Sex differences in placental structure and gene expression in ICR mice during embryonic development

The placenta is a vital organ for fetal development, providing structural support and mediating both nutrient exchange and maternal-fetal immune interactions. However, there are many aspects of murine placental development, which are used as a model organism, that are not fully understood compared to humans. In this study, we examined sex differences in placental development in ICR mice, a commonly used outbred strain, by analyzing the junctional zone (JZ) and labyrinth (LAB) from embryonic day (E) 8.5 to E18.5. At E14.5, the JZ area and its proportion relative to the total placental area (JZ + LAB) were significantly larger in females than in males. Consistently, expression levels of JZ-associated genes were also higher in female placentas at this stage. In contrast, such sex differences in placental morphology and gene expression were not observed in C57BL/6J mice, suggesting that observed features are unique to the ICR strain. Collectively, our findings demonstrate a female-biased expansion of the JZ in ICR mice at mid-gestation and underscore the presence of strain-specific patterns in sex-dependent placental development.