Successful human development is dependent upon a cascade of events following fertilization. Unfortunately, knowle dge of these critical events in humans is remarkably incomplete. Although hundreds of thousands of human embryos are cultured yearly at infertility centers worldwide , the vast majority fail to develop in culture or following transfer to the uterus. In this study, we sought to characterize global patterns of gene expression in individual, normal embryos during the first three days

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... embryonic life using microarrays; we then compared gene expression between normally growing and growth-arrested embryos using quantitative PCR. Our results documented several novel findings: First, we found that a complex pattern of gene expression exists; most genes that are transcriptionally modulated during the first three days following fertilization are not upregulated, as was previously thought, but are downregulated. Second, we observed that the majority of genes exhibiting differential expression during preimplantation development are of unknown identity and/or function. Third, we show that embryonic transcriptional programs are clearly established by day 3 following fertilization, even in embryos that arrested prematurely with 2-, 3-or 4-cells . This indicates that failure to activate transcription is not associated with the majority of human preimplantation embryo loss. Finally, taken together, these results provide the first global analysis of the human preimplantation embryo transcriptome , and demonstrate that RNA can be amplified from single oocytes and embryos for analysis by cDNA microarray technology, thus lending credence to additional studies of genetic regulation in these cell types, as well as in other small biological samples.