A high proportion of bovine blastocysts produced in vitro are mixoploid

Fluorescence in situ hybridization with chromosome 6- and chromosome 7-spec ific probes was used to assess the extent of chromosome abnormalities in de veloping bovine blastocysts at 7-8 days after insemination in vivo or in vi tro. Interphase nuclei (N = 10 946) were analyzed from 151 blastocysts prod uced in vitro and from 28 blastocysts recovered from superovulated animals. This revealed that 72% (109 of 151) of the in vitro-produced blastocysts w ere mixoploid, i.e., were a mixture of normal, diploid, and polyploid cells . However, only a small fraction of the total number of cells were chromoso mally abnormal. Of the mixoploid blastocysts, 83% (91 of 109) contained les s than 10% polyploid cells, 13% (14 of 109) contained 11-25% polyploid cell s, and only 4% (4 of 109) of the blastocysts had more than 25% polyploid ce lls per blastocyst. In contrast, a significantly lower proportion (25%) of mixoploidy was found in 28 bovine blastocysts developed in vivo (p < 0.0001 ). All of the mixoploid blastocysts that had developed in vivo contained le ss than 10% polyploid cells. No entirely aneuploid blastocysts, i.e., blast ocysts in which all cells had the same type of chromosome abnormality, were found in either of the groups. Taken; together, the most common chromosome abnormalities observed were diploid-triploid mixoploidies and diploid-tetr aploid mixoploidies. Thus, our results confirm earlier reports that morphol ogically normal bovine blastocysts developed in vivo are often mixoploids. We further show that in vitro-produced bovine blastocysts have a high rate of mixoploidy. Although the difference in mixoploidy rate detected in this study may not be general, it is an interesting phenomenon for further studi es.