Recent developments in embryo sexing and its field application.

This review focuses on polymerase chain reaction (PCR) sexing of bovine embryos in commercial situations with emphasis on new developments. Simplifications of the biopsy technique is one of the major simplifications over the last few years. The stabilization of the embryo by means of protein-free medium or scratches produced on the bottom of the Petri dish makes it possible to perform a biopsy with a single microinstrument. The traditional PCR sexing approach utilizes electrophoresis, which involves the risk of deoxyribonucleic acid (DNA) contamination of subsequent assays. Such contamination, resulting in females misdiagnosed as males, is avoided efficiently by using a non-electrophoretic method in which the sex is determined based on fluorescence of unopened tubes. However, female samples cannot be distinguished from blank samples in the non-electrophoretic assay, which thus relies on accurate transfer of biopsy into tubes. Nevertheless, an accuracy of about 95% can be reached with both approaches. High pregnancy rates (50-70%) can be reached with biopsied Grade 1 embryos, but there is evidence that pregnancy rates with Grade 2 embryos is 15-20% lower. Recent data indicate that pregnancy rates of 50% can be achieved with frozen-thawed biopsied Grade 1 embryos. In conclusion, recent developments in biopsy techniques, detection systems and freezing should increase interest in PCR sexing.