Coculture of ovine zygotes fertilized in vivo or in vitro and positive effect of CZB medium on the development of in vitro fertilized zygotes.

The development and quality of ovine zygotes derived from in vivo (IVOF) or in vitro fertilization (IVF) were compared after coculture on sheep oviductal cells. The same criteria were used to evaluate the coculture of IVF zygotes in CZB medium for 2 d followed by 199 medium for 6 d (CZB-199 coculture) or in 199 medium for 8 d (199 coculture). A higher overall developmental rate to blastocyst stages was obtained with IVOF (65.7%) than with IVF (23.2%) zygotes. More IVF zygotes reached blastocyst stages in CZB-199 (36.7%) than in 199 coculture (22.9%). The morphological aspect did not differ significantly between IVOF and IVF or between 199 and CZB-199 blastocysts. Histological examination revealed no significant difference in the pyknotic and mitotic indices and mean number of cells in the trophoblast and in the inner cell mass of hatched blastocysts between IVOF and IVF or between CZB-199 and 199 cocultures. According to criteria used in this study, the quality of blastocysts was equivalent, independently of fertilization or coculture systems. The use of CZB medium during the first cleavages increases the proportion of blastocysts.