Global activation of the embryonic genome is the most critical event in early mammalian development. After fertilization, a rich supply of maternal proteins and RNAs support development whereas a number of zygotic and embryonic genes are expressed in a stage-specific manner leading to embryonic genome activation (EGA). However, the identities of embryonic genes expressed and the mechanism(s) of EGA are poorly defined in the bovine. Using the Affymetrix bovine-specific DNA microarray as the biggest available array at present, we analyzed gene expression at two key stages of bovine development, matured oocytes (MII) and 8-cell-stage embryos, constituting the ultimate reservoir for life and a stage during which EGA takes place, respectively. Key genes in regulation of transcription, chromatin-structure cell adhesion, and signal transduction were up-regulated at the 8-cell stage as compared with 8-cell embryos treated with ␣-amanitin and MII. Genes controlling DNA methylation and metabolism were upregulated in MII. These changes in gene expression, related to transcriptional machinery, chromatin structure, and the other cellular functions occurring during several cleavage stages, are expected to result in a unique chromatin structure capable of maintaining totipotency during embryogenesis and leading to differentiation during postimplantation development. Dramatic reprogramming of gene expression at the onset of development also has implications for cell plasticity in somatic cell nuclear transfer, genomic imprinting, and cancer.gene expression ͉ microarray

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The development of totipotent bovine embryonic cell cultures has great value in cattle breeding. They provide: (1) a mechanism for making large numbers of clonal offspring by nuclear transfer; (2) an efficient gene transfer system through the use of selectable markers to select transgenic cells; and (3) a mechanism for site-specific gene transfer or deletion by homologous DNA sequence recombination. Bovine embryonic cell cultures have been established from blastocyst inner cell mass (ICM) cells, morulae and the precompaction 16-20-cell stage. All have exhibited similar morphology to mouse embryonic stem (ES) cells, pluripotency on differentiation and proliferation in culture. Culture systems have consisted of microdrop loose suspension short-term cultures or long-term cultures on bovine or murine fibroblast feeder layers, in either a microdrop or a culture dish. The relative merit of culture systems or media requirements for mitosis and prevention of differentiation have not been determined. At present, totipotency is also unknown for cultured cells of the 16-20-cell stage. For cultured ICM cells, totipotency was demonstrated by the birth of four calves from ICM cells cultured 27 days or less in a loose suspension microdrop. Advanced pluripotency and perhaps totipotency was demonstrated in one fetus in a recently reported study where morulae cells cultured in vitro were chimaerized with non-cultured cells. DNA fingerprinting to associate cell lines with offspring and karyotyping to ascertain chromatin normalcy is important in ES cell research. Data pertaining to the use of each are presented.

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