Silvia Azaña, an embryologist at inviTRA, explains the characteristics of a normal and abnormal egg in this video:
Determining egg quality is not a simple task. Nevertheless, during IVF treatment, it is possible to evaluate the morphology of the eggs through a microscope. However, these structural alterations that can be observed under the microscope do not indicate or provide information initially about whether the egg has chromosomal alterations or not.
In any case, let's take a closer look at what the appearance of a normal oocyte would be like and what types of usite dysmorphisms might possibly be found during its evaluation.
A normal oocyte must properly exhibit a rounded shape and, in addition, have all of its structures clearly defined:
A homogeneous cytoplasm without any foreign bodies that attract attention inside it.
There should be one polar body in the perivitelline space, which is slightly flattened and contains homogeneous material.
A perivitelline space that is barely noticeable, only in the area where the polar body is located.
And a uniform zona pellucida of adequate thickness.
The presence of the first polar body indicates that the egg is ready for fertilization. These eggs are said to be in metaphase II.
As for the alterations, the ones we might encounter in the cytoplasm of the oocyte are the following:
A granular content.
Refractory bodies.
Necrotic bodies,
Smooth endopplasmic reticulum and vacuoles.
And cluster, a granular and dark area.
Conversely, the changes that can occur in the usite related to the polar body include the following points:
Multiple polar corpusles. In other words, there is more than one.
A fragmented polar corpusle.
And an amorphous or very flattened polar corpusle.Regarding the perivitelline space, it is the area found between the zona pellucida and the oocyte itself. This should barely be noticeable, only hinted at in the area where the polar body is located. Therefore, the alterations related to the perivitelline space are as follows:
A wide perivelline space or with irregular areas which are both wide and narrow.
Presence of debris in the perivelline space.In conclusion, the zona pellucida is the outer layer enveloping the ovum. This should have a proportional thickness of approximately 15 to 20 microns.
The alterations that could be found in the zona palucida of an usite would be the following:
Amplitude too thick or narrow.Lax or overly dense pollucid zone.
Irregular zona pellucida, with areas thinner than others.
Partitioned zona pellucida, forming like an independent section.
And a long, clear zone.
These dysmorphisms can make it difficult for the eggs to be fertilized. Nonetheless, using intracytoplasmic sperm injection (ICSI) could allow for their fertilization. It is also possible that thickened, elongated or partitioned zona pellucida may present significant problems for embryo hatching. This occurs when the embryo, in the blastoyst stage, detaches itself from the zona pellucida to implant in the endometrium. In such cases, the embryologist would evaluate the option of assisted hatching.
