Ultrastructural analysis of preimplantation lethal yellow (Ay/Ay) mouse embryos.

Embryos recovered at 62 and 80 h post coitum from reproductive tracts of yellow (Ay/a) and black (a/a) female mice mated to Ay/a males were examined ultrastructurally to define the developmental defect of lethal Ay homozygotes. No abnormalities were observed in five embryos from control matings (female a/a x male Ay/a). Of 24 morulae and blastocysts from Ay/a x Ay/a matings, six were observed to possess some morphological aberration. Two of the six abnormal embryos were morulae and contained isolated blastomeres which had developmental features typical of younger embryos; remaining cells of these embryos were normal. The third, an early blastocyst, contained a degenerating trophoblast cell' other cells of this embryo were also abnormal but not in an advanced stage of degeneration. The fourth abnormal embryo (late cleavage stage) was in an advanced stage of degeneration affecting all blastomeres. Finally, the remaining two abnormal morulae had a unique nucleolar morphology and an unusual abundance of intra-cisternal A particles. Presumably, one or more of the six abnormal embryos from Ay/a x Ay/a matings were Ay homozygotes. However, no single ultrastructural alteration characteristic of Ay/Ay embryos was found.