Immunohistochemical study on the development of serotoninergic neurons in the chick: II. Distribution of cell bodies and fibers in the spinal cord.

Developmental changes of serotonin (5-hydroxytryptamine) neurons and fibers in the spinal cord of the embryo and posthatching chick were studied with immunohistochemical techniques with the aid of an antibody against serotonin. The first serotonin-immunoreactive fibers were found in the marginal layer of the cervical and lumbar spinal cord on embryonic days 6 and 8, respectively. There was a time lag of a few days between the first appearance of serotonin fibers in the marginal layer (embryonic days 6-8) and the time of penetration of serotonin fibers into the mantle layer (embryonic day 8 or older). The developments of serotonin innervation in the rostral parts of the spinal cord precedes that of caudal regions. Serotonin fibers penetrating into the mantle layer of the lumbar spinal cord were first found in lamina VII on embryonic day 8, whereas there were no serotonin-immunoreactive fibers in lamina IX by embryonic day 10. Large differences were found between embryonic day 16 and posthatching day 5 with regard to the density of serotonin varicosities and fibers in lamina IX, where profiles of soma and large-sized dendrites were heavily covered with varicosities. Laminae I and II first received serotonin fibers on embryonic day 16 and had a much denser innervation by posthatching day 5. There were no traces of serotonin fibers in lamina III in the stages examined up to posthatching day 5. Serotonin fibers were located in the lateral and ventral marginal layers in all specimens examined; only a few fibers were found in the dorsal marginal layer. Although few, serotonin-immunoreactive cell bodies were found in an area around the central canal of all animals from embryonic day 8 to adult. Some of these were located in the ependymal layer and sent processes toward the central canal; there were a small number of cells with long, fine processes. Serotonin-immunoreactive fibers in the spinal cord were not altered in regions rostral to the spinal transection, whereas all the serotoninergic fibers of the supraspinal origin were eliminated in the spinal cord caudal to the gap.