Mitochondria in human oogenesis and preimplantation embryogenesis: engines of metabolism, ionic regulation and developmental competence

Abstract

Mitochondria are the most abundant organelles in the mammalian oocyte and early embryo. While their role in ATP production has long been known, only recently has their contribution to oocyte and embryo competence been investigated in the human. This review considers whether such factors as mitochondrial complement size, mitochondrial DNA copy numbers and defects, levels of respiration, and stage-specific spatial distribution, influence the developmental normality and viability of human oocytes and preimplantation-stage embryos. The finding that mitochondrial polarity can differ within and between oocytes and embryos and that these organelles may participate in the regulation of intracellular Ca 21 homeostasis are discussed in the context of how focal domains of differential respiration and intracellular-free Ca 21 regulation may arise in early development and what functional implications this may have for preimplantation embryogenesis and developmental competence after implantation.

Brilliant Cresyl Blue stain selects largest oocytes with highest mitochondrial activity, maturation-promoting factor activity and embryo developmental competence in prepubertal sheep

“…Therefore, these data suggest that mitochondrial activity is a determinant factor of quality and changes in mitochondrial activity can alter oocyte quality in a remarkable way. In cattle (Tarazona et al 2006), horses , humans (Van Blerkom 2004) and pigs (Torner et al 2004) an increase in mitochondrial activity after IVM was described. In cattle, Torner et al (2008) observed higher mitochondrial activity in BCBK oocytes than in BCBC.…”

No mitochondrial DNA deletions but more D-loop point mutations in repeated pregnancy loss

“…Such mitochondrial dysfunction affecting ATP production during development may result in empty sacs and other features of fetal loss. Thus , it is possible that some mtDNA mutations cause a developmental arrest before pregnancy is clinically recognized [45].…”

An oocyte score for use in assisted reproduction

“…These are mostly observed either to one side of the cytoplasm or in the centre. Oocyte granularity is correlated with the localisation of mitochondria and may represent domains of high ATP request that are necessary for the normal development of embryos [52,53]. However, granularity focused in the centre of the oocyte is considered a dysmorphism [50].…”