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 nit rogen (LN2). This may consequently be a source of infection. This report de scribes an attempt to overcome this problem by applying sterile conditions for the recently introduced Open Pulled Straw (OPS) vitrification method. T he whole vitrification procedure is performed in a laminar hood using filte red liquid nitrogen for cooling. The embryo-containing OPS is then placed i nto 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 liqui d nitrogen, the end cut and then the OPS is quickly removed and warmed dire ctly in the culture medium. The procedure is fairly simple, safe and does n ot decrease the in vitro survival rates of in vitro produced bovine blastoc ysts. The possibility to perform sterile OPS vitrification is a prerequisit e for using this technique in accordance with the sanitary regulations and may result in a significant extension of its application.