Molecular actions guiding neural regeneration in planarian

Planarian is among the simplest animals that possess a centralized nervous system (CNS), and its neural regeneration involves the replacement of cells lost to normal 'wear and tear' (cell turnover), and/or injury. In this review, we state and discuss the recent studies on molecular control of neural regeneration in planarians. The spatial and temporal expression patterns of genes in intact and regenerating planarian CNS have already been described relatively clearly. The bone morphogenetic protein (BMP) and Wnt signaling pathways are identified to regulate neural regeneration. During neural regeneration, conserved axon guidance mechanisms are necessary for proper wiring of the nervous system. In addition, apoptosis may play an important role in controlling cell numbers, eliminating unnecessary tissues or cells and remodeling the old tissues for regenerating CNS. The bilateral symmetry is established by determination of anterior-posterior (A-P) and dorsal-ventral (D-V) patterns. Moreover, neurons positive to dopamine, serotonin (5-HT), and gamma-aminobutyric acid (GABA) have been detected in planarians. Therefore, planarians present us with new, experimentally accessible contexts to study the molecular actions guiding neural regeneration.