EFFECTS OF COCULTURE AND EMBRYO NUMBER ON THE IN-VITRO DEVELOPMENT OFBOVINE EMBRYOS

It is generally accepted that culturing embryos in groups or with soma tic cells improves both the yield and quality of the blastocysts obtai ned. The aims of this study were 1) to compare the yield and quality o f the embryos obtained after culture in several number conditions and in several culture systems and 2) to assess the effect of co-culture s tarted at various stages of embryo development. Under cell-free cultur e conditions (modified synthetic oviduct fluid [mSOF] supplemented wit h 10% fetal calf serum [FCS] 48 h post insemination, the rate of Day 1 0 blastocysts was lower when embryos were cultured in smalt groups (1 to 6 per drop) than in large groups (4 versus 23%; P < 0.01). There wa s no group effect when embryos were co-cultured either with Buffalo ra t liver (BRL) cells in TCM 199, or in a culture system allowing the pr ogressive development of cumulus cells in mSOF, even if co-culture sta rted at 66 or 114 h post insemination. However, embryos cultured singl y had lower cell numbers than embryos;cultured in large groups when co -culture started at 114 h post insemination. This suggests that 1) som atic cells improve the development of singly cultured bovine embryos u p to the blastocyst stage after the 9-16 cell stage; 2) co-culture aff ects blastocyst cell number of singly cultured embryos by acting rough ly between the 5-8 and the 9-16 cell stage; and 3) cooperation between embryos could replace the effect of co-culture either on the yield of blastocysts or on blastocyst cell number. Blastocysts appeared signif icantly earlier in co-culture with cumulus cells in mSOF than in cc-cu lture with BRL cells in TCM 199 (detection of the blastocysts: 7.3+/-0 .1 d post insemination with cumulus cells versus 8.1+/-0.1 d with BRL cells; P<0.001) and had a significant higher number of cells (143+/-9 versus 85+/-11; P<0.001). This system thus seems suitable for the cult ure of small numbers of embryos resulting from in vitro maturation and fertilization of oocytes from individual donor cows. (C) 1997 by Else vier Science Inc.