A new study has found that the process of combining maternal and paternal genetic information is surprisingly highly error-prone.
Parental genomes during fertilisation
Estimates indicate that one in three fertilisations lead to a successful pregnancy. In fact, most embryos fail to progress beyond early development. Aneuploidy in embryos is the leading cause of miscarriage and infertility. Researchers currently understand that the majority of aneuploidy arises during the mitotic division of the embryo. Errors during mitotic division are linked to abnormal division events during early embryo development. However, the cellular origins of mitotic aneuploidy is unclear.
The zygote stage is of particular importance for healthy embryo development. During this stage, parental genomes are enclosed in two separate pronuclei in the fertilised egg (zygote). Researchers have proposed that clustered nucleoli at the interface of the two pronuclei are less likely to give rise to aneuploid embryos. However, due to ethical and legal reasons as well as lack of available biological material, studying human zygotes can be difficult.
Errors during embryogenesis
In this study, published in Cell, researchers utilised high-resolution live cell imaging of human zygotes and bovine embryos to identify potential causes of errors during early embryogenesis.
Here, they found that parental genomes clustered with nucleoli in each pronucleus within both human and bovine zygotes. They also showed that this clustering was required for the reliable unification of parental genomes after fertilisation. Parental chromosomes that did not cluster at the interface were lost when the parental genomes united, resulting in aneuploidy and subsequent developmental defects. The team uncovered that clustering is driven by key components of the cytoskeleton and nuclear envelope that control chromosome movement within the pronuclei. Therefore, emphasising that these two closely linked processes are essential for efficient embryo development.
These findings may be important for in vitro fertilisation in humans. This is because the accumulation of nucleoli at the interface could be an indictor for the likelihood of successful fertilisation.
Image credit: By Science Photo Library – canva.com