Why do planarian cells without centrioles divide and cells with centrioles do not divide?

Planarians present a unique cellular paradox: their proliferative stem cells (neoblasts) completely lack centrioles, while their post-mitotic, differentiated cells possess them. This review investigates the mechanisms and biological significance of this inverse correlation. We synthesize evidence demonstrating that neoblasts employ a robust, evolutionarily conserved acentrosomal pathway for spindle assembly, reliant on chromatin-mediated nucleation via RanGTP and motor protein-driven self-organization. This adaptation potentially confers advantages including enforced asymmetric division, metabolic economy, and a significantly reduced risk of centrosome amplification-driven genomic instability, which may underpin the planarians' extensive regenerative capabilities and resistance to tumors. Conversely, the quiescence of centriole-bearing somatic cells is not caused by the organelles themselves but is a consequence of terminal differentiation. These cells epigenetically silence the cell cycle machinery and repurpose centrioles as basal bodies for ciliogenesis. The presence of centrioles is thus a marker, not a driver, of the differentiated state. This system represents a profound uncoupling of the mitotic apparatus from the centriole, offering novel insights into stem cell biology, alternative modes of cell division, and providing conceptual frameworks for regenerative medicine and cancer research.