NUCLEAR TRANSFER IN CATTLE USING IN VIVO-DERIVED VS IN VITRO-PRODUCEDDONOR EMBRYOS - EFFECT OF DEVELOPMENTAL STAGE

To determine the best developmental stage of donor embryos for yieldin g the highest number of clones per embryo, we compared the efficiencie s of nuclear transfer when using blastomeres from morulae or morulae a t cavitation, or when using inner-cell-mass cells of blastocysts as nu clear donors. This comparison was done both on in vivo-derived and in vitro-produced donor embryos, In experiment 1, with in vivo-derived do nor embryos, nuclei from morulae at cavitation supported the developme nt of nuclear transfer embryos to the blastocyst stage (36%) at a rate similar to that of nuclei from morulae (27%), blastomeres from morula e at cavitation being superior (P < 0.05) to inner-cell-mass cells fro m blastocysts (21%). The number of blastocysts per donor embryo was si gnificantly (P < 0.05) higher when using nuclei from morulae at cavita tion (15.7 +/- 4.1) rather than nuclei from morulae (9.8 +/- 5.5) or b lastocysts (6.3 +/- 3.3). With in vitro-produced donor embryos (experi ment 2), nuclei from morulae yielded slightly more blastocysts (32%) t han nuclei from morulae at cavitation (29%), both stages being superio r to nuclei from blastocysts (15% development to the blastocyst stage) . Morulae at cavitation yielded a higher number of cloned blastocysts per donor embryo (11.5 +/- 5.9) than did morulae (9.3 +/- 3.2) and bla stocysts (3.3 +/- 1.4). Transfer of cloned embryos originating from in vivo-derived morulae, morulae at cavitation, and blastocysts resulted in four pregnancies (10%), three pregnancies (7%), and one (17%) preg nancy on day 45. The corresponding numbers of carves born were 3 (4%), 3 (7%), and 0, respectively. After transfer of blastocysts derived fr om in vitro nuclear donor morulae (n = 16) and morulae at cavitation ( n = 7), two (20%) and two (50%) recipients, respectively, were pregnan t on day 45. However, transfer of seven cloned embryos from in vitro d onor blastocysts to three recipients did not result in a pregnancy. Us ing in vitro-produced donor embryos, calves were only obtained from mo rula-stage donors (13%). Our results indicate that the developmental s tage of donor embryos affects the efficiency of nuclear transfer, with morulae at cavitation yielding a high number of cloned blastocysts. ( C) 1996 Wiley-Liss, Inc.