Cytological and ultrastructural comparisons of t12-t12 and normal mouse morulae.

Abstract

AbstractCytological and ultrastructural observations were made of normal (BALB/c) mouse morulae and embryos homozygous for the t12 mutation, which is lethal at the late morula stage. Normal and t12/t12 embryos are indistinguishable cytologically until the late morula stage, at which time cytoplasmic and nuclear differences become apparent. Different cells within individual normal morulae exhibit differences in cytoplasmic basophilia, and the cells appear closely applied to one another. Differences in cytoplasmic basophilia are not apparent between different cells of t12/t12 morulae, and the cells appear more rounded and less closely applied. Nucleoli of normal morulae are irregular in shape, with projections which often reach the nuclear envelope, while nucleoli of t12/t12 morulae exhibit extreme rounding.Nucleoli of normal embryos exhibit a net‐like nucleolonema, with fibrillar and granular elements, and several pars amorpha. Nucleoli of t12/t12 morulae possess the same structural elements but the nucleolonema appears contracted. Nuclei of mutant embryos often contain several agranular electron‐dense bodies, 0.1–0.7 μ in diameter. These are infrequently observed in normal morulae. Ribosomes exist primarily in clusters in normal morulae, but large numbers of free ribosomes are observed in t12/t12 morulae. Many structures are common to both normal and mutant morulae, such as: fibrous strands, crystalloids, granular endoplasmic reticulum, vacuolated mitochondria, cytoplasmic vesicles, tight junctions, nuclear pores, and intranuclear annulate lamellae.Since the abnormalities exhibited by t12/t12 morulae can be interpreted as degenerative changes, they do not resolve the mode of action of the t locus or the primary cause of t12/t12 lethality.