BEAUTY’S ONLY CELL-DEEP: How the embryologist selects the best embryo for transfer and the new technology that will enhance this process.

It has been 3 decades since the first generation of courageous and visionary patients used the revolutionary technique of In Vitro Fertilization (IVF) to achieve pregnancy. Virtually everything in assisted reproductivetechnology has evolved significantly and dramatically since the birth of Louise Brown in 1978, and every change has been sought with two objectives in mind: to maximize the chances of a patient’s getting pregnant,and to make the process easier and more accessible for patients.  

For instance, in the earliest days, egg retrieval required a trip to the operating room for a laparoscopy undergeneral anesthesia. And, many cycles had to be cancelled when patients experienced an endogenous LH surge, causing them to ovulate ahead of the scheduled egg retrieval. Finally, without ICSI, there was no solution to infertility for many men with low sperm concentrations or low motility, and the concept of using testicular sperm was still years away from consideration.

One of the earliest innovations to IVF was the introduction of controlled ovarian hyperstimulation. The administration of gonadotropins made the collection of multiple eggs possible, and consequently, there were often multiple embryos from which to choose for transfer to the uterus. Indeed, there was a time when transferring three or four cleavage stage (2 to 8- cells) embryos was routine even in young patients.  

Now, with the development and integration into practice of so many innovations in both the clinical and  laboratory aspects of IVF, the transfer of a single embryo can often be prescribed for the patient, with the promise of the same high pregnancy rate once sought by transferring twice or more as many embryos.

Selection of the “best” embryo, that is, the embryo that is most likely to implant and establish the pregnancy, is built on a time-honored foundation, but is being improved and honed using some emerging and very interesting tools. The Reproductive Science Center of New England is proud that we are involvedin the appraisal of such a new tool. As a background, it may be helpful to understand  what has been the traditional paradigm for selecting embryos for intra-uterine transfer

Many embryologists evaluate the quality of an embryo using three basic parameters: the stage of development of the embryo, or number of cells it contains; the symmetry in size and shape among the cells, and the amount of fragments within the embryo. On day 2 of development (two days after retrieval and one day after fertilization of the egg has been confirmed), an embryo should be at the 2 to 4-cell stage and on day 3, at the 6 to 8-cell stage. The constituent cells, or blastomeres, should be evenly shaped and sized, and there should be a minimum of fragments within the embryo. Fragments are pieces or bits of cytoplasm that may be generated during cell division and high degrees of fragmentation may reflect poor developmental potential. Despite the array of embryo morphology grading systems among IVF laboratories, it is likely that these factors are always considered.

To further refine the process of embryo selection, embryologists considered other developmental factors in addition to these. For instance, the time of first cell division and arrangement of intra-nuclear structures called nucleoli during confirmation of fertilization (called the Z-score) have been proposed as indicators ofembryo quality.

With the design of media for the culture of eggs and embryos at specific stages of their development, the nurturing of embryos beyond day 3 became possible for many laboratories, and with this, the opportunity to implement another technique for selecting the embryo with highest implantation potential. If maintained in culture beyond day 3, some embryos will acquire the morula stage at day 4, and then a distinctive stage at day 5, when the embryo becomes a blastocyst. Not all embryos are capable of reaching this stage, undergoing arrest owing to genetic or other cellular deficiencies within the egg or embryo, or arising from the sperm cell which fertilized the egg.

By allowing the entire group or cohort of a patient’s embryos to continue development through day 5, one or two blastocysts, depending on the patient’s age or other indications for treatment, can be selected for transfer, offering the assurance offered by the sustained advancement and growth. Some IVF clinics use the
day 5 approach exclusively, while others use a flexible approach to day 3 or day 5 transfer.

Whether day 3 or day 5 embryos are chosen for transfer, the selection process is still largely made on the basis of morphology, or, how each embryo looks under microscopic inspection.  

Exciting innovations to how embryos are selected are in development and emerging into clinical application. Embryologists will soon be able to harness some of the technological advancements made in other areas of cell biology.  

One of these is called metabolomics. “Omics” refers to a field or area of knowledge and metabol-omics refers to knowledge of a cell’s metabolic activity. The theory, and hope, is that by measuring an embryo’s metabolic activity, and comparing it to that of embryos known to have implanted and created a pregnancy,  the embryologist will be able to identify with a higher probability than using morphology alone, the embryo with the maximal implantation potential. This ability should lead to higher pregnancy rates and the increased transfer of only a single embryo during IVF treatment to help the patient avoid a multiple
gestation.  
 
One of the advantages of metabolic assessment is that is it not invasive, meaning, it does require the embryo itself to be assessed through biopsy, for instance. Instead, the culture medium in which the embryo was grown is sampled and evaluated using scientific instrumentation to detect changes in the levels of multiple
metabolites. It is hypothesized that embryos with high implantation potential can be distinguished from embryos with low implantation potential on the basis of comparing their metabolomic profiles.  
 
Another example of “omics” applied to clinical embryology is genomics and proteomics. In these cases, the expression of genes, as measured by molecules that are coded for by those genes, is evaluated.  
 
The future promises to be bright with the addition of these tools to the IVF laboratory once the techniques are customized to single cells (eggs and sperm) and embryos, validated, and ultimately introduced to the embryology laboratory. With the application of these bio-informatic methods, we hope that our patients will
glean the ultimate benefit of maximally high pregnancy rates through their IVF therapy.

********  
Kathryn J. Go, Ph.D. joined the Reproductive Science Center in 2006 after serving as the Director of the IVF Laboratory at Pennsylvania Hospital/Pennsylvania Reproductive Associates, Philadelphia, PA for 21 years.  Dr. Go received her Bachelor’s degree in Biology and Doctor of Philosophy in Molecular Biology from the University of Pennsylvania and completed a post- doctoral fellowship in the Department of Physiology and Biochemistry at the Medical College of Pennsylvania.  Board-certified as a high complexity laboratory director, Dr. Go has served two terms on the Executive Council of the Society of Assisted Reproductive Technology (SART) of the American Society of Reproductive Medicine (ASRM), was a member of the Reproductive Biology Resource Committee of SART and the College of American Pathologists, consultant to the ASRM Office of Government Affairs,  Chair of the Reproductive Biology Professional Group of the ASRM, and was on the faculties of the University of Pennsylvania and Thomas Jefferson University.  Dr. Go is currently President of the New England Fertility Society, has an avid interest in the assisted reproductive technologies and the pursuit and implementation of new techniques in the laboratory to improve patient outcomes, and is the author of articles on quality control and clinical findings, as well as reviews on novel research areas in reproductive
biology.