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

The major events of female germline establishment, from primordial germ cell (PGC) specification to assembly of primordial follicles, occur during embryonic/fetal development. This study presents a single-cell RNA-sequencing atlas of the bovine fetal ovary at four gestational timepoints: estimated day 50, and timed pregnancies at days 70, 90, and 120, capturing the progression of PGCs through commitment, meiotic entry, and early oocyte growth. Fourteen transcriptionally distinct cell populations were identified, including stromal, epithelial, endothelial, immune, somatic support cell, and germ cell lineages. Sub-clustering of the germ cell population resolved six developmental stages (PGCs, transitioning oogonia, proliferative oogonia, committed oogonia, meiotic prophase I oogonia, and oocytes), while that of the somatic support cell compartment revealed five granulosa cell subtypes (steroidogenic, pre-granulosa 1, pre-granulosa 2, pre-granulosa 3, and epithelial cells). Trajectory analysis reconstructed the developmental path of PGCs to oocytes, with sequential activation of meiotic and oocyte-specific gene programs. Representation of all six germ cell stages at day 120 pointed to asynchronous oogenesis in the fetal ovary, which was validated and shown to be region-specific by protein immunolocalization. Intercellular signaling networks between germ cells and the somatic niche were mapped, revealing strong interactions through BMP, KIT, IGF, IGFBP, WNT, and MDK pathways with temporal specificity across gestational ages. The bovine germ cell and pre-granulosa cell subtypes demonstrate significant transcriptional parallels with similarly-staged cells from human fetal ovaries, establishing the cow as a reliable model for human germ cell and ovarian development. Collectively, these data provide a developmental roadmap for bovine oogenesis at the single-cell resolution that advances fundamental understanding of gametogenesis and informs strategies for advanced assisted reproduction.