Cross-species identification of conserved cell-type specific mechanisms during early placenta development in ruminants.

The majority of pregnancy loss in ruminants occurs during the first two months of gestation, and a failure in placenta development is a major cause of pregnancy loss in cattle after day 20. Gaining a cell-type level understanding of normal placental development is essential for uncovering how this critical organ, responsible for nutrient exchange, gas transfer, and waste removal, fails during pregnancy loss. This study integrated single-cell RNA sequencing (scRNA-seq) from sheep and cattle during early placental development. Nineteen distinct cell populations were identified across species, with mesenchymal, epithelial, and trophoblast cells showing largely conserved expression profiles. Interestingly, two trophoblast clusters were unique to cattle, with one expressing IFNT2 (uninucleate) and another expressing CSH2 and PAG17 (binucleate). Genes associated with epithelial-to-mesenchymal transition (EMT), such as SNAI1, SNAI2, ZEB1, VIM, CDH1, and CLDN4, showed dynamic and prominent expression patterns in trophoblasts. Pseudotime and cell-cell signaling analyses supported the occurrence of EMT in uninucleate trophoblasts. Gene ontology comparisons revealed similarities between ruminant and human extravillous trophoblasts, suggesting conserved EMT across placental types. Collectively, these findings highlight EMT as a potentially critical process in early ruminant placentation.