Ultrastructural studies of tw32/tw32 mouse embryos.

Homozygous tw32/tw32 mouse embryos, obtained from both spontaneously ovulated and superovulated T+/tw32 females mated to T+/tw32 males, have a lethal period which extends from the 8-12 cell stage to the late morula stage. Most of the homozygous mutant embryos die at the early morula stage and are characterized by excessive amounts of cytoplasmic lipid, mitochondrial abnormalities, binucleated cells and nuclear lipid droplets. The excessive cytoplasmic lipid and nuclear lipid droplets distinguish 35-50% of the embryos (presumably tw32 homozygotes) from their litter-mates prior to the lethal period. The remainder of the distinguishing characteristics appear in the later (8-cell to late morula) tw32/tw32 embryos in frequencies high enough to be considered phenotypic expressions of the mutant genome. The present study indicates that the non-complementary tw32 and t12 alleles are in fact separate T locus recessive alleles.