Mitochondrial transfer between oocytes: potential applications of mitochondrial donation and the issue of heteroplasmy

The developmental competence of mouse and human early embryos appears to be directly related to the metabolic capacity of a finite complement of mater nally inherited mitochondria that appear to begin to replicate after implan tation, Mitochondrial dysfunctions resulting from a variety of intrinsic an d extrinsic influences, including genetic abnormalities, hypoxia and oxidat ive stress, can profoundly influence the level of ATP generation in oocytes and early embryos, which in turn may result in aberrant chromosomal segreg ation or developmental arrest. Deletions and mutations in oocyte mitochondr ial DNA may subtend metabolic deficiencies or replication disorders in some infertile women and in women of increased reproductive age. Here, we descr ibe methods for (i) the compartmentalization of mouse and human oocyte mito chondria into unique cytoplasts enriched for these organelles, and (ii) the ir transfer by microinjection into intact recipient oocytes, Metabolically active mitochondria in donor and recipient metaphase II stage oocytes were labelled with mitochondria-specific fluorescent probes, and the fate and lo cation of donated mitochondria in recipient oocytes were followed by conven tional epifluorescence and scanning laser confocal fluorescence microscopy, The net ATP content of undisturbed and recipient oocytes from the same coh ort(s) was measured quantitatively at timed intervals after mitochondrial i njection. The results demonstrate the feasibility of isolating and transfer ring mitochondria between oocytes, an apparent increase in net ATP producti on in the recipients, and the persistence of activity in the transferred mi tochondria, The findings are discussed with respect to mitochondrial functi on and dysfunction in mammalian oocytes and embryos, and to the potential c linical applications of mitochondrial donation as they relate to the creati on of heteroplasmic embryos.