Embryonic development of the mammalian caudal neural tube.

In the literature, some controversy still exists about the embryonic developmental processes involved in the formation of the caudal neural tube. Therefore, a three-dimensional and histological study concerning the normal development of the caudal neural tube was performed on both mouse and human embryos. Three developmental processes can be distinguished in caudal neural tube development: caudal neuropore closure, secondary neurulation, and degeneration and differentiation of the secondary neural tube. Caudal neuropore closure occurs at the level of somite 32-34 in both species. Therefore, primary neurulation leads to the formation of all spinal cord segments and ganglia. Secondary neurulation involves cell deposition from a cluster of neurectodermal cells at the caudal end of the closed neural tube, directly around a lumen, the lumen always in contact with the lumen of the primary neural tube. This process leads only to the formation of the primordia of the filum terminale and ventriculus terminalis and, possibly, part of the conus medullaris. Secondary neurulation is followed by a period characterized by degeneration and differentiation of the secondary neural tube. Its lumen and neural tissue will disappear, whereas part of the secondary neurectodermal cells differentiate to a fibrous layer comparable and continuous with the marginal layer of the primary neural tube. This fibrous layer represents the future filum terminale. The embryological processes indicated above can be helpful in the interpretation of congenital anomalies affecting the caudal spinal cord and spine.