Peroxisomes in the mouse ovary and their alterations during follicular development and oocyte maturation

Peroxisomes are the site of important metabolic pathways as they contain enzymes involved in the β-oxidation of fatty acids, the degradation of reactive oxygen species and the synthesis of ether-lipids and steroid hormone precursors. They are present in virtually all eukaryotic cells, yet they demonstrate considerable heterogeneity of number and protein composition. The presence of peroxisomes and concomitant activity of peroxisomal enzymes was previously documented in ovarian cells and it was suggested that peroxisomal oxidative stress management through catalase plays a pivotal role during oogenesis. In the present article, an exhaustive analysis is presented of the changes in peroxisome abundance during folliculogenesis in mouse oocytes. To this end, a quantitative assessment of the number of peroxisomes was conducted through morphometric analysis in the oocytes in follicular and oestrus cycle stages. Furthermore, a qualitative investigation was undertaken into the distribution of peroxisomal proteins involved in their biogenesis, membrane transport and lipid/hormone and reactive oxygen species metabolism in secondary and tertiary follicles. Given the metabolic connection between peroxisomes and mitochondria, the study was complemented with immunofluorescence stainings for reactive oxygen species and mitochondrial electron transport chain complexes and antioxidative enzymes. Our results confirmed the presence of peroxisomes in all follicular cells, including the oocyte. They further prove that peroxisomal abundance and protein composition are subject to variation depending on follicular and oestrus cycle stage and follicular cell type. Most strikingly, the study provided evidence of the near absence of catalase and of oxidative stress marker 8-oxo-2'-deoxyguanosine in the oocyte.