The embryo's genomic integrity is monitored and safeguarded by the fertilized egg cells within 24 hours of fertilization, reports a recent study.
The DNA present in the sperm and egg cells are the genetic blueprints of the zygote, the single-cell embryo. After fertilization, the male parental DNA initiates the modification of its "epigenetic memory" of its sperm state.
Reprogramming To Totipotent Embryo
In the meantime, the fertilized egg that comes into play provide proteins to erase this memory to a large extent and helps in generation of a totipotent embryo, which paves way for rise of a new individual. Though the reprogramming of fertilized egg to totipotent embryo is an important process of fertilization, it has not been studied in detail by far.
Kikuë Tachibana-Konwalski, the study's senior author from Institute of Molecular Biotechnology, noted that it takes weeks for cell cultures to be reprogrammed into induced pluripotent cells. On the contrary, the zygotes are reprogrammed to totipotent cells in just 24 hours.
Safeguarding Epigenetic Memory
Furthermore, it is noted that the fertilized eggs don't just participate in the reprogramming process of male DNA but also closely monitor and protect the genetic integrity of the material. As soon as the sperm enters the ovum, the tightly packed male chromatin is unraveled and arranged around the histone protein scaffolds, noted Sabrina Ladstätter, the study's first author.
It is found in the mice study that as soon as the sperm enters the egg the reprogramming of the sperm DNA begins. The fertilized egg triggers demethylation of sperm DNA and erases the epigenetic memory inherited from the father, thereby paving way for creation of new epigenetic memory.
The process of demethylation could cause damage to the genetic material which could result in embryo loss, chromosome fragmentation and infertility. However, the fertilized egg cells surprisingly do more than just reprogramming. In addition to initiating and monitoring the reprogramming of epigenetic memory, the egg cells also fix the damages caused during demethylation.
The lesions caused in the parental DNA triggers the activation of check points in the zygote that stops cell division until the damages are completely rectified. Eventually, the lesions are fixed and reprogramming is done in just 24 hours, say in one cell cycle, making sure the epigenetic memory is intact.
Epigenetic Memory Reprogramming In Vitro
That being the case, there are possibilities that the fertilization taking place in vitro may not undergo such processes as efficiently as in natural conditions.
"It will be exciting to explore how cell culture conditions enhance the zygote's intrinsic surveillance and repair mechanisms, thus leading to better quality embryos and potentially more successful pregnancies," noted Tachibana-Konwalski in a press release.
The study is published in the journal Cell.