Prolonged pause during follicle growth contributes to offspring throughout reproductive lifespan

Ovulation results from cyclical recruitment of quiescent oocytes for growth, hence the primordial follicles established during development comprise the lifelong reserve. In mouse fetal oocytes, we lineage traced a subset with earlier onset and slower progression through meiotic prophase using Sycp3CreERT2/+; Rosa26nT-nG/+ females. Initially GFP labeled first meiotic entrants survived at elevated rates and contributed to primordial follicles as well as the first wave of growing follicles in neonates. However, by puberty, only oocytes in the first wave of growing follicles remained. To identify the transcriptional signature of GFP+ oocytes in growing follicles, we performed Bulk RNA sequencing, which revealed remarkable similarity between the transcriptomes of GFP+ (first meiotic entrants) and RFP+ (the rest of the oocytes), with only 12 genes showing differential expression. Overall design: MII oocytes from Sycp3CreERT2/+; Rosa26nT-nG/+ female mice were collected and categorized into three groups (GFP high, GFP low, and RFP) based on their GFP and RFP fluorescence signals. The oocyte numbers used for the three replicates in each group were as follows: GFP high: 15, 15, 16; GFP low: 49, 52, 57; and RFP: 88, 87, 100. Total RNA isolated from oocytes was enriched for poly-A mRNA followed by library preparation for RNA-seq. Following quality control, libraries were pooled and sequenced on an Illumina platform.