Developmental study of the shortened spinal cord in the adult tiger pufferfish, Takifugu rubripes (Teleostei)

The spinal cords of vertebrates are generally divided into the cord proper and the minute filum terminale. While the spinal cord extends the entire le ngth of the vertebral canal in the adult tiger puffer, Takifugu rubripes, t he cord proper is greatly reduced in length and almost all of the canal is occupied by the filum terminale, which is tape-like rather than thread-like . The dorsal and ventral roots of the spinal nerves extend, respectively, a bove and below the filum terminale; as a whole, these form a massive cauda equina. Supramedullary cells are found in the rostral half of the medulla o blongata caudal to the cerebellum. In 4-mm long tiger puffers, the spinal c ord is cylindrical and supramedullary cells are found in the rostral half o f the cord. In 7-mm puffers, the longitudinally arranged ventral roots appe ar ventrally in the middle portion of the spinal cord. In 15-mm puffers, th e dorsal and ventral roots run longitudinally along the spinal cord and hav e noticeably increased in number. Supramedullary cells are located in the r ostral 15% of the cord. In 21-mm puffers, the spinal cord in large part bec omes dorsoventrally flattened. In 30-mm puffers, the spinal cord becomes mu ch flatter, and supramedullary cells now are located mainly in the medulla oblongata. These observations indicate that formation of the shortened spin al cord proper is due to at least two developmental processes. First, the e longation of the spinal cord proper is remarkably less than that of the ver tebral canal. Second, the bulk of the spinal cord proper is translocated to the cranial cavity, where it is transformed into part of the medulla oblon gata. (C) 2000 Wiley-Liss, Inc.