KARYOPLAST-CYTOPLAST VOLUME RATIO IN BOVINE NUCLEAR TRANSFER EMBRYOS - EFFECT ON DEVELOPMENTAL POTENTIAL

To evaluate the effect of karyoplast-cytoplast ratio on the developmen t of nuclear transfer embryos, karyoplasts from day 4, day 5, and day 6 embryos were transferred to oocytes enucleated with different volume s of cytoplasm: Type I, removal of a small volume of cytoplasm equival ent to the first polar body, Type 2, removal of a volume of cytoplasm approximately equal to the volume of the respective karyoplast, and Ty pe 3, removal of half of the oocyte volume. In addition, the effect of experimental reduction of karyoplast cytoplasm was investigated in da y 4 and day 5 karyoplasts. Intact day 4 karyoplasts fused to Type 3 cy toplasts did not support development to blastocysts, whereas these kar yoplasts yielded blastocysts in combination with Type 1 (7%) and Type 2 cytoplasts (12%). After experimental reduction of cytoplasmic volume in day 4 karyoplasts, blastocysts (10%) were also obtained after fusi on with Type 3 cytoplasts, probably due to reduction of cytoplasmic ch imerism. With day 5 karyoplasts, blastocyst rate was higher in combina tion with Type 2 (34%) than with Type 1 (19%) and Type 3 cytoplasts (1 6%; P < 0.05). The use of day 6 intact karyoplasts resulted in a signi ficantly (P < 0.05) higher proportion of blastocysts when fused with T ype 2 (38%) or Type 1 cytoplasts (34%) than with Type 3 cytoplasts (16 %). These results suggest that enucleation of oocytes with a volume si milar to that of the respective karyoplast creates better conditions f or cell cycle interactions with all types of karyoplasts than enucleat ion with minimal or large volume of cytoplasm. (C) 1997 Wiley-Liss, In c.