The role of pigment cells in the brain of ascidian larva.

Abstract

The functions of melanin in the pigment cells, the ocellus and the otolith, of ascidian larvae were studied by their swimming behavior and cell morphology with and without 1-phenyl-2-thiourea (PTU), an inhibitor of vertebrate tyrosinase. Melanin formation in both the otolith and the ocellus of PTU-treated larvae at 12 hours of development was completely inhibited. These larvae were unable to swim because of abnormal tail development, but expression of rhodopsin in the outer segments of the photoreceptor was normal. In the PTU-treated larvae at 15 hours of development, melanin formation in the ocellus was inhibited, but that in the otolith seemed to be normal. The photic behavior of these larvae was normal, as was rhodopsin expression in the outer segments. However, the treated larvae lost upward swimming behavior. Synchrotron radiation X-ray fluorescence images showed that metallic elements of K, Ca, and Zn in the statocyte of larva were greatly decreased by PTU treatment, which may result in lowering the specific gravity of the pigment mass. SEM observations showed that the statocyte of Ciona intestinalis was supported by three parts, a foot-piece of the statocyte itself and two fibrous spring-like structures produced from protuberances. All three structures were synaptotagmin-positive. Movement of the statocyte would be detected by these three structures and thus would be responsible for the gravitational orientation.