Whole Structure of the Photoreceptors in the Ascidian Larva Visualized by an Antibody Against Arrestin (Ci-Arr)

The anterior brain vesicle of ascidian larvae contains two distinct pigment cells. Ultrastructure of these pigment cells has been shown that the anterior pigment cell is an otolith for perception of gravity and the posterior pigment cell is an ocellus for light reception. The larva has remarkably simple central nervous system (CNS) composed of about 330 cells. We focused to study neural networks of visual systems. In the present paper, we report the whole structure of the photoreceptors of the ascidian larva visualized by an antibody against arrestin. Visual arrestin is the key protein for the termination of phototransduction and one of the abundant proteins in photoreceptors. Recently, we cloned an arrestin homologue gene, Ci-arr and the expression of Ci-arr was found to be restricted to the photoreceptors in the ocellus. To study the whole structure of the photoreceptors in the larva, we prepared an antibody against Ci-Arr. It is found that anti Ci-Arr antibody specifically stains the photoreceptors, including the cell bodies, the axons, and the nerve terminals. The photoreceptor cell bodies lies in row outside the pigment cup which penetrate the pigment cell and is continuous with the outer segments of the photoreceptor cell, inside the concavity of the pigments. The axons form bundle into a single tract. The tract extends toward the midline, where the nerve terminals diverge and seem to form synapses