Sensory cutaneous papillae in the sea lamprey (Petromyzon marinus L.): II. Ontogeny and immunocytochemical characterization of solitary chemosensory cells.

Solitary chemosensory cells (SCCs) and their innervating fibers are located in the respiratory system of many vertebrates, including papillae on lamprey gill pores. In order to gain stronger insight for the role of these chemosensory cells, we examined immunocytochemical and innervation characteristics, as well as abundance at the different stages of the lamprey life cycle. The SCCs were distinguished from the surrounding epithelial cells by calretinin and phospholipase C140 immunoreactivity. Nerve fibers extended into the gill pore papillae, as far as the SCCs and serotonergic fibers extended from the underlying dermis into the papillar base. Gill pore papillae were absent and SCCs were sparse during the larval stage and in newly transformed lamprey. Few SCCs were located on small nub-like papillae during the parasitic juvenile stage, but SCCs were abundant on prominent papillae in migrating and in spawning adults. These findings show similarities between the SCCs in lampreys and other vertebrates and suggest that gill SCC function may be important during the feeding juvenile and the adult stages of the lamprey life cycle.