Single-cell roadmap of bovine oogenesis and somatic niche interactions during fetal ovarian development

The intricate process of female germline development, from the initial specification of primordial germ cells to the formation of primordial follicles, takes place during embryonic and fetal development. In this study, researchers utilized single-cell RNA-sequencing to create an atlas of the bovine fetal ovary at four distinct gestational stages, namely day 50 and timed pregnancies on days 70, 90, and 120.

This investigation unveiled 14 distinct transcriptionally diverse cell populations, encompassing stromal, epithelial, endothelial, immune, somatic support cell, and germ cell lineages.

By employing sub-clustering techniques, the germ cell population was effectively organized into six developmental stages, which included primordial germ cells, transitioning oogonia, proliferative oogonia, committed oogonia, meiotic prophase I oogonia, and oocytes. Similarly, the somatic support cell compartment was segmented into five granulosa cell subtypes - steroidogenic, pre-granulosa 1, pre-granulosa 2, pre-granulosa 3, and epithelial cells.

Through trajectory analysis, the team was able to reconstruct the developmental pathway of germ cells into oocytes, showcasing the sequential activation of both meiotic and oocyte-specific gene programs.

This research demonstrated that by day 120, all six germ cell stages were present within the fetal ovary, indicating asynchronous oogenesis within this developmental stage. This finding was further corroborated by protein immunolocalization, and was shown to be region-specific. Additionally, the study mapped intercellular signaling networks between germ cells and the somatic niche, revealing strong interactions through various pathways such as BMP, KIT, IGF, IGFBP, WNT, and MDK, with temporal specificity varying across gestational ages.

The findings from this research establish the bovine germ cell and pre-granulosa cell subtypes as having significant transcriptional parallels with similarly-staged cells found in human fetal ovaries. This discovery highlights the bovine fetal ovary as a reliable model for studying human germ cell and ovarian development. Taken together, these data provide a comprehensive, single-cell resolution developmental roadmap for bovine oogenesis, furthering our fundamental understanding of gametogenesis and informing potential strategies for advanced assisted reproduction techniques.