The language of embryology is evolving – a guide to understanding time-lapse nomenclature

New assessment methods call for new assessment measures. Having a common nomenclature can form the basis for clearer communication of embryo assessment and evaluation. The nomenclature described in this blog post is what we have found to facilitate ease of agreement and understanding between time-lapse users.

The need for a common nomenclature

With the introduction of time-lapse monitoring of embryos into a clinical setting comes the gain of new levels of information about embryo development. This allows for more precise assessments due to continuous monitoring and hence other ways of grading embryos.

The advanced precision with which embryo development can be assessed with time-lapse means that new parameters are being assessed in clinics utilising time-lapse technology. This introduces a new terminology in the language of embryology because traditional nomenclature does not adequately cover what can be observed with time-lapse.

As for any terminology, agreement on term definition and assessment is important. All users within a clinic – or a clinic chain – need to use the terminology in the same way and aligned understanding and application should be assured within these settings. Such consistency is an important step on the way to gain the full benefits of time-lapse technology and also for relying on the data that result from annotations, analysis and evaluation.

Also outside the clinic or the clinic chain we should all be speaking the same language when it comes to embryology with time-lapse. This will ease communication on time-lapse experiences across clinics and facilitate true comparison and discussion of results and workflow in the community. This was pointed out by Ciray et al in 2014¹ who also suggested a terminology for time-lapse assessment.

Nomenclature should reflect what is practical and at the same time should help the user gain informative insights into embryo development during culture and subsequently.

My colleagues and I have worked with time-lapse technology - and as importantly, time-lapse users – for many years. Based on our experience we have set up guides that we find support the consistent use and user-oriented workflow of embryo assessment with time-lapse. Parameters that are frequently annotated are described below. “Annotating” refers to the process of registering each of these parameters individually, which leads to a value and a time-stamp for each registered parameter in a database.

Morphokinetics - Morphology at a given time

For assessment of specific timings (“t”) of embryonic stages until the 8-cell stage we suggest to annotate those at the first image frame for which this stage is observed.

The variables most used in this part of development are the time of syngamy (“tPNf”) as well as annotation of cell cleavages (“t2”, “t3”, “t4” etc.).  tPNf denotes the visual impression of syngamy, ProNuclear fading, and therefore should be annotated at the first image in which both pronuclei have faded. For cleavage we recommend annotating each specific timing at the first image frame at which the true separation of cell membranes is observed.

The reason for choosing this approach is that always annotating specific timings at the first image frame in which a developmental step is observed is the most distinct, objective and reproducible sign of the step.

For assessment of specific timings of later stages, one has to take into account that development to these stages occur as gradual processes. Therefore, the nomenclature used has to define at which distinctive feature during the gradual process each parameter should be annotated. We suggest the following definitions which all require observing the sequence of development in the proximity of the parameter in question:

Time of morula (tM): the first image in which a compacted morula includes all the blastomeres that will later take part in the formation of the blastocyst. This solves the question of how to handle partial compactions as excluded cells can be accounted for.

Time of starting blastulation (tSB): the first time that a sign of cavity formation is observed. As the blastocoel cavity grows during blastulation, going back in the image sequence from a definite blastocyst stage can be helpful to attain this annotation.

Time of full blastocyst (tB): the last image before expansion starts. This is recognised as the last image before the zona pellucida is pushed by the growing blastocyst. This is a very distinct hallmark during blastocyst development and therefore easy to annotate precisely and consistently.

Time of expanding blastocyst (tEB): blastocyst expansion can go on for several hours and therefore a defined characteristic during this process is necessary to obtain accuracy during embryo analysis. Importantly, this should be informative on another level than previous parameters as otherwise annotation would be dispensable. Therefore, we characterize tEB as the time at which the blastocyst has expanded so much that the zona pellucida has reached half of its original thickness, which can be measured and thus represents a truly objective assessment.

Time of hatching blastocyst (tHB): the first image at which a sign of hatching is observed.

Dynamic morphology

The embryo is constantly developing. Therefore morphology is not static and observation of its dynamics facilitates a more accurate impression of embryo morphology.

When utilising the time-lapse sequence at the stage at which a morphologic feature should be annotated, changes in morphology can be observed and accounted for in the annotation. One example is the assessment of the nuclear status of blastomeres.In order to accurately assess this parameter it should be noted that observation too close in time to a cleavage often obscures the impression of nuclear status as nuclear membranes are disintegrated during blastomere cleavage.

The same holds true for fragmentation and relative blastomere sizes, parameters that slightly change in their appearance from the immediate time after cleavage to the middle of the cell cycle of a given cleavage stage.

Deepen your knowledge of time-lapse and annotations

If you follow our blog you know that time-lapse is a popular topic. My colleauge, Dr. Silversand, has previously written about how you can benefit from time-lapse technology in your lab. Here you also have the opportunity to download a white paper on the same topic. 

The use of annotations varies between clinics. We asked time-lapse users how they work with annotations and results from the survey are summarised in this blog post.

My colleauge Reidun Kuhlman explains the benefits of consistent annotation in her blog post. The final reading suggestion is a blog post about how working with annotations can be more convenient with the use of a desicion support tool. 

REF

1. Ciray, H.N., et al., Proposed guidelines on the nomenclature and annotation of dynamic human embryo monitoring by a time-lapse user group. Hum Reprod, 2014. 29(12): p. 2650-60.