Immunohistochemical study on the development of serotoninergic neurons in the chick: I. Distribution of cell bodies and fibers in the brain.

The ontogenetic development of the serotoninergic system in the embryonic as well as in posthatching chick brain was studied with an indirect immunohistochemical technique with the aid of a specific antibody to serotonin (5-hydroxytryptamine). By embryonic day 4, rostral and caudal groups of serotonin-immunoreactive cell populations appeared in the mesencephalon and rostral and caudal rhombencephalon. At this stage, the rostral group had a considerable number of labelled cells that sent axons toward more rostral parts of brain, whereas the caudal group consisted of a small number of scattered serotonin-immunoreactive cells. The number of serotonin-positive cells increased with development, such that by embryonic day 8 almost all the serotoninergic cell groups found in the adult chick were already present. Serotoninergic-positive cells appeared in the paraventricular organ of the diencephalon as early as embryonic day 10. Judging from the cytoarchitectural organization of serotonin-immunoreactive cells, all of the serotoninergic cell groups in the chick brain seemed to be fully developed by embryonic day 16. On embryonic day 4, serotonin-immunoreactive fibers were found to enter into the marginal layer of the mesencephalon. Subsequently, serotonin-positive fibers ascended in the marginal layer of the brainstem up to the levels of the diencephalon and to the telencephalon on embryonic day 6 and 8, respectively. Serotonin-positive fibers, which first began to penetrate into the mantle layer on embryonic day 8, reached to the rostral pole of the telencephalon by embryonic day 10. In general, serotonin fibers were found in almost all brain regions by embryonic day 16. However, "terminal formation" in some nuclei did not seem to begin until the late embryonic or posthatching period. These observations indicate that the initial development of serotoninergic cell groups occurs during the first half of the 20th day of the incubation period of the chick. However, a longer time, ranging from early embryonic to posthatching stages, is necessary for the complete development of the serotoninergic projections.