Embryo, embryo - definition

According to the Infertility Treatment Act, an embryo or human embryo is a group of cells resulting from the extracorporeal fusion of a female and a male reproductive cell (egg and sperm), from the completion of the fusion of the germ cell nuclei (karyogamy) until implantation in the uterine mucosa.

An indispensable part of the procedure in vitro is the incubation and evaluation of embryos by embryologists. It is they who, based on careful observation of embryo development and morphology, decide which embryos can be used for transfer. This is why it is so important to choose the right infertility treatment clinic. So that every step of the treatment is handled by experienced embryology specialists.

Where to look for information on embryos?

InviMed clinic patients are informed about the development of the embryos needed for transfer by the attending physician, who provides them with detailed embryological data if necessary. At the end of the cycle, patients receive an IVF discharge card with a summary. This includes embryological data on:

oocyte retrieval (divided into immature oocytes - MI, GV, mature oocytes - MII and degenerated oocytes),
concentration, motility and semen volume,
the in vitro fertilisation technique used,
oocyte fertilisation (with oocyte division due to the presence of PN pre-nuclei),
the number of oocytes frozen and embryos frozen,
embryo development (figures on e.g. number of embryos transferred, frozen, rejected),
transfer (date of transfer and number, stage and age of embryos),
supporting techniques and genetic testing.

Extended medical records can be obtained from the InviMed clinic upon request. Due to the special protection of medical data, also embryological information regarding the puncture, methods, embryo culture process and the materials and media used can only be collected in person at the clinic where the procedure was carried out.

What do the embryo classes tell us about?

The class of a given embryo determines its chances of further development, successful implantation and, consequently, the birth of a child. - Embryos are classified on the basis of their external (morphological) structure and rate of development (morphodynamics) on a given day. The result of the classification is also a testimony to their quality. An embryo showing poor quality, has very little potential for further development and consequently implantation, therefore the success of the IVF procedure is largely influenced by the knowledge and experience of the embryologist - explains Robert Szachoń, head of the InviMed laboratory in Katowice.

Classification of embryos and their markings

Only mature oocytes, i.e. oocytes at the MII stage, i.e. in metaphase II of division, are used for in vitro fertilisation. The entry of an oocyte into the MII stage is evidenced by the presence of the first directional corpuscle. If six oocytes have been successfully retrieved during the LP and each oocyte is at stage MII - all of them will be fertilised. With the moment of fertilisation, i.e. the introduction of the sperm into the oocyte, the incubation and observation of the embryo by the embryology team begins. Depending on the day of culture, the different stages of embryo development are assessed:

Day 1: assessment of oocyte fertilisation approximately 17 hours after the sperm enters the oocyte. At this stage, we observe the appearance of markers indicating correct (or incorrect) fertilisation of the ovum. These are the so-called PN pre-nuclei (pronucleus):

1 PN
2 PN - correctly fertilised ovum on the first day
3 PN
>3 PN

In addition to determining the presence of PN pre-nuclei, the way in which the precursor bodies are distributed in the zygote is also assessed. On this basis, a correctly fertilised 2PN oocyte is classified as:

Class 1 - symmetrical distribution of forewings, highest rating,
Class 2 - asymmetrical distribution of pre-nuclei and precursor bodies,
Grade 3 - asymmetrical pre-nuclei, with one or complete absence of precursor bodies, lowest grade.

In addition, after approximately 26 hours, the so-called first division is assessed - this indicates the proper transformation of the zygote into an embryo.

Day 2: assessment of embryo morphology at approximately 44 hours after the procedure. The embryo should be at the 4 blastomere stage.

Day 3: assessment of embryo morphology at approximately 68 hours after the procedure. The embryo should be at the 8 blastomere stage.

The evaluation of embryos from day 2 to day 3 is further extended by an assessment of the degree of fragmentation. The assessment uses 3 classes indicating the degree of fragmentation present:

Class 1 - mild fragmentation representing less than 10% of embryo volume - the highest embryo class means the highest prognosis,
Class 2 - medium fragmentation representing between 10% and 25% of embryo volume,
Class 3 - high fragmentation representing more than 25% of embryo volume - the lowest embryo class means the lowest prognosis.

Only those embryos with a normal rate of development for the day of culture and a score of 1-2 are eligible for transfer. Embryos in score 3 cannot be transferred or frozen.

Day 4: assessment of embryo morphology at approximately 92 hours after the procedure. The embryo should have reached the morula stage.

Day 5: assessment of embryo morphology at approximately 116 hours after the procedure. The embryo should have reached the blastocyst stage.

Assessment of blastocyst stage

On days 5 and 6 of culture, embryos should reach the blastocyst stage. The assessment of the blastocyst is already more complicated and is based on the nomenclature developed by Gardner (Gardner and Schoolcraft, 1999).

On days 5 and 6 it is assessed :

The stage of development of a given blastocyst:

Grade 1: early blastocyst,
Grade 2: blastocyst,
Grade 3: dilated blastocyst,
Grade 4: hatching blastocyst.

In addition, embryologists assess each of these structures in terms of two parameters:

Trophoblast (TE) A,B,C. It is responsible for the formation of the placenta and the nutrition of the embryo.
The embryonic node - the embryoblast (ICM) A,B,C. It is from this that the embryo proper is formed. It consists of pluripotent stem cells, which give rise to all the cells of the body.

The assessment of blastocysts scores A, B, C. The highest potential blastocysts are those scoring AA, AB, BA, BB regardless of the stage of development measured on a scale of 1 to 4. A score of C in any of the above structures (TE, ICM) indicates a low potential blastocyst that should not be transferred and subjected to vitrification (rapid freezing).

All assessments of promising blastocysts at Day 5 are shown in the table below:

-Transfer takes place on day 3, 5 or 6 of embryo culture if the patient's clinical condition allows it. If there are medical reasons that do not allow the transfer to take place, embryos of the appropriate grade are vitrified, and the transfer takes place in a frozen cycle. Recent scientific evidence supports not transferring embryos in a fresh cycle - that is, the cycle in which cell collection and fertilisation occurred. Performing transfers in a frozen cycle increases the percentage of clinical pregnancies, concludes embryologist Robert Szachoń.

As you can see, the embryo evaluation procedure is extremely complex and complicated. This is why it is so important that it is only handled by experienced embryology specialists. InviMed clinics employ the best experts to ensure that embryo evaluation is carried out in a correct and structured manner.

Embryological consultation

Once you have the IVF discharge sheet or the embryo culture run sheet in front of you, you can always ask the attending doctor about its interpretation. In most cases, however, the data on the sheet is already fully understandable, because the information about the embryos and their class designations is presented to the couple on the day of transfer or at previous appointments. If you have any doubts, the doctor from the InviMed clinic will answer all your questions and dispel your doubts.

InviMed Infertility Treatment Clinics also allow their patients to meet free of charge with an embryologist, who also helps them to understand the more complicated, extended embryology documentation. This is where you can expect to find embryo classification data.

***

If you have additional questions for the embryologist, please add them in a comment to this article or send them to InviMed in a private message on Facebook - based on the questions collected, we will prepare an interview with our expert. Remember, when adding a comment, you do not have to give your name, it can be just your first name or nickname.