Myeloschisis in early human embryos.

Four early human embryos with open myeloschisis are reported. These are a thoracolumbar myeloschisis in Carnegie developmental stage 12, a cervical myeloschisis in stage 13, and two lumbosacral myeloschisis in stage 14. All of them are the smallest human embryos with this type of malformation ever reported. In these embryos, the neuroectodermal junction is smooth and there is no microscopic evidence that the neural tube is forced open after its proper closure. The presence of the lesion in such early embryos, especially in one of stage 12, implies that the lesion evolved from the neural plate which never closed. Cellular polarity and the limiting membrane in the lesion are generally well preserved in spite of apparent overgrowth of the neural tissue. These findings do not support the hypothesis of LEMIRE et al. that the defective external limiting membrane predisposes to a loss of cellular polarity and resultant neural overgrowth.