Exploring the Abnormal Characteristics of the Ovaries During the Estrus Period of Kazakh Horses Based on Single-Cell Transcriptome Technology

The ovary is among the earliest organs to undergo age-related degeneration, limiting the reproductive potential of elite horses and constraining the growth of the equine industry. Follicular development during estrus is a key determinant of fertility, yet the molecular mechanisms underlying its decline, particularly at the level of specific ovarian cell types, remain poorly understood in equids. Here, we constructed a single-cell transcriptomic atlas to investigate ovarian changes in Kazakh horses. Using single-cell RNA sequencing (scRNA-seq), we profiled 112,861 cells from follicle-containing and follicle-absent ovaries, identifying nine distinct ovarian cell types and their subtypes, each with distinct gene expression signatures. Functional enrichment analyses revealed cell type-specific engagement in biological pathways, including ECM-receptor interaction, PI3K-Akt signaling, and oxytocin signaling. Gene expression patterns indicated tightly regulated processes of ovarian activation and cell differentiation. Notably, stromal cells exhibited high expression of <i>ROBO2</i>, <i>LOC111770199</i>, and <i>TMTC2</i>, while smooth muscle cells (SMCs) were marked by elevated levels of <i>CCL5</i>, <i>KLRD1</i>, and <i>NKG7</i>. Moreover, cell-cell interaction analyses revealed robust signaling interactions among SMCs, endothelial cells, neurons, and proliferating (cycling) cells. Together, these findings provide a comprehensive single-cell transcriptomic map of normal and abnormal ovarian states during estrus in Kazakh horses, offering novel insights into the cellular mechanisms of follicular development and identifying potential diagnostic biomarkers and therapeutic targets for ovarian quiescence in equids.