Onset of notochord shrinkage in human embryos

The notochord is a fundamental element of chordates and a major organizer of developmental processes. In humans, the notochord shrinks and eventually degenerates during development, with the exception of the nucleus pulposus in the intervertebral disc. However, the timing of notochord shrinking is only mentioned based on fragmentary embryonic data or with no reference, although the degenerative change of the notochord means the loss of its functional role. The morphological pattern of the human notochord during the Carnegie Stage (CS) 13-23 was assessed to determine the timing of the notochord shrinkage. The results showed that a decrease in the diameter of the notochord and the appearance of the vacant space between the notochord and the perichordal tube occurred at the cervicothoracic region at CS 17 and subsequent stages, and the appearance of the vacant space between the notochord and the perichordal tube was noted at the lumbar region at CS 18 and subsequent stages. Therefore, our study concluded that the notochord starts to shrink at CS 17-18 (39-45 days post-fertilization).