Ultrastructural observations of lethal yellow (Ay/Ay) mouse embryos.

Ay/Ay embroys were identified by the presence of large exluded blastomeres (Pedersen, 1974) and examined cytologically and ultrastructurally. Cell organelles, inclusions and junctions in the excluded blastomeres were compared with those of non-excluded cells of Ay/Ay embryos and control embryos. Excluded blastomeres always had the fine structural characteristics of earlier developmental stages and may have arrested at the 4- to 8-cell stage of slightly later. Interior cells (inner cell mass) were observed in all mutant blastocysts. Non-excluded cells of Ay/Ay embryos were normal until degenerative changes appear in the late blastocyst stage. The mode of action of the +Ay gene was not determined, but evidence from this study and others indicate that the effects of +Ay gene action occur over a wide range of time in early cleavage and implantation.