Oocytes

The morphology of the oocyte can be described by its subcellular phenotypic constituents: the zona pellucida, the perivitelline space, the polar bodies and the cytoplasm. There are infinite variations of these structures and the best way to describe them is through micrographic pictures.

Zona pellucida (ZP)

The thickness of the ZP may vary from <15 µm to > 20 µm. It may also present a septum or a dark color.

Perivitelline space (PVS)

The perivitelline space (PVS) may be large or nearly absent. It may contain fragments.

First polar body (PB1)

PB1 is considered as normal when it is oval and intact, with a smooth or rough surface. Fragmented or giant PB is considered as abnormal.

Cytoplasmic inclusions and clusters

• vacuoles: fluid-filled refringent round cavities. They may be isolated or numerous.
• refractile body: inclusions of different sizes containing lipofucsin
• endoplasmic reticulum (SER) cluster: appears as a flat elliptical disk in the centre of the ooplasm
• dark cluster: a cluster of organelles that appears as a dark area in the ooplasm

Cytoplasmic granularity gradients

A granulated area is frequently observed in the animal (PB1 region) or vegetal poles (opposite to the animal pole) but also in a meridional region extending from the vegetal to the animal pole.

Parameters of the EQC test

The 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 convenient 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 thinner ZP than others. It is usually a characteristic 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 aspects
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

Perivitelline space

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

 

Parameters	Morphological aspects
Small	the ooplasma is adherent to the ZP and the perivitelline space is almost inexistent
Large	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 appears when the first meiotic division has been completed. It consists of a small cell within the PVS, close to the oocyte nucleus, and may undergo a second division. It usually rapidly degenerates and appears as a fragmented structure.

 

Parameters	Morphological aspects
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 achieve size of about half of that of an oocyte

 

Cytoplasm

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 aspects
Fragmented	the cytoplasm shows signs of fragmentation or appear as an undefined mass with absent or degraded plasma membrane
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 aspects
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 autofluorescing lipofucsin (appear yellowish)
None	absence of the above-mentioned structures