Mitochondria are present in all eukaryotic cells and generate the vast majority of cellular energy through the process of oxidative phosphorylation (OXPHOS), which takes place within the electron transfer chain (ETC). The ETC is encoded by two genomes, the chromosomal and the mitochondrial (mtDNA) genomes. MtDNA replication is mediated by chromosomally-encoded genes, which translocate to the mitochondrial genome.

Primordial germ cells possess ~ 200 copies of mtDNA per cell and these copies are clonally expanded during oogenesis so that mature, fertilisable, metaphase II oocytes possess ³ 250,000 copies. However, those oocytes that fail to become developmentally competent have significantly fewer copies of mtDNA.

Following fertilization, mtDNA replication is strictly regulated through DNA methylation of the nuclear-encoded mtDNA-specific replication factor, Polymerase Gamma. During early preimplantation development, mtDNA copy number is significantly reduced with no mtDNA replication taking place. At the blastocyst stage, mtDNA replication is initiated but this is restricted to the trophectodermal cells. The inner cell mass cells, which have the potential to give rise to all cell types of the body, do not replicate mtDNA and continue to reduce mtDNA copy number so that, prior to gastrulation, they possess fewer copies of mtDNA. This continual reduction results in the establishment of the ‘mtDNA-set point’, which enables all differentiating cells to acquire the appropriate numbers of mtDNA copy to meet their specific demands for ATP generated through OXPHOS in order that they can perform their designated cellular functions. Consequently, cells with a high requirement for OXPHOS-derived ATP, such as neurons, posses thousands of copies of mtDNA per cell whilst cells will a low requirement for OXPHOS-derived ATP, such as endothelial cells, have far fewer copies of mtDNA.

Although mtDNA is normally inherited from the population present in the oocyte at fertilization, some of the more sophisticated assisted reproductive techniques violate this mode of transmission. For example, nuclear transfer, where a donor cell is fused to an enucleated oocyte, can result in two or more populations of mtDNA being transmitted to the offspring with the potential of 0 to 59% of the offspring’s total mtDNA originating from donor cell. This arises as the nuclear-encoded mtDNA replication factors are prematurely expressed during preimplantation development unlike their in vitro fertilised counterparts. Transmission of mtDNA in this manner can have severe consequences for the offspring’s health. However, embryos and live offspring can now be generated that do not possess donor cell mtDNA. This has important implications for the generation of embryonic stem cells to treat disease and for the use of pronuclear and spindle transfer to prevent the transmission of mutant mtDNA from one generation to the next.