Genetic and biochemical activities in preimplantation embryos

Light and electron microscopic analysis of bovine embryos derived by in vitro and in vivo fertilization

PURPOSE

The aim of this study was to examine the morphological features of in vitro fertilization-derived bovine embryos (IVt) and compare them with those of in vivo fertilization-derived (IVv) ones.

RESULTS

Light microscopy showed the blastomeres of IVv embryos to have a tendency to be rounder up to the 16-cell stage and to form a more compact mass at the morula stage than in their IVt counterparts. Electron microscopy revealed that as development progressed, some structures (such as microvilli, phagosomes/lysosomes, intercellular junctions and intermediate filaments) appeared or reappeared while others (such as lipid droplets, vesicles with flocculent materials, cortical granules, nuclear annulate lamellae, nuclear envelope blebs) decreased or disappeared. These changes were observed about one cell stage later in IVt than in IVv embryos. Other structures were present in both IVt and IVv embryos, and they morphologically either changed (such as mitochondria, endoplasmic reticulum, Golgi apparatus and nucleoli) or did not change (cytoplasmic annulate lamellae). In contrast to previous reports, vacuolated nucleoli in both IVt and IVv embryos were observed from the two-cell stage.

CONCLUSIONS

It was concluded that (1) the development of bovine embryos to the blastocyst stage is like that of other mammalian embryos; (2) IVt and IVv embryos did not show consistent differences in morphological features; (3) although IVt embryos appear delayed in development, this may reflect different definition of age in vivo and in vitro.

Activation of mouse ribosomal RNA genes at the 2-cell stage

The Ag-AS method, developed by Goodpasture and Bloom (1975) strains transcriptionally active nucleolus organizer regions (NORs) on the chromosomes and in the interphase nuclei. Metaphases and interphase nuclei of early mouse embryos (unfertilized eggs, pronucleus stages, 2-, 4-, 8-cell stages, and morulae) were subjected to silver-staining. First staining of a single chromosome bearing an NOR was observed at the 2-cell stage. At the 4-cell stage 4--6 chromosomes, and at the 8-cell stage invariably all the 6 chromosomes known to bear NORs, respond positively to silver-staining. These results indicate that during mouse embryogenesis ribosomal RNA genes start to function at the 2-cell stage. The polar body does not respond to silver-staining, which supports the view that the polar body genome remains inactive.

Nucleic acid synthesis in preimplantation rabbit embryos. III. A "dark period" immediately following fertilization, and the early predominance low molecular weight RNA synthesis

This report stems from efforts to characterize the products of transcription during the initial stages of cleavage in the rabbit embryo, since previous studies have suggested that RNA synthesis is required during this period for the continuation of normal development. Early cleaving embryos were exposed to radiolabeled precursors of nucleic acids in vitro and RNA extracted and analyzed by polyacrylamide gel electrophoresis and BD-cellulose chromatography. No unequivocal evidence of RNA synthesis could be obtained during the first three cleavage divisions; all apparent "embryonic" RNA synthesis could be ascribed to contaminating follicle cells. After the fourth cleavage, however, there is readily detectable incorporation of radioisotope into RNA by embryos which have been denuded of follicle cells. Two-thirds of the cytoplasmic RNA labeled during a 4-hour period is 25-30 X 10(3) daltons in size, and some of this low molecular weight RNA can be aminoacylated and shown to be transfer RNA. A low level of ribosomal RNA synthesis is detectable at this stage. The contribution, if any, of the rabbit embryonic genome to development prior to the 16-cell stage remains to be defined.