Embryology Glossary

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Oocyte/Egg: female gamete cell provided by the ovary.

• MII Oocyte/Egg: “Mature” egg stage characterized by the display of the Polar Body (PB) in the Perivitelline space. These oocytes are ready to perform ICSI or Vitrification.


The following egg stages, we cannot perform ICSI or cryopreserve them and they will be discarded.

• GV Oocyte/Egg: immature stage which presents an intracytoplasmic nucleus called the ‘Germinal Vesicle’. This type of eggs is not used in the ICSI process since they are not nuclear nor cytoplasmatic matured.
• MI Oocyte/Egg: immature stage of the eggs that have neither a visible GV nor a Polar Body (PB). It is in the process of maturation, but still, the polar body is not extruded, thus the ICSI cannot be performed.
• EZ: “Empty Zona” when there is no cytoplasm within the egg, this phenomenon is rare, but it can happen and shows that some cells tried to create an oocyte but failed in the creation of cytoplasm.
• BZ: “Broken Zona” when the Zona pellucida “shell” is broken, meaning there is no egg inside of it. The reason is attributed to the poor quality of the egg.
• DEG: it is the Degenerated form of the oocyte/egg in which the cell has undergone apoptosis.
• Parthenogenetic Egg: This egg has already started the mitotic divisions by itself without use of sperm. These embryos need to be discarded.

What to Expect on Day 1 - Fertilization Check:

DAY 1 = Zygote stage

• 2PN: This means Normal fertilization and is assessed by the presence of two Pronuclei (PN) and two Polar bodies (PB). One PN comes from the mother and the other from the father. This stage is meant to continue the culture and see if they continue to the next mitotic divisions.

The following eggs/embryo stages are not optimal to see during the fertilization check. However, we keep them in culture to monitor future cell division and see if they reach the blastocyst stage. It is highly recommended to perform PGT-a testing on these embryos to confirm their euploidy, as we could not observe normal fertilization of the 2PN.
• PB: This stage shows a Polar Bodies (PB) but no Pronuclei (PN). This could occur due to the accelerated/late fusion of the Pronuclei (PN). Also, it may be a lack of fertilization. This stage is kept under culture to see if they divide. PGT-a is highly recommended to do in these embryos to confirm the euploidy of it.
• 1PN: When the fertilized egg shows just one Pronuclei (PN), it can result from errors in the fertilization process with asynchrony in PN formation or PN fusion. This type of fertilization is kept in culture to see if they divide. PGT-a is highly recommended to do in these embryos to confirm the euploidy of it.
• 2 Cell: This is an embryo stage of two cells that could be derived from an accelerated fusion of the Pronuclei and division of the zygote. In this situation, the embryos can continue their development and see if they reach the blastocyst stage. PGT-a is highly recommended to do in these embryos to confirm the euploidy of it.

The following eggs or embryos are discarded because they are not compatible with life if fertilized or due to degradation.

3PN: It is abnormal fertilization and known as a Triploid zygote due to the presence of three Pronuclei (3PN), which differs in origin when fails in extrusion of the second Polar Body (digenic) or two spermatozoon entry into the oocyte (diandry) if conventional insemination was performed.
>3PNs: It is abnormal fertilization where more than 3 Pronuclei are shown, and the causes can come from failed cytokinesis (cell division) or binucleate spermatozoon. In this kind of fertilization and polyploidy found, the genetic abnormalities are higher and not compatible with life.
DEG: Some fertilized eggs (zygotes) after ICSI can undergo degeneration. In this case, the Degraded eggs must be discarded. Most of the cases can be related to poor egg quality.

What to expect on day 5:

The ideal stage for day five is to have the blastocyst or at least the early stages of it. Between 45-55% of the embryos that showed a 2PN fertilization on day one reach the blastocyst stage. This stage can be reached between days five, six, or day seven.
The earlier an embryo reaches the blastocyst stage, its chances of successful implantation are higher. However, the blastocyst grade and euploidy also influence the implantation rate. For instance, a blastocyst on day 5 has a better implantation rate than one on day 6, which is better than one on day 7. It is crucial to consider these factors when evaluating the likelihood of successful implantation.

On your embryology report for day five, you can have the following stages.