Contrary to the current belief, the spinal cord of the rat does not terminate with the conus terminalis, but its basic components (central canal, gray matter, white matter) continue in the filum terminale (FT). Proceeding caudally in the conus terminalis, first the motoneuron cell column discontinues in the ventral horn. More caudally the dorsal horns separate from the intermediate zone, and discontinue. The ensuing filum terminale consists of the slit-like central canal lined by ciliated ependymal cells, the periventricular gray matter and the peripheral white matter. Uniform small size neurons and glial cells populate the gray matter.
Some of the small size neurons in the FT are located immediately attached to the ependymal cell lining of the central canal in periventricular location. Neural stem cells or neural progenitor cells reside preferentially close to the central canal. Adult spinal cord in culture yielded proliferating stem cells under appropriate circumstances. When different spinal cord segments were compared, lumbar/sacral cord that included the caudal end of the spinal cord (coccygeal segments) gave fourfold greater frequency in stem cells than thoracic cord. These and other results as well as the small size of the neurons seem to raise the possibility that nerve cells in the FT may represent neurons in an early phase of commitment and differentiation.
Ultrastructural analysis revealed various types of axodendritic and axosomatic synapses as well as fine unmyelinated axons. The white matter consists mainly of myelinated nerve fibers. The neuronal components of the filum terminale, if they occur also in the human spinal cord, should be involved in the diagnosis and treatment of various diseases, e.g. tethered spinal cord syndrome, vascular malformations and disraphysm.