Effect of Initial Cumulus Morphology on Meiotic Dynamic and Status of Mitochondria in Horse Oocytes during IVM

Contents The aim of this investigation was to examine the chromatin configuration of the nucleus, pattern of mitochondrial aggregation and mitochondrial activity in parallel studies in the same horse oocytes. Horse oocytes recovered by ultrasoundguided follicle aspiration in vivo were classified according to two main initial cumulus morphologies as having compact or expanded cumulus. The percentage of oocytes with a diplotene meiotic configuration at the time of recovery from the follicles was highest in compact oocytes. Oocytes with expanded cumulus layers at the time of recovery matured more rapidly in vitro and reached a proportion >50% at the metaphase II stage (M 2) sooner during in vitro maturation (IVM), than did compact oocytes. The mitochondrial aggregation pattern changed from finely distributed (Type 1) through crystalline (Type 2) to an aggregated, granulated appearance (Type 3) during IVM. The pattern of mitochondrial aggregation at the time of recovery was associated with the initial cumulus morphology of the oocyte, in that compact oocytes had a higher proportion of Type 1 aggregation, whereas expanded oocytes had a higher proportion of Type 3. The fluorescence intensity of metabolic active mitochondria, measured by fluorescence intensity (Em 570) per oocyte after MitoTracker CMTM Ros orange labelling, increased in the oocytes during IVM and depended on initial cumulus investment. Oocytes with the granulated type of aggregated mitochondria Type 3 had the highest level of metabolic activity and were in more progressed stages of meiosis (A 1–M 2). Oocytes initially having expanded layers of cumulus reached significantly higher levels of mitochondrial activity after IVM than did oocytes initially having compact cumuli. During resumption of meiosis the mitochondrial activity of oocytes with initially expanded cumulus increased continuously up to M 2, whereas in oocytes from compact cumulus–oocyte complex (COC), the activity declined after A 1/T 1 stages of meiosis.