Molecular and cellular aspects of planarian regeneration

Neoblasts, which are classically defined as prospective totipotent stem cells, supporting planarian regeneration, now have been identified by expression of the DjvlgA gene coding for a vasa-type ATP-dependent RNA helicase. DjvlgA-positive cells are distributed in the mesenchymal space from head to tail, participating in formation of organ rudiments and the blastema during regeneration. Interestingly, neoblasts begin to transcribe tissue-specific genes in a position-dependent manner, while they are still in the mesenchymal space. This is prior to migrating to the organ rudiments or blastema, and at a time when they are not yet morphologically distinguishable from neoblasts. We speculate that the mRNAs transcribed in the committed cells may be trapped in a complex with RNA helicase(s), forming a 'chromatoid body' and are not translated into protein until they migrate to the rudiment. After formation of the rudiments, these committed cells may receive a signal for organogenesis and then start to translate these mRNAs as well as to express pattern formation genes for organogenesis.