Sterile application of the open pulled straw (OPS) vitrification method

A major limit to the application of certain vitrification techniques is the direct contact between the medium containing the embryo and the liquid nitrogen (LN2). This may consequently be a source of infection. This report describes an attempt to overcome this problem by applying sterile conditions for the recently introduced Open Pulled Straw (OPS) vitrification method. The whole vitrification procedure is performed in a laminar hood using filtered liquid nitrogen for cooling. The embryo-containing OPS is then placed into a 0.5 ml plastic straw pre-cooled in the vapor of liquid nitrogen. The straw is hermetically sealed and stored in non-filtered liquid nitrogen. At warming, the large straw is kept vertically and half-immersed in the liquid nitrogen, the end cut and then the OPS is quickly removed and warmed directly in the culture medium. The procedure is fairly simple, safe and does not decrease the in vitro survival rates of in vitro produced bovine blastocysts. The possibility to perform sterile OPS vitrification is a prerequisite for using this technique in accordance with the sanitary regulations and may result in a significant extension of its application.