The aim of this study was to test the Brilliant Cresyl Blue (BCB) stain to select prepubertal sheep oocytes for in vitro blastocyst production. Oocyte diameter, mitochondrial activity, maturation-promoting factor (MPF) activity and mRNA relative expression (RE) of genes related to metabolism (ATPase Na+/K+ transporting α 1 (ATP1A1) and cytochrome c oxidase subunit 1 (COX1)) and constitutive function of the cell (cytoplasmic polyadenylation-element-binding protein (CPEB) and S100A10) were assessed. Immature oocytes were exposed to different BCB concentrations (13, 26, 39 and 52 μM) and classified according to their cytoplasm colouration as grown BCB+ (blue cytoplasm) and growing BCB− (colourless cytoplasm). Staining oocytes with 13 μM BCB during 60 min allows selection of (BCB+) the largest (123.66 μm) and most competent oocytes to develop to the blastocyst stage (21%) with a higher number of cells (69.71±6.19 s.e.m.) compared with non-stained BCB− oocytes (106.82 μm, 9% and 45.91±3.35 s.e.m. respectively). Mitochondrial activity, assessed by MitoTracker Orange CMTMRos probe, was significantly higher in BCB+ than in BCB− oocytes after in vitro maturation (3369 and 1565 AU respectively). MPF activity was assessed by CDC2 kinase activity assay showing significantly higher activity at metaphase II stage in BCB+ than in BCB− oocytes (1.479±0.09 and 1.184±0.05 optical density respectively). The genes analysed in this work, ATP1A1, COX1, CPEB and S100A10, did not show significant effect in mRNA RE between BCB selected oocytes. In conclusion, BCB stains larger and more competent oocytes to develop to the blastocyst stage with more active mitochondria and MPF activity and higher blastocyst cell number.

Contents: Sheep and goat production is an important economic activity in Spain with an increasing interest in milk production. Multiovulation and Embryo Transfer (MOET) and In vitro Embryo Production (IVEP) are assisted reproductive technologies aimed at increasing the genetic diffusion of females. In vitro embryo production is a multi‐step methodology comprising the following procedures: (i) In vitro Maturation (IVM) of oocytes recovered directly from the follicles, (ii) In vitro Fertilization (IVF) or co‐incubation of capacitated spermatozoa with in vitro matured oocytes and (iii) In vitro culture (IVC) of zygotes up to the blastocyst stage. In vitro embryo production from oocytes recovered from prepubertal females is called JIVET (Juvenile in vitro Embryo Transfer) and allows shortened generation intervals and increased genetic gain. Embryo production together with embryo cryoconservation would allow large‐scale embryo marketing, a pathogen‐free genetic movement and easier and cheaper germplasm commercial transactions. Commercial Embryo activity in small ruminants is low compared to cows in the European Union (data from the European Embryo Transfer Association) and in the world (data from the International Embryo Transfer Association). There is less IVEP research in small ruminants compared to other livestock species. The aim of this review was to provide an overview of the current status of IVEP of small ruminant with an emphasis on (i) description of the main methodologies currently used for IVM, IVF and IVC of embryos (ii) comparing procedures and outputs from JIVET and IVEP of adult females and (iii) the future research perspectives of this technology.

Two experiments were designed to determine the ability of in vitro-cultured blastocysts at different stages of development to survive the vitrification procedure using cryotop devices. Day 7 and Day 8 embryos were classified as non-expanded, expanded or hatching and/or hatched blastocysts. In the first experiment, we examined the survival rate of vitrified–warmed blastocysts after 3 h incubation in synthetic oviducal fluid (SOF) medium. In the second experiment, vitrified–warmed blastocysts were evaluated using the terminal deoxyribonucleotidyl transferase-mediated dUTP–digoxigenin nick end-labelling (TUNEL) technique to detect nuclei with damaged DNA. In both experiments, results for cow and calf blastocysts were compared. No differences in survival rates were observed after vitrification of Day 8 expanded (52.4%) and hatched (50%) cow blastocysts or Day 8 expanded (54.5%) and hatched (59.4%) calf blastocysts. When embryos were vitrified on Day 7, survival rates of 78.4% and 66.7% were observed after warming expanded and hatched cow blastocysts, respectively, compared with rates of 80% and 76.9%, respectively, for calf blastocysts. Lowest survival rates were recorded for non-expanded blastocysts (26%–54%) compared with the other developmental stages, particularly those vitrified at Day 8 (=40%). The DNA integrity index obtained after vitrification–warming was comparable to that for control fresh blastocysts, regardless of the length of embryo culture, the developmental stage of the embryo or the source of the oocytes. Our findings suggest that the cryotop vitrification method is particularly useful for the cryopreservation of blastocysts presenting with a high degree of expansion (expanded or hatched blastocysts), particularly when vitrification is performed after 7 days of in vitro embryo culture.