Description of 2-PN stage zygotes

The kinetics of PN formation can be followed by time-lapse microphotography. A small movie can be viewed here.

PN centering

PN are usually located at or near the centre of the zygote but may be peripheral in some zygotes.

PN proximity

After their formation, the female PN moves toward the male PN until they are in close proximity. The 2 PN may appear abutted, in contact or still separated in some zygotes.

PN orientation

The abutted PN rotate in the cytoplasm and their axis may appear aligned with the second polar body or misaligned to a certain extent.

Nucleolar precursor bodies (NPB)

NPB number
The total NPB number may vary from more than 14 to less than 6. A progressive coalescence of NPB is probably responsible for a decrease in NPB number combined to an increase in their size.

NPB number asymmetry
Asymmetry in NPB number between the 2 PN is observed in most of the zygotes and probably reflect different patterns of development between the male and female PN. This asymmetry may be however limited or very important.
NPB polarisation
NPB tend to polarise towards the line separating the 2 PN. This polarisation may be very strong, intermediate or totally absent. It may progress differently in the two pronuclei.

Cytoplasmic halo

Cyoplasmic organelles, probably mitochondria, move towards the centre of the zygote around the 2PN, resulting in a clear halo at the zygote periphery. The size of the halo vary according to the zygotes. It may be absent in some zygotes.

Effects of zygote phenotypes on embryo development and implantation

Effect on implantation
PN centring Detrimental if peripheral
PN proximity Detrimental if distant
PN orientation Controversial
NPB polarisation Beneficial if strong
NPB asymmetry Detrimental if important (NPB>3)
Cytoplasmic halo Beneficial


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