Sex differences in the development and progression of cardiovascular disease manifest across multiple life stages. These differences are associated with variations in cardiovascular morphology and function between the sexes. Although estrogens and sex hormones are associated with sex differences in cardiovascular diseases in reproductive adults, the molecular mechanisms of cardiovascular sex differences during development are largely unknown. Thus, we investigated sex differences in cardiovascular development. We used a newly developed coronary arteriogram system to visualize the morphology of the coronary arteries in murine anterior surface ventricles at embryonic day 17.5 by injecting nanoparticle ink at a constant pressure. No sex difference was found in the length of ventricle. Based on the boundary value of the distribution of that length, the hearts were divided into “long” and “short” groups and the diameters of the left coronary arteries were analyzed. The mean diameter of the coronary arteries was significantly smaller in females than in males only in the group with the longer length of ventricle. This ventricular size-specific sex difference was observed in the presence of vasodilators such as NOC7 (1-Hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene). When NOC7 was perfused into the left coronary arteries of embryonic day 17.5 mice, females with longer ventricles showed larger left coronary arteries than males. These sex differences in vasodilation capacity suggest that factors related to drug reactivity such as signaling pathways are present at a late embryonic stage. These results indicate that sex differences in the functional morphology of the left coronary arteries exist at a late embryonic stage in mice. NEW & NOTEWORTHY This study introduces a novel coronary angiography method for analyzing murine embryonic hearts, revealing sex differences in coronary artery morphology and contractile function in the late stage of the fetal period. By categorizing heart components based on size, we unveil nuanced insights into sexual dimorphism during this critical fetal period. This work contributes insights into the early origins of sexual dimorphism in coronary vessels, laying the foundation for further understanding of cardiovascular development.

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