Standardizing Cell Ontology terms for cross-study integration in the female reproductive tract.

The female reproductive tract is essential for fertility, pregnancy, and overall health, yet many of its cellular components remain poorly defined. Recent advances in single-cell and spatial transcriptomics have begun to reveal this complexity, but inconsistent naming of cell types and states has limited the ability to compare findings across studies. To address this challenge, an international group of reproductive biologists and ontology experts collaborated to harmonize annotations within Cell Ontology, focusing on the ovary, fallopian tube, and uterus. Standardized terms are proposed for major epithelial, stromal, and germ cell populations, supported by marker gene sets and anatomical linkages. This framework provides a shared reference that can be used to harmonize existing datasets and guide annotation of future studies, enabling consistent classification of oocytes, granulosa, theca, luteal, epithelial, stromal, and immune cells across tissues. By establishing a unified taxonomy, this work lays the foundation for integrating datasets, supporting cross-tissue comparisons, and advancing the understanding of reproductive biology, health, and disease.