Qualification system for oocytes – what does it look like?
We all know that egg quality has a major impact on the overall health and viability of the embryo. What most patients don’t know, however, is what exactly makes an egg “good”. In order to understand the complexities of grading oocytes, we must understand a few basic facts about their origins, their biology and what exactly goes on inside the egg.
Luckily, three IVF experts agreed to explain the methods and criteria they use to evaluate eggs and we present it in 3on1 #IVFANSWERS.
Our experts are:
- Dr Harry Karpouzis, Pelargos IVF, Greece
- Dr Minerva Ferrer, Crea Assisted Reproduction Medical Center, Spain
- Dr Renata Huttelova, IVF Cube, Czech Republic
Answer from Dr Karpouzis
The oocyte grading system allows us to evaluate the eggs retrieved during a collection. They aren’t used as often as embryo grading systems, and the reason for that has to do with what happens to embryos on day three. On day three of embryo development, the effects of the male genetic material on the embryo are fully visible; thus, even the best eggs can still be greatly affected by the malefactor.
Despite that, there is research that shows that the quality of the oocyte can indicate its chances for fertilisation; oocytes displaying proper morphological qualities are more likely to undergo successful fertilisation when compared to abnormal eggs. Additionally, implantation rates and pregnancy rates have been shown in studies to be affected by the quality of the egg.
The first step in grading an oocyte’s quality is simple – we simply examine them under a microscope and judge its visual qualities – the colour, the size and shape, the morphology; all of these factors play an important role in evaluating egg quality. Then, we look inside the oocyte and examine its cytoplasm – whether there are any vacuoles or granules, which are indications of poor egg quality. We also look at the zona pellucida, which surrounds the oocyte and helps with fertilisation; later on, it helps the embryo implant. Its thickness is an important factor in evaluating egg quality. Following that, we take a look at the polar bodies, which are the initial nuclei of the oocyte before fertilisation.
We also evaluate the perivitelline space – the space between the oocyte and the zona pellucida. Finally, we judge the size of the oocyte – ideal eggs must be within very specific dimensions. The combination of all of these factors gives us a specific grading which evaluates whether an egg is okay to fertilise.
Answer from Dr Ferrer
Upon egg retrieval, the IVF facility begins the work of evaluating their quality. At this stage, mature oocytes are separated from immature ones – the latter usually making up 20% of a single cycle’s retrieval. Once mature oocytes are isolated, morphological examinations begin – first, the eggs are judged by their size, as large oocytes are commonly associated with chromosomal abnormalities. Following that, eggs are screened for the presence of dark cytoplasm, vacuoles or other aggregates within, as all of these are indicators of abnormal oocyte development.
Answer from Dr Huttelova
When oocytes are retrieved, we start by evaluating the status of their cumulus cells and their expansion. Fully mature oocytes are surrounded by several layers of expanded cumulus cells – the stage of their expansion and the number of cells give us information about the quality of the oocyte within. Unexpanded and compacted cumulus oophorus usually surround immature eggs, which are not ready to be fertilized; those oocytes are then discarded.
In case of an ICSI treatment, the cumulur oophorus is removed, allowing us to examine the nuclear maturity and the morphology of the oocyte. Mature oocytes should be in metaphase II and display polar bodies within their perivitelline space.
Some eggs can show a nuclear structure called the germinal vesicle in their cytoplasm – in that case, polar bodies are not visible. Approximately 5% of all retrieved eggs have neither visible polar bodies nor germinal vesicles. These oocytes are called metaphase I oocytes. Oocytes in the germinal vesicle stage or in metaphase I are immature and not ready for fertilisation.