Effects of maternal age and ovarian stimulation regimen on mitochondrial copy number and function in the cumulus-oocyte-complex (#76)
Mitochondria are the most abundant organelles within the oocyte and are fundamental for energy metabolism, spindle formation and chromosome segregation. Therefore it is likely that a reduction in mitochondria number or function may explain the decreased quality, developmental potential and increased frequency of chromosomal non-disjunction in oocytes from older women, especially when coupled with IVF stimulation regimens. The aim of this study was to delineate, using a novel bovine model of human IVF stimulation, the effects of ovarian stimulation regimens and maternal age on mitochondrial DNA copy number and function of the cumulus-oocyte-complex. Seven young (4 years old) and five old (11 years old) cloned cows with an identical genetic background, and managed as one herd, were subjected to repeated rounds of ovarian stimulation; four natural (hCG), three mild (FSH and hCG) and two heavy stimulated (GnRH agonist, FSH and hCG) cycles. Individual cumulus-oocyte-complexes were collected. The mitochondrial DNA copy number and expression profiles of 17 key genes involved in the control of mitochondrial biogenesis and function in cumulus cells and oocytes were determined using quantitative real-time RT-PCR. No effect of maternal age was evident on mitochondria number in oocytes or cumulus cells (P>0.1). Increasing ovarian stimulation was however associated with a decrease in mitochondria DNA copy number in oocytes (P<0.001), but not in cumulus cells. Changes in the expression of several key mitochondrial genes, including those involved in mitochondrial biogenesis and oxidative stress, such as TFAM and TXN2, were identified in oocytes and cumulus cells with respect to maternal age and/or stimulation regimen. Our data, in a model with reduced genetic noise, suggests that both maternal age and stimulation regimens can influence mitochondrial function of the cumulus-oocyte-complex. Thus changes in mitochondrial characteristics may result in decreased oocyte quality and reduce success of IVF treatment, particularly in older women.