Structure of the Planarian Central Nervous System (CNS) Revealed by Neuronal Cell Markers

TLDR

Planarians are among the most primitive animals and possess a central nervous system. The study aims to elucidate the origin and evolution of the planarian CNS by isolating a neural marker gene and mapping its expression. Using in situ hybridization, the authors mapped the ventral CNS, revealing a cephalic ganglion mass and paired ventral nerve cords, with an inverted U‑shaped brain‑like structure comprising nine dorsal branches, eye‑associated axons, and taste‑sensory auricles. The planarian CNS’s anterior‑posterior layout mirrors that of vertebrate embryonic primary neurons, implying an early evolutionary blueprint predating the dorsal shift of the CNS.

Abstract

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 planarian 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, and 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 region of the inverted U-shaped brain. The 6(th)-9(th) branches cluster more closely and form auricles on the surface which may function as the sensory organ of taste. We found that the gross structure of the planarian CNS along the anterior-posterior (A-P) axis is strikingly similar to the distribution pattern of the "primary" neurons of vertebrate embryos which differentiate at the neural plate stage to provide a fundamental nervous system, although the vertebrate CNS is located on the dorsal side. These data suggest that the basic plan for the CNS development 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 ventral to the dorsal side.