Studies of cleavage stage mouse embryos are reported, with particular emphasis upon nucleolar fine structural and functional changes. Multiple fibrillar primary nucleoli are present in the early 2-cell embryo. In late 2-cell embryos, some of these nucleoli acquire a peripheral zone of granules, while others reticulate, forming nucleoli composed of fibrillo-granular cortices and fibrillar cores. The nucleoli of early 4-cell embryos are composed only of fibrils. In the middle of the 4-cell stage, some of the nucleoli acquire a peripheral granular zone, while others reticulate. The reticulated nucleoli of both the late 2-cell and 4-cell embryos can be considered, on the basis of their fine structure, to be definitive nucleoli. Early 8-cell and morula embryos usually contain only two definitive nucleoli per nucleus. H-5-uridine-pulsed embryos contain label localized in the nucleus, particularly over definitive nucleoli. Nucleolar labeling increases at each successive developmental stage. Beginning at the 8-cell stage, re-incubation in nonradioactive medium results in a significant decrease in nucleolar labeling and an increase in cytoplasmic labeling suggesting that more ribosomal RNA is transferred from the nucleus to the cytoplasm at the later cleavage stages.
A statistical study of embryos obtained from both spontaneously ovulated and superovulated +/P females, mated inter se, shows that the range of the lethal phenocritical period of the t l Y allele in a homozygous condition is from the 8-12 cell stage to the early blastocyst stage. The majority of t12 homozygotes are developmentally arrested as late morula and the nuclei of these embryos contain lipid droplets and fibrillo-granular bodies. These same inclusions are found i n other tI2 embryos which are developmentally arrested either earlier or later than the late morula stage and distinguish 30-40% of the embryos (presumably t l x homozygotes ) from their litter-mates at the 2-and 4cell stages. Ultrastructural-cytochemical studies of the fibrillo-granular bodies show that the fibrillar areas are sensitive to pepsin and the granules ts rihonuclease and are thus structurally and chemically similar to definitive nucleoli. Binucleate cells are also present in a high frequency of t't homozygous embryos. This condition is considered an additional phenotypic expression of the genotype.Prior to developmental arrest, the nuclear and cytoplasmic organelles of t l Z embryos do not differ from those of similarly staged litter-mates or control +/+ embryos.Homozygous mutant embryos examined shortly following developmental arrest contain cells ranging from structurally normal to degenerative. Asynchronous cell death is common to all t 1 2 homozygous embryos. A chronological description of degenerative cellular changes is presented.
The actual and potential activities of the cyochrome system were studied in cleavage-stage mouse embroys. Activities were determined by assaying embroys for total ATP and the rates of [32-P]ATP synthesis both before and after their incubation in medium supplemented either with an energy coupling site inhibitor (antimycin, amytal or cyanide) or with the FADH-linked substrate, succinate. The data indicate that there are three major shifts in the mode of ATP production during preimplantation stages: the first, between the two-cell and late four-cell stages; the second, between the eight-celland late morula stages; and the third, between the late morula and late blastocyst stages. These data are discussed in relation to studies on the energy metabolism of cleavage and blastocyst stage mouse embryos.
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