Transcription of paternal Y-linked genes in the human zygote as early as the pronucleate stage

Global activation of the embryonic genome occurs at the 4- to 8-cell stage in human embryos and is marked by continuation of early cleavage divisions in the presence of transcriptional inhibitors. Here we demonstrate, using reverse transcriptase-polymerase chain reaction (RT-PCR), the presence of transcripts for two paternal Y chromosomal genes, ZFY and SRY in human preimplantation embryos. ZFY transcripts were detected as early as the pronucleate stage, 20-24 h post-insemination in vitro and at intermediate stages up to the blastocyst stage. SRY transcripts were also detected at 2-cell to blastocyst stages. The expression of SRY and ZFY at these early stages and the faster cleavage rate of male embryos observed in many mammalian species focuses attention on the role of events in sex determination prior to gonad differentiation.