MOLECULAR REPRODUCTION AND DEVELOPMENT 61:234±248 (2002) Consequences of Bovine Oocyte Maturation, Fertilization or Early Embryo Development In Vitro Versus In Vivo: Implications for Blastocyst Yield and Blastocyst Quality DIMITRIOS RIZOS, FABIAN WARD, PAT DUFFY, MAURICE P. BOLAND, AND PATRICK LONERGAN* Department of Animal Science and Production and Conway Institute for Biomedical and Biomolecular Research, University College Dublin, Lyons Research Farm, Newcastle, County Dublin, Ireland ABSTRACT The aim of this study is to examine the effect of bovine oocyte maturation, fertilization or culture in vivo or in vitro on the proportion of oocytes reaching the blastocyst stage, and on blastocyst quality as measured by survival following vitrification. In Experiment 1, 4 groups of oocytes were used: (1) immature oocytes from 2± 6 mm follicles; (2) immature oocytes from > 6 mm follicles; (3) immature oocytes recovered in vivo just before the LH surge; and (4) in vivo matured oocytes. Significantly more blastocysts developed from oocytes matured in vivo than those recovered just before the LH surge or than oocytes from 2±6 mm follicles. Results from > 6 mm follicles were intermediate. All blasto- cysts had low survival following vitrification. In Experi- ment 2, in vivo matured oocytes were either (1) fertilized in vitro or (2) fertilized in vivo by artificial insemination and the resulting presumptive zygotes recovered on day 1. Both groups were then cultured in vitro. In vivo fertilized oocytes had a significantly higher blastocyst yield than those fertilized in vitro. Blastocyst quality was similar between the groups. Both groups had low survival following vitrification. In Experiment 3a, presumptive zygotes produced by in vitro maturation (IVM)/fertilization (IVF) were cultured either in vitro in synthetic oviduct fluid, or in vivo in the ewe oviduct. In Experiment 3b, in vivo matured/in vivo fertilized zygotes were either surgically recovered on day 1 and cultured in vitro in synthetic oviduct fluid, or were nonsurgically recovered on day 7. There was no difference in blastocyst yields between groups of zygotes originating from the same source (in vivo or in vitro fertilization) irrespective of whether culture took place in vivo or in vitro. However, there was a dramatic effect on blastocyst quality with those blastocysts produced following in vivo culture surviving vitrification at significantly higher rates than their in vitro cultured counterparts. Collectively, these results indicate that the intrinsic quality of the oocyte is the main factor affecting blastocyst yields, while the conditions of embryo culture have a crucial role in determining blastocyst quality. Mol. Reprod. Dev. 61: 234±248, 2002. ß 2002 Wiley-Liss, Inc. Key Words: oocyte development; in vitro fertiliza- tion; ovum pick-up; embryo; gamete biology INTRODUCTION The development of bovine oocytes to the blastocyst stage following maturation, fertilization and culture in vitro is limited to about 30±40%. There is now a growing amount of evidence to suggest that while culture conditions can impact on the developmental potential of the early embryo, the intrinsic quality of the oocyte is the key factor determining the proportion of oocytes developing to the blastocyst stage (Sirard and Blondin, 1996). In support of this idea, oocytes derived from large follicles are more competent than those derived from small follicles (Pavlok et al., 1992; Lonergan et al., 1994). Also, a greater proportion of in vivo matured oocytes reach the blastocyst stage compared to those produced in vitro (Greve et al., 1987; Leibfried-Rutledge et al., 1987; Marquant-Le Guienne et al., 1989; McCaffrey et al., 1991; Van Soom et al., 1992). While still enclosed within the follicle, especially the one destined to become the dominant follicle, the oocyte undergoes signi®cant changes that play a key role in its acquisition of developmental competence. A number of ultrastructural and molecular changes occurring dur- ing oocyte development have been linked to develop- mental competence (Assey et al., 1994b; Hyttel et al., 1997). Also, in vitro maturation (IVM) has been associated with certain abnormalities in the oocyte (Hyttel et al., 1986, 1988, 1989). In addition to the proportion of oocytes developing to the blastocyst stage, the quality of these embryos is important. Despite extensive research in terms of ß 2002 WILEY-LISS, INC. DOI 10.1002/mrd.1153 Grant sponsor: EU Commission; Grant number: QLK3-CT-1999- 00104; Grant sponsor: Greek State Scholarships Foundation. *Correspondence to: Patrick Lonergan, Ph.D., Department of Animal Science & Production, University College Dublin, Lyons Research Farm, Newcastle, County Dublin, Ireland. E-mail: pat.lonergan@ucd.ie Received 18 May 2001; Accepted 1 August 2001