Genomic Imprinting: History and Embryology

Published on October 1, 2007 Reviewed on September 15, 2015   29 min

Other Talks in the Series: Epigenetics

Hi, my name is Davor Solter. And today, I'll talk about genomic imprinting, how it was discovered, and what its effects are on embryonic development. Before we go into details, it is necessary to understand what genomic imprinting really means. Genomic imprinting is a process, well,
I'm sure you can read it yourself. It is important, however, to remember that imprinted genes are expressed monoallelically either from the maternal or paternal allele, depending on the gene and this hemizygous expression has obvious genetic consequences. The discovery of imprinting was made possible through interaction of several elements. One was the age-old question in mammalian development, namely, why paternal genetic embryos do not develop. Before going into the description of parthenogenesis, just a few necessary words about normal fertilization.
The ovulated egg is encased in a proteinaceous envelope, the zona pellucida, shown by green arrows. It has completed first meiotic division and extruded the first polar body, blue arrow. It is arrested in the metaphase of the second meiotic division, red arrow, awaiting fertilization by the sperm, shown by the black arrow. Following fertilization, the first polar body is degraded, green arrow. Second meiotic division is completed and the second polar body is extruded, blue arrow. Sperm and oocyte haploid genomes are contained within the male and female pronucleus shown by red arrows. The pronuclei undergo synthesis and then common metaphase plate, red arrow. And subsequently, zygote divides into two-cell embryo, blue arrow. In parthenogenesis, the oocyte is activated by various means. In order to produce a diploid embryo, one way is to suppress the extrusion of the second polar body, which instead forms a pronucleus-like structure which is shown by red arrows. DNA synthesis ends and division proceeds as a normal fertilized egg. Alternatively, following egg activation, the second meiotic division is completed normally, but again to preserve diploidy, first mitotic division is now suppressed, resulting in an egg with a diploid set of chromosomes, which then proceeds to divide. Thus, fertilization results in a diploid embryo containing both maternal and paternal chromosomes, green arrows, while parthenogenesis also results in diploid embryos but these contain only a maternal genetic contribution, blue arrows. Two hypotheses are proposed to explain the inability of parthenogenetic embryos to complete development, one suggesting that chromosomal duplication results in excessive homozygosity, thus, expression of hidden lethal genes. And another suggesting that the sperm contributes some essential non-genetic material or physiological stimuli which are crucial for normal development. In order to resolve this issue, we needed the appropriate technique of nuclear transfer and this, we developed.