How to grade a zygote?

14The EQConline test will give you several possible answers fot the various characteristics of a 2 pronuclear zygote. You can make only one choice.
Although you may not be used to grade the zygotes in your day-to-day activity, grading is a convienient way of communicating zygote characteristics both to colleagues and medical staff. In our view, it this is an important parameter to monitor in an External Quality Control scheme.


PN position

The position of the pronuclei (PN) change over time. They appear usually at the periphery where the spermatozoon entered into the oocyte. The aster created by the sperm centriole will bring the female and male pronuclei together and the two will the migrate together towards the center of the oocyte, where the events leading to the first mitotic division will take place.

Parameters Morphological aspect
Periphery clearly situated near the ooplasm
Intermediate no more in the periphey, but still not at the centre
Centered in the middle of the cell

PN proximity

The male pronucleus originates from the sperm head, whereas the female pronucleus appears after the second meiotic division. They contain the male and female chromosomes respectively. These two organelles are brought together by the aster originating from the sperm centriole. The two pronuclei may thus appear separated, close or abutted depending at which moment the zygote is observed. One should keep in mind that this is a dynamic process, which does not represent a stable characteristic of the zygote.

Parameters           Morphological aspect
Separated the two PN are separated by a some cytoplasmic material
Close the two PN are touching, but have not builded yet a common frontier. Depending on the orientation of the oocyte, this stage may be mistakingly taken for the two other ones
Abutted the two PN exhibit a clearly visible common border

 PN sizes

The male pronucleus (the one situated most distantly from the polar bodies) may appear slightly bigger. In most situation, this difference is barely visible and the two pronuclei exhibit sizes that are similar.

Parameters Morphological aspect
Strongly unequal the two PN exhibit sizes that are clearly different
Slightly unequal the sizes of the two PN is slightly different
Equal the two PN are of comparable size

 NPB number

The nucleolar precursor bodies (NPB) are intranuclear structures that will lead to the formation of nucleoles. Their number and sizes vary from species to species, and in humans from zygote to zygote. They are correlated with early RNA synthesis activity. Three classes have been considered here (<6, 6-14, >14).

Parameters Morphological aspect
<6 1-5 NPB counted in the two PN
6-14 6-14 NPB counted in the two PN
>14 more than 14 NPB counted in the two PN

NPB symmetry

The number of nucleolar precursor bodies (NPB) may not be identical in the two PN. Three situations are considered here, where the number of NPB may be equal, different by less or more than 3 units.

Parameters Morphological aspect
∂<3 the NBP numbers in the two PN differ by less than 3
∂≥3 the NBP numbers in the two PN differ by 3 or more
Symmetrical the same number of NPB is found in the two PN

 NPB polarisation

The nucleolar precursor bodies may be evenly distributed throughout the PN or may tend to concentrate along the common border between the two PNs. Three situations are considered here.

Parameters  Morphological aspect
Non polarised the NPBs are evenly distributed
Polarised the NPBs are aligned along the common border
Intermediate the NBPs are not evenly distributed, but clearly tend evenly or unevenly to concentrate near the common border

Cytoplasmic halo

Following fertilization the cytoplasm may retract as a result of the migration of the organelles towards the two PN. This leads to the formation of clear zone at the periphery of the zygote, known as cytoplasmic halo. The extent of this halo can be large, small or inexistent.

Parameters   Morphological aspect
None no cytoplasmic halo is visible
Small a small cytoplasmic halo is visible near the oolemma
Large a large and clearly visible cytoplasmic halo appears on more than one fourth of the circonference