THE IMPACT OF CELLULAR FRAGMENTATION INDUCED EXPERIMENTALLY AT DIFFERENT STAGES OF MOUSE PREIMPLANTATION DEVELOPMENT

It has been demonstrated previously that removal of acellular debris f rom the preimplantation mouse embryo is beneficial for subsequent deve lopment to the hatched blastocyst stage. We have studied the impact of cellular fragmentation induced in the mouse embryo during the late pr onuclei and 8-cell stages on the hatching frequency and total cell num ber at the blastocyst stage. At the late pronuclei stage about one-qua rter of the cytoplasm was removed from embryos in the experimental gro up, in four to six steps, thus creating four to six cytoplasts that we re subsequently returned as anucleated fragments under the zona pelluc ida. Embryos with one-quarter of the cytoplasm removed and with intact cytoplasm after partial zona dissection (PZD) served as controls, At the 8-cell stage, embryos with their nucleoplast removed from two blas tomeres served as an experimental group, Groups of embryos with part o f the cytoplast removed from two blastomeres (nucleated fragments), em bryos with two blastomeres removed and embryos after PZD alone served as controls, After manipulation all embryos were left in culture and a nalysed at about 100 h after human chorionic gonadotrophin administrat ion. Fragments induced at the late pronuclei stage did not participate in compaction and were often spontaneously expelled from the embryo d uring hatching. Neither embryo hatching rate nor total cell number was affected when compared with zygotes with reduced cytoplasm, Although both nucleated and anucleated fragments induced at the 8-cell stage pa rticipated in recompaction, hatching was not compromised and there was no interference in further development as assessed by the cell number or hatching rate at the blastocyst stage, as compared with embryos wi th blastomeres removed. We conclude that anucleated cellular fragments formed in an otherwise healthy embryo, both before and after acquisit ion of the ability for compaction, are benign and that their removal p rovides no benefit for embryo development, at least to the hatched bla stocyst stage.