Criopreservação de embriões bovinos: boas práticas nas técnicas de vitrificação e direct transfer

Abstract

Bovine embryo cryopreservation is a fundamental step in modern embryo transfer programs, enabling long-term storage, transportation, and commercialization of high-value genetic material. Given Brazil's position as the world's largest bovine embryo transferor, as reported by the International Embryo Technology Society (IETS, 2023), standardization of techniques and adoption of best practices are essential to ensure consistent reproductive outcomes. This study consists of a literature review on two of the main bovine embryo cryopreservation techniques: vitrification and Direct Transfer (DT). Vitrification is based on the ultrarapid solidification of highly concentrated cryoprotectant solutions, achieving cooling rates above 20,000°C/min with devices such as the Open Pulled Straw (OPS) and Cryotop, completely preventing intracellular ice crystal formation and providing high embryo survival rates, especially for in vitro-produced (IVP) embryos. Direct Transfer, in turn, involves slow-freezing embryos in ethylene glycol (EG) solutions at concentrations compatible with direct inoculation into recipient females after thawing, eliminating laboratory cryoprotectant removal steps and providing exceptional practicality for field use. The review covered the cryobiological principles underlying both techniques, the types and mechanisms of action of permeating and non-permeating cryoprotectants, vitrification and slow-freezing protocols, available devices and their cooling rates, warming and embryo rehydration processes, reproductive outcomes reported in the scientific literature, and best practices regarding laboratory conditions, biosafety, liquid nitrogen handling and storage, quality control, traceability, and prevention of operational errors. The results showed that, for in vivo-produced embryos, vitrification and DT yield equivalent pregnancy rates (54% to 69% for Grade I embryos). For IVP embryos, vitrification is biologically superior to conventional DT; however, the Embrapa Vitrificação Direta (VD) protocol, which combines vitrification with Direct Transfer, demonstrated pregnancy rates statistically comparable to those of fresh embryos (33% vs 44%), representing a significant advance for commercial field application. It was concluded that both techniques are complementary and strategically important for Brazilian bovine reproductive biotechnology, and that the adoption of standardized operating procedures is an indispensable condition for reproducibility and excellence of results.