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Development of the posterior neural tube (PNT) in human embryos is a complicated process which involves both primary and secondary neurulation. Normal development of the human PNT should be understood to elucidate the pathogenesis of spinal neural tube defects, but there have been some discrepancies among previous reports. We examined histologically 20 human embryos around the stage of the posterior neuropore closure and found that the developing PNT can be divided into three parts: (1) the most rostral region which corresponds to the posterior part of the primary neural tube; (2) the junctional region of the primary and secondary neural tubes; and (3) the caudal region which emerges from the neural cord. In the junctional region, the axially condensed mesenchyme (AM) intervened between the neural plate/tube and the notochord. The AM appeared to be incorporated into the most ventral part of the primary neural tube, and no cavity was observed in the AM. Interestingly, we found three cases of human embryos with lumbosacral myeloschisis in which the open primary neural tube and the closed secondary neural tube overlapped dorso-ventrally. The open and closed neural tubes appeared to be part of the primary and the AM-derived secondary neural tubes, respectively. Thus, these findings suggest that in embryos with lumbosacral myeloschisis, the AM may not be incorporated into the ventral part of the primary neural tube but aberrantly differentiate into the secondary neural tube containing cavities, leading to dorso-ventral overlapping of the primary and secondary neural tubes. These findings suggest that the AM in human embryos plays some role in normal and abnormal development of the human posterior neural tube.