Specification of the interstitial cell lineage in the fetal mouse ovary requires balanced Notch signaling

The ovary contains two major somatic lineages, granulosa cells and interstitial cells, that arise from progenitors within the coelomic epithelium. However, how these two lineages diverge during ovarian development remains unclear. By analyzing joint single-nucleus transcriptomic and chromatin accessibility profiles of murine ovarian cells at the onset of ovary formation, we identified two somatic progenitor populations from the coelomic epithelium distinguished by expression of the nuclear receptors Nr5a1 and Nr2f2. Based on their transcriptomic trajectories, the Nr5a1+ epithelial cells preferentially transitioned toward the granulosa lineage whereas the Nr2f2+ epithelial cells differentiated into mesenchymal populations. This lineage relationship was supported by Nr2f2 lineage tracing experiments that fetal Nr2f2+ progenitors contribute to ovarian interstitial cells postnatally. To define the molecular features underlying this divergence, we performed differential gene expression and chromatin accessibility analyses and found that Nr2f2+ epithelial cells, but not Nr5a1+ cells, were enriched for Notch pathway components and Notch effector motifs. Consistently, lineage tracing of Notch-responsive cells marked Nr2f2+ interstitial cells in postnatal ovaries, whereas ectopic Notch activation in Nr5a1+ cells promoted expansion of the interstitial population accompanied by reduced granulosa cells. By integrating motif analysis with accessible chromatin-gene linkage, we also identified downstream targets regulated by Notch effectors in Nr2f2+ cells, which showed concordant changes upon ectopic Notch activation. These findings demonstrate that somatic cell fate is established early during ovarian development, with active Notch signaling specifying the interstitial lineage and a balanced Notch activity required for proper somatic lineage establishment. Significance StatementProper differentiation of somatic cell types in the fetal ovary lays the foundation for future ovarian function in adulthood. Understanding how each cell type is formed is essential for developing methods to intervene in ovarian diseases caused by cellular dysfunction. Given that common somatic progenitors give rise to both supporting and interstitial lineages, a main unanswered question is how these two lineages diverge apart from each other during ovarian development. By integrating joint single-nucleus transcriptomic and chromatin accessibility assays with lineage tracing, single cell RNA-sequencing, and mouse genetic models, we demonstrate the role of Notch signaling in specifying the interstitial lineage and separating it from the supporting cell fate.