Monoamines in the brain of the lancelet, Branchiostoma lanceolatum. A fluorescence-histochemical and electron-microscopical investigation.

Three types of monoamine-containing neurones and fibres can be discriminated in the brain of the lancelet. Two types of elongated cerebrospinal fluid-contacting neurons, located in the ventral and the dorsolateral part of the brain, exhibit formaldehyde-induced catecholamine fluorescence. These neurones contain dense-core vesicles 75-100 nm in diameter. Their apical portion possesses cilia, displaying a 9 X 2 + 2 arrangement of their internal tubuli, and projecting into the ventricle. Basal processes from the ventrally situated perikarya abut upon the meninx and may discharge their catecholamines into the circulatory system. Fibres exhibiting catecholamine fluorescence originate from the dorsolaterally situated perikarya and run ventrocaudally to the neuropil, where they form numerous swellings of the bouton en passant type. A third type of perikarya in the posterior part of the brain displays specific green fluorescence. Further, neurones characterized by a specific yellow fluorescence are present in the anterior part of the brain and the anterior part of the neural tube. The rapid photodecomposition of the latter fluorophore indicates that these cells contain an indolamine.