The identification of genes and deduced pathways from the mature human oocyte can help us better understand oogenesis, folliculogenesis, fertilization, and embryonic development. Human metaphase II oocytes were used within minutes after removal from the ovary, and its transcriptome was compared with a reference sample consisting of a mixture of total RNA from 10 different normal human tissues not including the ovary. RNA amplification was performed by using a unique protocol. Affymetrix Human Genome U133 Plus 2.0 GeneChip arrays were used for hybridizations. Compared with reference samples, there were 5,331 transcripts significantly up-regulated and 7,074 transcripts significantly down-regulated in the oocyte. Of the oocyte up-regulated probe sets, 1,430 have unknown function. A core group of 66 transcripts was identified by intersecting significantly up-regulated genes of the human oocyte with those from the mouse oocyte and from human and mouse embryonic stem cells. GeneChip array results were validated using RT-PCR in a selected set of oocyte-specific genes. Within the up-regulated probe sets, the top overrepresented categories were related to RNA and protein metabolism, followed by DNA metabolism and chromatin modification. This report provides a comprehensive expression baseline of genes expressed in in vivo matured human oocytes. Further understanding of the biological role of these genes may expand our knowledge on meiotic cell cycle, fertilization, chromatin remodeling, lineage commitment, pluripotency, tissue regeneration, and morphogenesis.

We generated a database of the human oocyte transcriptome by comparing the transcripts in the oocyte with the reference samples, which contain mRNA from several somatic tissues.Human oocytes were obtained from three patients undergoing an assisted reproductive treatment (ART) at the Unit of Reproductive Medicine of Clinica Las Condes, Santiago, Chile. It is important to emphasize that the routine in vitro fertilization protocol at Clinica Las Condes calls for fertilizing only those oocytes that will be transferred into the uterus of the patient. Therefore, there is always a surplus of oocytes. We then had the opportunity to use specific criteria to select donors as follows: (i) <35 years old, (ii) reproductively healthy with regular ovulatory cycles, (iii) male factor as the only cause of infertility, and (iv) considerable number of developing follicles that assured spared oocytes. The experimental protocol was reviewed and approved by a local independent Ethics Review Board. All donors signed informed consent. At the time this manuscript was submitted, all three donors had already conceived; two of them got pregnant during the ART cycle in which our samples were collected, and the third one got pregnant after a spontaneous cycle with artificial insemination using donated sperm. Ovarian stimulation, oocyte retrieval, and cell lysis were performed as described in Supporting Materials and Methods, which is published as supporting information on the PNAS web site. Three groups of 10 oocytes each were used. Reference RNA (100 ?g) was prepared by mixing 10 ?g of total RNA from each of 10 different normal human tissues, including skeletal muscle, kidney, lung, colon, liver, spleen, breast, brain, heart, and stomach (Ambion). Transcriptional profile of each sample was probed by using Affymetrix Human Genome U133 Plus 2.0 GeneChips.