SEROTONIN-CONTAINING CELLS IN THE NERVOUS-SYSTEM AND OTHER TISSUES DURING ONTOGENY OF A LANCELET, BRANCHIOSTOMA-FLORIDAE

Serotonin-containing cells are described by immunohistochemistry throu ghout lancelet ontogeny. Such cells are first detected in the 2-day la rva: these are (1) enterochromaffin cells in the inner epithelium of t he gut and (2) anterior serotonergic neurons at the rostral end of the nerve cord. In the 6-day larva, relatively low levels of serotonin ap pear in ventro-lateral perikarya and cell processes of intraspinal ser otonergic neurons scattered along the nerve cord. In the 18-day (early metamorphic) larva, antero-lateral serotonergic neurons are detected near the rostral end of the nerve cord as two small, bilateral cluster s of perikarya with axons that descend the nerve cord; at later develo pmental stages, these axons extend almost to the posterior end of the body. In the 21-day (mid-metamorphic) larva, serotonin can no longer b e detected in the anterior serotonergic neurons, but serotonin-contain ing cells are found subjacent to the inner epithelium of the digestive caecum and in the peribranchial epithelium covering the primary gill bars. In the discussion, we suggest that the anterior serotonergic neu rons may play a role in larval photoreception and that the antero-late ral serotonergic neurons may be homologous to vertebrate hindbrain neu rons with axons descending the spinal cord to modulate undulation (if this homology is valid, the anterior limit of the lancelet hindbrain w ould be roughly 100 mum behind the rostral tip of the nerve cord).