Long-Term Effects of Exposure to Methylnitrosourea on Blastocysts following Transfer to Surrogate Female Mice

The effect of in vitro (reimplantation exposure of mouse embryos to N-methyl-N-nitrosourea (MNU) on development subsequent to embryo transfer into pseudopregnant surrogate mothers has been investigated. The offspring developed from mouse blastocysts exposed to MNU or to solvent were examined carefully for 1 year after birth and then examined with X-rays and complete autopsies. Metaphase chromosomes were examined in some of these animals. Exposure to MNU at the morula stage did not affect progression to the blastocyst stage in vitro; however, exposure at the blastocyst stage resulted in a profound reduction in the live birth rate. Offspring developed from blastocysts exposed to MNU in vitro were found to have a 3-fold-higher crude mortality rate than offspring developed from blastocysts exposed to solvent. There were no gross dysmorpho-genic effects of exposure, nor were any major histological abnormalities associated with MNU exposure. No chromosomal abnormalities were discovered in either surviving group of offspring.