Chromatin remodelling and histones mRNA accumulation in bovine germinal vesicle oocyte

In several mammalian species, a major remodelling of the germinal vesicle chromatin is known to occur towards the end of the oocyte growth. Various chromatin configurations have been identified. However, the mechanisms involved in this remodelling process are yet not completely understood. In the bovine species, four distinct and progressive states of chromatin compaction have been characterized and are linked to a gradual acquisition of the developmental potential. Germinal vesicle oocytes were collected and separated in four groups according to their degree of chromatin condensation, ranging from a diffused state to a fully compacted configuration. To better understand the molecular changes undergoing in the oocyte during that critical period, transcriptomic analysis was performed with the EmbryoGENE microarray platform (custom Agilent 44K) in order to identify mRNA modulations occurring during the remodelling process. An important proportion of genes showed a reduced mRNA level as the chromatin becomes more compacted, which strongly correlates with the decreased transcriptional activity at the end of oocyte growth. However, among the transcripts presenting an increased mRNA level, many were associated with the histone genes. Depending on the specific histone (H2A, H2B, H3, H4 or linker H1), an important mRNA accumulation occurs in the oocyte before ovulation. This dataset then offers a unique opportunity to picture the stock of accumulated histone mRNAs either to complete the build-up of a compacted chromatin, but also to ensure the protamine-histone replacement following fertilization and the completion of the first three cell cycles.