How to grade an oocyte?

O_07_256-2The EQC test will present you several possible answers for the various characteristics of an oocyte. You can make only one choice. Although you may not be used to grade the oocytes in your day-to-day activity, grading is a convienient way of communicating oocyte characteristics both to colleagues and medical staff. In our view, this is an important parameter to monitor in an External Quality Control scheme.

Zona pellucida

The zona pellucida (ZP) may vary in thickness and rigidity. Some patients have clearly thiner ZP than others. It is usually a characteristics associated with the patient more than with the oocyte. Although thickness may vary slightly around the oocyte, the following categories can be defined.

Parameters          Morphological aspect
Thin the ZP is less than 15 µm
Thick the ZP is greater or equal to 25 µm
Normal the ZP is around 20 µm (15-25)
Abnormal the ZP is quite irregular both in thickness and in shape

Periviteline space

The periviteline space surrounds the ooplasm. It is generally clearly visible, at least in the vicinity of the polar bodies. It may contain fragments.

Parameters                  Morphological aspect
Small the ooplasma is adherent to the ZP and the periviteline space is almsot inexistent
Large the the ooplasma is clearly retraced away from the ZP leaving a clearly identifiable space all around the cytoplasm
With fragments presence of clearly identifiable fragments
Normal the ooplasm is slightly retracted away from the ZP at least in the vicinity of the polar body

Polar body

The polar body appear when the first meiotic division has been completed. It appears as an small cell close to the oocyte and may undergo a second division. It usually rapidly degenerates and appears as a fragmented structure:

Parameters                            Morphological aspect
Unclear or absent the polar body may not always be clearly identifiable, depending on how the oocyte is oriented. It is absent in a metaphase I oocyte (before the first meiotic division)
Intact clearly identifiable smooth round/oval cell lying close to the oocytes
Fragmented the polar body material is irregular in shape and show clear signs of disorganization
Giant usually intact, giant polar bodies a clearly recognizable, they may achive size of about half of that of an oocyte

Cytoplasmic texture

The cytoplasm of the oocyte exhibits usually a granular texture. Depending on the oocyte, this texture may appear homogeneous or exhibit a gradient in the granulated texture. In some cases, it may contain a dark cluster and exhibit sign of fragmentation.

Parameters                         Morphological aspect
Fragmented the cytoplasm shows signs of fragmentation or appear as an undefined mass with absent or degraded oolema
Homogeneous the cytoplasmic material appears homogeneously distributed with little signs of granulation
Polarised zones more or less granulated are visible as two opposing hemispheres
Dark cluster a dark zone is clearly identifiable inside the cytoplasm

Inclusions

The cytoplasm may contain several remarkable structures.

Parameters                    Morphological aspect
Vacuole one large or several small vacuoles bounded with a clearly identifiable membrane
SER cluster smooth endoplasmic reticulum appearing as clear spherical zones inside the cytoplasm. The boundaries of these structures may not be easily identified as in the case of vacuoles
Refractile body Refractile bodies constitute one of the main morphological abnormalities in human oocytes and are made of auto fluorescing lipofucsin (appear yellowish)
None absence of the above mentioned structures