Morphology of the distal tip of the spinal cord in Alligator mississippiensis.

Secondary neurulation is a common feature of vertebrate development, which in non-mammalian and non-anuran vertebrates, results in the formation of a caudal spinal cord. The present study was undertaken to describe the terminal end of the caudal spinal cord in a crocodylian, a group chosen for their unique status of a living-tailed archosaur. The caudal spinal cord of Alligator mississippiensis terminates near the intervertebral joint between the fourth and fifth terminal vertebrae. Prior to this termination, the dorsal root ganglia get proportionately larger, then stop before the termination of the spinal cord; and the gray matter of the spinal cord is lost producing an unusual morphology in which an ependymal-lined central canal is surrounded by only white matter which is not divided into a cauda equina. The inner layer of the meninges (the pia-arachnoid) courses over the distal end of the spinal cord and forms a ventral attachment, reminiscent of a very short Filum terminale; there is no caudal cistern. The dura extends beyond the termination of the spinal cord, continuing for at least the length of the fourth terminal vertebra, forming a structure herein termed the distal meningeal sheath. During its course, the distal meningeal sheath surrounds a mass of mesothelial cells, then terminates as an attachment on the dorsal surface of the vertebra.