Structure of the parapineal organ of the adult rainbow trout,Salmo gairdneri Richardson

Summary The parapineal organ of the teleost Salmo gairdneriRichardsonsu1 was investigated with the light and electron microscopes. It is a small cell mass, 0.1–0.3 mm in diameter, containing a narrow lumen and consistently situated to the left of the pineal stalk and dorsal to the left habenular nucleus. It is connected with the habenular nucleus through a conspicuous parapineal tract. The parapineal organ continues to grow at least until the fish reaches sexual maturity and shows no sign of cellular degeneration at the age of two years.The parapineal tissue consists of supporting cells and nerve cells; the latter give rise to the axons of the parapineal tract. Furthermore, a small number of receptor cells of the type existing in the pineal organ is present. No morphological evidence was obtained to suggest a sensory or secretory function of the parapineal organ.The existence of the parapineal organ in the adult pike, Esox lucius, L., and of a connection between the pineal tract and the habenular commissure in Salmo gairdneri is briefly reported. The results are discussed in the light of existing literature.Work done with the aid of a research scholarship from the Alexander von Humboldt Foundation, Bad Godesberg, Germany. —The electron microscope used in this study was placed at the disposal of Prof. Oksche by the Deutsche Forschungsgemeinschaft. —I wish to thank Prof. Oksche for the facilities made available at his institute and for his helpful interest in my work.