Spinal dysraphism Myeloschisis 2016-02-25T21:26:13Z The severe form of a neural tube defect where the open neural tube appears as a flattened, plate-like mass of nervous tissue with no overlying membrane. UMLS:C0266507 SNOMEDCT_US:360530005 Myeloschisis is a cleft spinal cord arising from a failure of neural tube closure. The placode is defined as a plate of embryonic epithelial cells constituting a primordial cell group from which the spinal cord arises and is often seen in a myelomeningocele. In a myelomeningocele, an exposed placode is often a myeloschisis. In other words, a myeloschisis is a collapsed myelomeningocele. When the placode in the myelomeningocele sac is a cleft spinal cord, this anomaly is classified as myeloschisis rather than myelomeningocele. Open neural tube defect