison that two non-invasive methods (genetic analysis of blastocoel fluid and spent media) do not yet provide sufficiently reliable results for clinical application. Sakkas too concluded that as yet these non-invasive methods could not reach the level of predictability of the more traditional invasive tests of aneuploidy - nor of course of conventional morphology.

It was interesting too to hear the views of andrologist Jackson Kirkman-Brown, co-ordinator of ESHRE's SIG Andrology, who underlined the aim of emerging sperm selection techniques as 'to waste none of the eggs we have'. Among these techniques he emphasised sperm selection according to motility evident in the female reproductive tract. High viscosity mucus, he said, 'is very strong at selecting sperm', and better sperm will make a better embryo. And because sperm swim along surfaces and have a propensity for 'corners', there is here an implicit model for sperm election according to motility. All such tests, however, are experimental, and the first priority is still 'to optimise the man'.