Serotonin‐containing Cells in the Nervous System and Other Tissues During Ontogeny of a Lancelet, Branchiostoma floridae

Serotonin‐containing cells are described by immunohistochemistry throughout lancelet ontogeny. Such cells are first detected in the 2‐day larva: these are (1) enterochromaffin cells in the inner epithelium of the gut and (2) anterior serotonergic neurons at the rostral end of the nerve cord. In the 6‐day larva, relatively low levels of serotonin appear in ventro‐lateral perikarya and cell processes of intraspinal serotonergic 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 clusters of perikarya with axons that descend the nerve cord; at later developmental stages, these axons extend almost to the posterior end of the body. In the 21‐day (mid‐metamorphic) larva, serotonin can no longer be detected in the anterior serotonergic neurons, but serotonin‐containing 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 neurons may play a role in larval photoreception and that the antero‐lateral serotonergic neurons may be homologous to vertebrate hindbrain neurons with axons descending the spinal cord to modulate undulation (if this homology is valid, the anterior limit of the lancelet hindbrain would be roughly 100 μm behind the rostral tip of the nerve cord).