Identification of "spinal enlargements" correlating with paired and unpaired fins in zebrafish

Cervical and lumbar enlargements involving several spinal segments are present in the spinal cord of tetrapods, reflecting the heavy motor and sensory innervation of limbs. Such spinal enlargements are not apparent in teleost fishes. However, teleosts possess paired pectoral and pelvic fins that are homologous to forelimbs and hindlimbs, respectively, and modest spinal enlargements might be present in teleosts as well. In the present study, therefore, we have investigated the innervation of different fins by spinal nerves in zebrafish. We then investigated the changes in transverse sectional areas of the spinal cord and gray matter, referring to the levels of spinal cord innervating different fins. These analyses revealed that enlargements of the spinal cord and gray matter are indeed present for pectoral and pelvic fins that are paired appendages like limbs in tetrapods. In addition, enlargements are also present for the dorsal, anal, and caudal fins. The present study thus suggests that spinal enlargements are present also in teleosts, although they are modest and can only be detected by analyses at the histological level. The present study also indicates that enlargements can be formed not only for paired fins that are homologous to limbs of tetrapods but also for unpaired fins. That is, spinal enlargements are present for all appendages or fins in teleosts.