Dynamic spiral artery remodeling in early human pregnancy: an analysis of specimens collected with a standardized protocol†

Human uterine spiral artery remodeling (SAR) is a tightly regulated process involving complex interactions between interstitial and endovascular extravillous trophoblasts (iEVTs and enEVTs) and diverse maternal decidual cell populations. However, the intrinsic spatiotemporal dynamics of SAR in human placentation remain poorly understood, largely due to the limited availability of high-quality maternal-fetal interface specimens. Electively terminated early pregnancies offer a valuable resource for studying SAR in situ, yet inconsistent methods for distinguishing fragmented villous and decidual tissues have hindered reproducibility and interpretation. Herein we present a standardized protocol for the classification and characterization of high-quality maternal-fetal interface specimens from elective terminations by integrating stereomicroscopic evaluation with confirmation by immunohistochemistry and immunofluorescence microscopy. Combined with multiplex immunofluorescence imaging with cell-type-specific markers, this approach enabled precise spatial mapping and quantification of key morphological and cellular events in SAR from gestational weeks 5-10. Our analyses reveal that SAR initiates as early as week 5 with extraluminal recruitment of natural killer (NK) cells, followed by the formation of tightly packed EVT plugs within the lumens of spiral arteries in the decidua compacta; these plugs progressively extend deeper into the vessels and gradually loosen as gestation progresses. Notably, enEVTs appear to acquire NK cell-like phenotypes that may facilitate the displacement of endothelial and smooth muscle cells, promoting progressive vessel dilation. In summary, we provide a robust and reproducible method for assessing physiological SAR in early human pregnancy, promoting the adoption of our methodology in future studies of pathological SAR and related pregnancy disorders.