Holding equine oocytes at room temperature for 18 hours prior to in vitro maturation maintains their developmental competence

The collection of equine oocytes both from ovaries of slaughtered mares and from live donors is a time- consuming technique since cumulus-oocytes complex (COCs) are tightly attached to the follicle wall. In both cases extensive manipulation of ovaries and flushing of single follicle are required to collect the oocytes. For this reason the collection of large number of oocytes from slaughter- house ovaries or from ovum pick up of many live donors in the same day or far from the laboratory where the oocytes will be fertilized, implicates that the COCs reach metaphase II stage (Met II) at different timing and often during the night-time. Holding treatment of immature oocytes that could delay the oocyte maturation process and simulta- neously synchronize them to have more homogenous groups at the desired time during more convenient working hours would be advantageous. In this work we examined chromatin configuration, maturation and developmental competence of equine oocytes collected from slaughterhouse ovaries either placed into culture immediately compared to holding for18 h at room temperature prior to in vitro maturation (IVM).