DEVELOPMENT OF SEROTONINERGIC SYSTEM IN THE BRAIN AND SPINAL-CORD OF THE CHICK

(1) Development of serotonin positive cells and fibers was immunohistochemically studied by the use of an antibody against serotonin. (2) Serotoninergic neurons were first observed in the immature rhombencephalon raphe nuclei on embryonic day (E)4, where two clusters of serotonin positive neurons were located: one observed at the rostral part of the rhombencephalon corresponding to the dorsal raphe nuclei had many serotonin positive cells: the other located at the caudal part of the rhombencephalon corresponding to the medullary raphe nuclei of the adult animals had only a small number of serotoninergic cells. (3) By E8 the number of serotonin positive cells in the brain stem increased, and virtually all the raphe nuclei found in an adult animal were located. (4) Serotonin positive fibers in the marginal layer reached up to the diencephalon and telencephalon on E6 and E8, respectively. (5) Serotonin positive cells were found beside the midline regions in the ventral part of the spinal cord of the embryonic as well as posthatching chick. (6) Because almost all the serotoninergic fibers in the spinal cord originated from the brain stem raphe nuclei, propriospinal serotonin positive cells were considered as phylogenetic vestiges. (7) Serotoninergic fibers were first found in the marginal layer of the cervical and lumbar spinal cord on E6 and E8, respectively. (8) There was a waiting period of a few days before they penetrated into the mantle layer. (9) Terminal arbolization of the serotoninergic fibers started from late embryonic periods (E16 <), and was maximized within one week of hatching. (10) Thereafter the density of serotonin positive fibers decreased in all the regions of the spinal cord. (11) Developmental changes of the density of serotonin determined with a high performance liquid chromatography were the same as those determined through immunohistochemistry. Namely the density of serotonin increased linearly from E6 to hatching period, and reached the maximum value one week posthatching. (12) The density of the serotonin in the adult spinal cord was about half of the maximum value. (13) It is to say that the densities of serotonin and serotoninergic fibers transiently increased around one week posthatching. (14) Following the transient increase serotoninergic fibers were eliminated from the neuropil, the fibers were localized in the specific regions of the motor nucleus: motor neuron pools of extensor muscles of the hip joint in the lumbosacral spinal cord. (15) Present study demonstrated that there are four stages in development of the raphe-anterior horn projection: (a) pathway formation in the marginal layer at early embryonic days, (b) terminal formation in the mantle layer (ventral horn) at peri-hatching period, (c) transient increase of serotonin positive fibers around one week posthatching, and (d) localization of serotoninergic fibers at specific motor neuron pools.