STRUCTURE OF THE PLANARIAN CENTRAL-NERVOUS-SYSTEM (CNS) REVEALED BY NEURONAL CELL MARKERS

Planarians are considered to be among the most primitive animals which developed the central nervous system (CNS). To understand the origin and evolution of the CNS, we have isolated a neural marker gene from a planarian, Dugesia japonica, and analyzed the structure of the planar ian CNS by in situ hybridization. The planarian CNS is located on the ventral side of the body, and composed of a mass of cephalic ganglions in the head region and a pair of ventral nerve cords (VNC). Cephalic ganglions cluster independently from VNC, are more dorsal than VNC, an d form an inverted U-shaped brain-like structure with nine branches on each outer side. Two eyes are located on the dorsal side of the 3(rd) branch and visual axons form optic chiasma on the dorsal-inside regio n of the inverted U-shaped brain. The 6(th)-9(th) branches cluster mor e closely and form auricles on the surface which may function as the s ensory organ of taste. We found that the gross structure of the planar ian CNS along the anterior-posterior (A-P) axis is strikingly similar to the distribution pattern of the ''primary'' neurons of vertebrate e mbryos which differentiate at the neural plate stage to provide a fund amental nervous system, although the vertebrate CNS is located on the dorsal side. These data suggest that the basic plan for the CNS develo pment along the A-P axis might have been acquired at an early stage of evolution before conversion of the location of the CNS from the ventr al to the dorsal side.