Comparative analysis of rhesus macaque and human placental organoids highlights evolutionary differences in placentation.

Throughout evolution, the placenta has diversified in structure and composition while maintaining its essential role in supporting fetal development. Trophoblasts, cells responsible for nutrient exchange and immune modulation, are a conserved feature of all placentas. Although primate placentas share morphological similarities, species-specific differences in gene expression remain poorly characterized, largely due to the lack of tractable in vitro models. To address this gap, we developed rhesus macaque placental organoids representing trophoblast and maternal-derived decidual cell types and compared them with human placental organoids. Using integrated single-cell and single-nucleus RNA sequencing, we identified shared and species-specific transcriptional programs across corresponding trophoblast lineages. We further reconstructed lineage trajectories to multinucleated syncytiotrophoblast and extravillous trophoblast populations, revealing conserved differentiation pathways alongside divergent gene expression signatures. This work establishes in vitro models of the nonhuman primate placenta and defines molecular distinctions between human and rhesus trophoblasts, offering insights into evolutionary adaptations underlying placental development.