Selection and Deselection Morphokinetic Criteria
In addition to enabling detailed, time-stamped monitoring of the morphokinetics of embryo development, time-lapse technology also allows the identification of aberrant embryo cleavage events, which may not be observable using “snapshot” traditional static microscopy methods. Potentially the most valuable example of aberrant cleavage, and one of the most well reported in terms of its correlation with viability, is the phenomenon of a multiple, as opposed to dichotomous, cleavage of a blastomere to three or more daughter cells. Several studies have demonstrated the ability of time-lapse to identify such aberrant cleavage divisions and highlight the reduced implantation potential of these embryos, compared with their counterparts that mitotically divide into two daughter cells. In a cohort of 1659 transferred embryos, the incidence of this “direct division” (occurring within 5 hours) was 13.7%. and the implantation rate of these embryos was markedly and statistically significantly lower than for embryos w'ith a normal cleavage pattern (1.2% vs. 20.2%, respectively) . Some investigations into this phenomenon have considered whether this erroneous cleavage phenomenon could be the cause or effect of aneuploidy within the cell . Since the first reports of this phenomenon, the nomenclature of this common example of early erroneous cell division has been proposed in an expert panel publication. An aberrant cleavage from a single cell directly to three daughter cells has been defined as trichotomous mitosis. This can be distinguished from “rapid" cleavages, which appear as an accelerated division event within 5 hours of the previous cell division . The hypotheses of Leese et al., that preimplantation embryos that are “quiet” metabolically have greater viability than those that are not, have recently been revisited using retrospective and prospective data on metabolic and kinetic activity of preimplantation embryo development. The group considered that there may be optimal ranges that may influence embryogenesis. They proposed that these may be a “Goldilocks zone,” within which embryos with maximum developmental potential can be categorized . This would sit well with what is being revealed from the increasing number of published time-lapse studies. Recently, researchers have searched for novel indicators of embryo viability, only accurately recorded using time-lapse imaging. A large unpublished study at CARE Fertility Group (Nottingham, UK) examined close to 2500 blastocysts with known implantation outcomes. Unpublished logistic regression analysis demonstrated a significantly higher clinical pregnancy rate (48.9% vs. 35.9%; p < 0.0001) when morulae were fully compacted, with no excluded material, compared to their counterparts.
Early adopters of time-lapse and its implementation for embryo selection were Basile, Meseguer, and colleagues, who used a hierarchical approach to time-lapse algorithm development whereby embryos received a classification based primarily on reaching developmental milestones and the relative timings associated with them . They reported significant differences between implanted and not implanted embryos for six early morphokinetic variables. The most significant, for implantation prediction, being the time that the embryo reached the five-cell stage (t5), and the duration of the second cell cycle (cc2). The resulting selection model also included three exclusion criteria, based on their negative association with implantation. These were rapid early division (from two to three cells), uneven blastomeres, and multinucleation at the four-cell stage. Since then, further time-lapse models have been published with variation in the time-lapse variables of significance, outcome measures, and the timings of specific developmental events [18-20].