Mitochondria as regulatory forces in oocytes, preimplantation embryos and stem cells.

In addition to their role in generating energy (ATP), other possible regulatory functions for mitochondria in the mature oocyte, preimplantation stage embryo and differentiating embryonic stem cell are discussed. The question of the numerical size of a normal mitochondrial complement in the mature oocyte, which has been suggested to be a critical factor in the determination of oocyte and embryo developmental competence, is addressed in the context of mitochondrial DNA copy numbers and the apparent number of organelles detected in living mouse and human blastocysts with mitochondria-specific fluorescent probes. The existence of a spatially stable subplasmalemmal domain of high-polarized mitochondria in the mature oocyte and early embryo is proposed to form a microzone of differential activity that may locally influence the normality of fertilization. High-polarized mitochondria may also influence developmentally significant activities in the peri-implantation blastocyst and differentiating embryonic stem cell. The work discussed indicates that mitochondria may have multiple functions during early development, and specific areas of investigation required to confirm these possibilities are suggested.