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Efficient reprograming of the donor cell genome in nuclear transfer (NT) embryos is linked to the ability of the embryos to sustain full-term development. As the nuclear architecture has recently emerged as a key factor in the regulation of gene expression, we questioned whether early bovine embryos obtained from transfer of cultured fibroblasts into enucleated oocytes would adopt an embryo-like nuclear organization. We studied the dynamics of constitutive heterochromatin in the stages prior to embryonic genome activation by distribution analysis of heterochromatin protein CBX1 (HP1), centromeric proteins CENPA and CENPB, and histone H3 three-methylated at lysine 9. Then we applied descriptive, quantitative, and co-localization analyses. A dramatic reorganization of heterochromatic blocks of somatic donor cells was first observed in the late one-cell stage NT embryos. Then at two- and four-cell stages, we found two types of NT embryos: one displaying noncondensed heterochromatin patches similar to IVF embryos, whereas the second type displayed condensed heterochromatin blocks, normally observed in IVF embryos only after the eight-cell stage. These analyses discriminate for the first time two contrasted types of nuclear organization in NT embryos, which may correspond to different functional states of the nuclei. The relationship with the somatic nucleus reprograming efficiency is discussed.

To investigate the embryonic genome organization upon fertilization and somatic cell nuclear transfer (SCNT), we tracked HP1β and CENP, two well-characterized protein markers of pericentric and centromeric compartments respectively, in four types of embryos produced by rabbit in vivo fertilization, rabbit parthenogenesis, rabbit-to-rabbit, and bovine-to-rabbit SCNT. In the interphase nuclei of rabbit cultured fibroblasts, centromeres and associated pericentric heterochromatin are usually isolated. Clustering into higher-order chromatin structures, such as the chromocenters seen in mouse and bovine somatic cells, could not be observed in rabbit fibroblasts. After fertilization, centromeres and associated pericentric heterochromatin are quite dispersed in rabbit embryos. The somatic-like organization is progressively established and completed only by the 8/16-cell stage, a stage that corresponds to major embryonic genome activation in this species. In SCNT embryos, pericentric heterochromatin distribution typical for rabbit and bovine somatic cells was incompletely reverted into the 1-cell embryonic form with remnants of heterochromatin clusters in 100% of bovine-to-rabbit embryos. Subsequently, the donor cell nuclear organization was rapidly re-established by the 4-cell stage. Remarkably, the incomplete remodeling of bovine-to-rabbit 1-cell embryos was associated with delayed transcriptional activation compared with rabbit-to-rabbit embryos. Together, the results confirm that pericentric heterochromatin spatio-temporal reorganization is an important step of embryonic genome reprogramming. It also appears that genome reorganization in SCNT embryos is mainly dependent on the nuclear characteristics of the donor cells, not on the recipient cytoplasm.

Heat stress compromises bovine oocyte developmental competence, but the effects of high temperature during oocyte maturation on embryo chromatin organization is unknown. In this study bovine oocytes were exposed to heat shock (41°C) for 12 h during in vitro maturation and then submitted to in vitro fertilization. The heat shock did not affect (P > 0.05) the cleavage but reduced (P < 0.01) the blastocyst rate on Day 7 and Day 8. No effect (P > 0.05) on total cell number was found, but the heat shock increased (P < 0.05) the proportion of apoptotic cells in blastocysts at Day 8. Immunofluorescence analysis of H3K9me3 and HP1 was performed in embryos at 52 h post in vitro fertilization. An accumulation of H3K9me3 in the nuclei of embryos derived from heat-shocked oocytes at four-cell and eight-cell stages was found. Also, a non-expected higher proportion (P < 0.05) of four-cell stage embryos displaying nuclei with increased HP1 fluorescence was observed, suggesting an abnormal chromatin compaction in embryos from heat-shocked oocytes. Embryos at eight-cell stage derived from heat-shocked oocytes displayed lower (P < 0.05) relative amount of HSP40 transcripts than control ones. In conclusion, heat shock before fertilization has an effect on embryo chromatin, influencing the accumulation of H3K9me3 and HP1 in early embryos as well as further development.

Even though it generates healthy adults, nuclear transfer in mammals remains an inefficient process. Mainly attributed to abnormal reprograming of the donor chromatin, this inefficiency may also be caused at least partly by a specific effect of the cloning technique which has not yet been well investigated. There are two main procedures for transferring nuclei into enucleated oocytes: fusion and piezoelectric microinjection, the latter being used mostly in mice. We have, therefore, decided to compare the quality and the developmental ability, both in vivo and in vitro, of embryos reconstructed with electrofusion or piezoelectric injection. In addition, the effect of piezo setups of differing electric strengths was investigated. Along with the record of the rate of development, we compared the nuclear integrity in the blastomeres during the first cleavages as well as the morphological and cellular quality of the blastocysts. Our results show that the piezo-assisted micromanipulation can induce DNA damage in the reconstructed embryos, apoptosis, and reduced cell numbers in blastocysts as well as a lower rate of development to term. Even if piezo-driven injection facilitates a faster and more efficient rate of reconstruction, it should be used with precaution and with as low parameters as possible.