Fate of unfertilized ova in male rodent dominant lethal assays: Extension of the studies by Kratochvilova

Kratochvilova has described a technique whereby ova can be recovered from mated mice and their stage of division determined, This is of value to determine if reduced total implantations in a male dominant lethal (DL) germ cell mutation assay are due to pre-implantation loss of embryos, a presumed mutagenic event, or to chemically induced male infertility. Kratochvilova was not specific about the fate of unfertilized ova, but it was implied that they undergo a process of fragmentation that might be confused with the regular cleavage of fertilized ova. It became important for us to draw a firm distinction between ova fragmentation and regular ova cleavage in the rat. We therefore repeated the ova analyses of female mice mated with males exposed to iso-propyl methanesulphonate (iPMS): as described by Kratochvilova, Following that calibration study the technique was extended to the rat via ova cleavage analysis in mated female rats, coupled to a study of the normal decay of ova in virgin rats, Unfertilized ova are shown to undergo irregular fragmentations that can be clearly distinguished from normal cell division. It is concluded that the individual or combined incidences of single celled ova and fragmented ova (dependent on the cleavage stage of the concurrent control embryos) can provide a measure of male infertility as it relates to reduced implantations in DL assays. This ability to regard two morphological classifications of unfertilized ova as providing evidence for male infertility will simplify the conduct of ova analyses in both the mouse and the rat.