Helical Body Axis Orientations in Human Embryonic Development

Background In mouse embryos, the body axis typically follows a right‐handed helical pattern; however, a definitive orientation in human embryos has not been established. This study aimed to characterize the body axis orientation in human embryos (CS13–CS17) from the Kyoto Collection. Methods Embryos were classified as right‐helical (RH), left‐helical (LH), and middle (M) using MRI‐based morphological assessment. Results RH orientation was predominant at CS13, whereas it became comparable to LH at CS14. From CS15 to CS17, LH became dominant, nearly doubling the frequency of RH by CS15. The proportion of M‐pattern embryos increased with advancing Carnegie Stages, reaching 70% at CS17. As vertebral column chondrification begins at CS17–18, these findings suggest that the helical body axis is established before chondrogenesis, particularly during CS13–CS15. Internal organ laterality (stomach, heart, intestines, and liver) appeared consistent among body axis orientations in CS15–CS17 embryos. Conclusion The results demonstrate substantial variability in human embryonic body axis orientation, in contrast to the well‐defined pattern in mice, and provide insights into body axis formation in human embryos and their potential role in left–right asymmetry.