Chromosomal abnormalities and developmental kinetics in in vivo-developed cattle embryos at days 2 to 5 after ovulation

The frequency of chromosome abnormalities was investigated in cattle embryo s (n = 256) derived from superovulated heifers (n = 35) on Days 2, 3, 4,and 5 postovulation (PO). Interphase nuclei (n = 4358) were analyzed for chrom osome abnormalities using fluorescent in situ hybridization with chromosome 6- and chromosome 7-specific probes and the developmental rate was describ ed by scoring cell numbers. We found that 93%, 85%, 84%, and 69% of the emb ryos from Days 2, 3, 4, and 5 PO, respectively displayed a normal diploid c hromosome number in all cells. Of the embryos containing abnormal cells, mi xoploidy was significantly more frequent than polyploidy. The percentage of mixoploidy at Days 2, 3, 4, and 5 PO was 5%, 13%, 16%, and 31%, respective ly, whereas the percentages of polyploidy were 2%, 2%, 0%, and 0%, respecti vely. The mean number of cells per embryo was 4.7, 8, 11.5, and 48.3, respe ctively, at Days 2, 3, 4,and 5 PO. Thus, in vivo-developed embryos were sig nificantly more advanced than the in vitro-produced (IVP) embryos except fo r Day 2. In conclusion, a significantly lower frequency of chromosomally ab normal embryos, in particular displaying polyploidy early after fertilizati on, was seen in in vivo versus IVP embryos, and these chromosomal abnormali ties may be inherent to the process of IVP in cattle.