FOLLICLE AND OOCYTE STAGE THAT AFFECT IN-VITRO MATURATION AND DEVELOPMENT OF BOVINE OOCYTES

Success of in vitro maturation (IVM) and production of bovine embryos as related to aspects of follicle source and oocyte size were evaluate d. First, it was determined that bovine oocytes continue growing in al l follicular sizes studied, including >1- to 15-mm follicles. Populati ons of oocytes were collected from surface visible (peripheral) and co rtical follicles from the same ovaries. When the number of oocytes fro m both peripheral and cortical follicles was combined, the yield of oo cytes was approximately double that collected from 1 ovarian site alon e. Oocytes from cortical follicles were smaller than those from the su rface population, and the smaller cortical oocytes had a lower potenti al for both meiotic maturation and embryo development. Only cortical o ocytes with the largest diameters underwent IVM and subsequently devel oped to blastocysts at rates comparable to oocytes from peripheral fol licles. As the diameter of the oocytes recovered from peripheral folli cles increased, so did their developmental potential. When the stage o f the estrous cycle was observed, it was found to have no effect on de velopmental potential. Finally, oocytes which extruded polar bodies at an earlier time during maturation were, on average, larger than those which extruded polar bodies later. The results serve a practical purp ose in assisting selection of oocytes capable of developing into blast ocysts and they give useful correlates of oocyte competencies based on knowledge of follicle source and oocyte stage.