ABSTRACT
Following fertilization, most mRNA encoding proteins in the zygote is of maternal origin, but as the embryo advances in development, it becomes less dependent on maternal mRNA as more mRNA transcripts representing its own genome are expressed.[1] This maternal to embryonic transcript transition occurs at the 4-cell stage in pigs and 8-to 16-cell stage in sheep and cattle.[2] Advancing embryonic development results in genes being silenced, so that the genome loses “totipotency” or its ability to orchestrate development of a normal individual. However, the cloning of the sheep Dolly taught us that the nuclear genome in adult cells can be reprogrammed to restore totipotency following somatic cell nuclear transfer, although less efficiently than cloning by blastomere nuclear transfer using 8-to 16-cell sheep and cow embryos.[3]
Genome imprinting refers to the requirement for an embryo to have both maternal and paternal chromosomes (genomes) to develop normally. This is because of preferential expression of maternal genes in the embryonic disc for development of the embryo/fetus, while paternal genes are expressed preferentially in the trophectoderm for development of a functional placenta. Imprinted genes include those for insulin-like growth factor II from the maternal genome and insulin-like growth factor II receptor from the paternal genome, both of which are essential for the development of a normal conceptus.
