P-209 Development and characterization of human uterine myometrium in prenatal stage: insights from a single-cell atlas and three-dimensional imaging

Abstract Study question What’s the timepoint of human uterine myometrial formation and how dose it develop in the prenatal stage. Summary answer This study identified that the myometrium layer initially forms at postconceptional week (PCW) 11, ESR1+, IGFBP5, POSTIN may play pivotal role in myometrial development. What is known already Using immunohistochemistry staining, previous studies observed a faint expression of α-SMA in the outer mesenchyme at approximately 8-9 weeks of gestation and showed the early development of muscular layer in the uterine wall, and the expression of α-SMA becomes more prominent in the uterine corpus and cervix by 18 weeks, suggesting the maturation of smooth muscle cells may commence from 18 weeks of gestation. ESR-1 gene encodes estrogen receptor alpha, which plays a pivotal role in menstruation, pregnancy, and parturition. In adult female myometrium, estrogen performs functions in proliferation before term and contractile at term. Study design, size, duration This is an observational study. To investigate when exactly the smooth-muscle tissue appears during uterine development, we have collected 14 uterine tissue samples from 14 aborted fetuses across seven gestational time points from PCW 8 to PCW 15. Participants/materials, setting, methods Uterus from aborted fetuses from PCW8- PCW15 cover the fusion of Mullerian tube and development of Myometrium. The 10X genomics scRNA-seq and immune labeling were performed. In addition, we have integrated scRNA-seq data from adult uterine tissue for comparison. Main results and the role of chance We provide a single cell atlas of the human developing uterus, and we observed that smooth-muscle cells (SMCs), identified by ACTA2, TAGLN, MYH11, exhibited a significant increase in abundance starting from PCW 11, but weaker signals of SMCs were also detected at earlier stages. Using the iDISCO technique with continuous α-SMA labeling to examine the localization of α-SMA in the uterus between PCW10- PCW12, we observed the entire process of myometrial development in 3-D level. We utilized an oblique section of 3-D reconstruction uterus and extended it to 150 μm. Subsequently, apparent signals became visible in the middle of the uterine wall and the uterine fundus. Collectively, these findings suggest that the anatomical myometrium layer initially forms at PCW 11. We used the CytoTRACE to assess the capacity of differentiation of adult and fetal SMCs, SMCs come from fetus appeal a significant grander potential. Specifically, fetal uterine SMCs showed higher expression of ESR1, IGFBP5, and POSTN, while adult uterine SMCs exhibited higher expression of PDGFRB and IGFBP7. These results suggested ESR1+ SMCs could be the precursor of SMCs and SMCs in fetal uterus could be in a primary state. Limitations, reasons for caution While our study did not capture later stages of development or postnatal changes. Future studies are needed to understand the full spectrum of myometrial development. It is also important to note that the findings in this study are specific to human and may not directly translate to other species. Wider implications of the findings Our study provides a basis for studying abnormal myometrial development and its association with various reproductive disorders, such as the abnormal uterine development in female fetus with Turner syndrome. Potential biomarkers or therapeutic targets for uterine malformations and abnormal uterine contractility may also of research interest. Trial registration number not applicable