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 Ca2+homeostasis are discussed in the context of how focal domains of differential respiration and intracellular-free Ca2+regulation may arise in early development and what functional implications this may have for preimplantation embryogenesis and developmental competence after implantation.
Jonathan Van Blerkom, Heather Cox and Patrick Davis
Studies of mitochondria in mouse and human oocytes and preimplantation stage embryos have focused primarily on their metabolic capacity to generate ATP. However, it is becoming increasingly apparent that mitochondria are also regulatory agents in other processes involved in the establishment of developmental competence, including calcium homeostasis and apoptosis. The magnitude of the inner mitochondrial membrane potential, or its polarity (ΔΨm), is a physiochemical property of mitochondria related to levels of organelle activity, and differences in the magnitude and spatial distribution of high- and low-polarized mitochondria have been suggested to influence oocyte and early embryo competence. Here, we investigated mitochondrial polarity in normal and diapausing peri-implantation-stage mouse blastocysts, and their corresponding outgrowths, for indications of cell-type-specific regulatory functions or activities in which these organelles may be engaged. The results demonstrate that cell-type- and location-specific domains of differential ΔΨm exist in the peri-implantation blastocyst and remain unchanged during blastocyst outgrowth and during delayed implantation, which for the latter, is accompanied by the suppression of mitochondrial oxidative phosphorylation. Our findings demonstrate that cell-type-specific ΔΨm in the peri-implantation blastocyst is not an intrinsic property of the corresponding mitochondria but one that can be mediated by the dynamics of intercellular contact. Cells with high- or low-polarized mitochondria are differentially affected by photosensitization, with developmental consequences related to embryo behavior and outgrowth performance. Differences in polarity are discussed with respect to the participation of mitochondria in regulatory and morphogenetic processes in the normal peri-implantation embryo. The persistence of high ΔΨm in the diapausing embryo is suggested to be associated with the regulation of levels of cytoplasmic free calcium and the ability of the embryo to reactivate development when delayed implantation terminates.