Development of lethal yellow (Ay/Ay) mouse embryos in vitro

Preimplantation embryos from matings between mice heterozygous for the yellow allele (Ay) of the agouti locus were cultured in vitro and their development was compared with embryos from control matings (Ay/a × a/a). No morphological abnormalities were consistently observed at the eightcell stage, but after 24 hours of culture there were abnormalities in 16.8% of embryos from Ay/a × Ay/a matings. The abnormalities, which consisted primarily of arrest of several blastomeres and their exclusion from the embryo, were also present in 23.9% of embryos removed from the uterus as morulae and blastocysts. Those embryos that became abnormal during preimplantation stages were presumed to be the Ay/Ay homozygotes because their frequency approximated the Mendelian expectation of 25% in Ay/a × Ay/a matings but not in control matings. The morphological abnormalities indicated an effect of the Ay allele as early as the fourth cleavage division. Presumed Ay/Ay early blastocysts collapsed and died without hatching when cultured for five days under conditions that allowed normal embryos to hatch and grow out on the substrate. If the zona pellucida was removed from these blastocysts before culture, limited trophoblastic outgrowth occurred, but the outgrown cells were few and small and the inner cell mass disintegrated entirely. The Ay allele in the homozygous condition thus appears to be a cell lethal that allows growth up to the early blastocyst stage but prevents the normal postblastocyst growth and differentiation of either trophoblast or inner cell mass.